14.1. Introduction to Semiconductor Devices...14. Semiconductor Devices By Ritesh Hariram 14.1....

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14. Semiconductor Devices By Ritesh Hariram 14.1. Introduction to Semiconductor Devices Semiconductor devices are electronic components that exploit the electronic properties of semiconductor materials, principally silicon, germanium, and gallium arsenide, as well as organic semiconductors. Semiconductors are thus materials whose conductivity lies between a good conductor and a good insulator. 14.1.1. Component Data A datasheet, data sheet, or spec sheet is a document that summarizes the performance and other technical characteristics of a product, machine, component (e.g., an electronic component), material, a subsystem (e.g., a power supply) or software in sufficient detail to be used by a design engineer to integrate the component into a system. 14.1.2. Component Data Sources The following are examples of component data sources The internet has a wide variety of sources. Manufacturers make their data sheets public knowledge so getting a hold of this may be in your best interest. Most libraries have a Technical section which should have sources in this regard. Your local electronics hobby store. These stores have an abundance of data sheets. 14.1.3. How to read a data sheet? Consider the diagram below. The diagram shows a typical datasheet for a diode. As mentioned earlier datasheets can be sourced from a variety websites, textbooks and electronic stores. This particular datasheet clearly displays the company logo as well as a picture depicting what the physical component looks like. The datasheet has been modified to show you exactly where important information can be found. (Fairchild Semiconductor)
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  • 14. Semiconductor Devices

    By Ritesh Hariram

    14.1. Introduction to Semiconductor Devices

    Semiconductor devices are electronic components that exploit the electronic

    properties of semiconductor materials, principally silicon, germanium, and gallium

    arsenide, as well as organic semiconductors. Semiconductors are thus materials

    whose conductivity lies between a good conductor and a good insulator.

    14.1.1. Component Data

    A datasheet, data sheet, or spec sheet is a document that summarizes the

    performance and other technical characteristics of a product, machine, component

    (e.g., an electronic component), material, a subsystem (e.g., a power supply) or

    software in sufficient detail to be used by a design engineer to integrate the

    component into a system.

    14.1.2. Component Data Sources

    The following are examples of component data sources

    The internet has a wide variety of sources.

    Manufacturers make their data sheets public knowledge so getting a hold of

    this may be in your best interest.

    Most libraries have a Technical section which should have sources in this

    regard.

    Your local electronics hobby store. These stores have an abundance of data

    sheets.

    14.1.3. How to read a data sheet?

    Consider the diagram below. The diagram shows a typical datasheet for a diode. As

    mentioned earlier datasheets can be sourced from a variety websites, textbooks and

    electronic stores. This particular datasheet clearly displays the company logo as well

    as a picture depicting what the physical component looks like. The datasheet has

    been modified to show you exactly where important information can be found.

    (Fairchild Semiconductor)

  • 14.1.4. Pin Configuration

    In electronics, a pin out (sometimes written "pin-out") is a cross-reference between

    the contacts, or pins, of an electrical connector or electronic component, and their

    functions

    14.1.5. Typical Operating Values

    This is a value on the datasheet that describes the typical operating parameters of a

    component. It is wise that the user does not exceed these values as it would destroy

    the component.

    14.1.6. Working Temperature

    Electronic components are designed to work within certain temperature parameters.

    Working out of these parameters may either damage the component or not produce

    the desired result. It is important to consult the manufacturer’s datasheet in this

    regard.

    14.1.7. Equivalent Components

    These are components that perform the same function as the component you are

    researching. Often these can be found on certain manufacturer’s datasheets.

    14.1.8. Dual in Line Packages

    In microelectronics, a dual in-line package (DIP or DIL) is an electronic device

    package with a rectangular housing and two parallel rows of electrical connecting

    pins. The package may be through-hole mounted to a printed circuit board or

    inserted in a socket. Dual-in-line packages were developed in the 1960s when the

    restricted number of leads available on transistor-style packages became a limitation

    in the use of integrated circuits.

    Figure 1: Dual in Line Package

    14.1.9. Through Hole Components vs. Surface Mount (SMT)

    There are some points that make SMT different from Through Hole, which has been

    briefly summarized below:

    SMT has helped significantly in solving the space problems that were

    commonly noticed with the Through Hole mounting.

  • The pin count has increased greatly in SMT when compared to its older

    counterparts.

    In SMT, the components are leadless and are directly mounted to the board

    surface. In Through Hole the components have lead wires that are taken to

    the wiring boards via holes.

    The Pads on the surface in SMT are not used for connection of layers on the

    printed wiring boards.

    The components in the Though Hole Technology are larger which leads to

    lower component density per unit area. The packing density that can be

    achieved with SMT is very high as this allows mounting component on both

    sides, when needed.

    SMT has made possible the applications that seemed impossible with

    through-hole components.

    SMT is suitable for high volume production that gives access to lowered cost

    of unit assemblies which is not possible with through-hole technology.

    With SMT acquiring higher circuit speed is easier due to the reduced size.

    SMT fulfils one of the prime marketing requirements while assisting in making

    high performance circuits in a very small size.

    SMT has a disadvantage as the capital investment involved in its machinery

    and production is higher.

    Designing, production, skill and technology required in implementing SMT is

    very advanced when compared to through-hole technology.

    Figure 2 through-hole component

    Figure 3: Surface mounted components

  • 14.2. Semiconductors

    14.2.1. Electron current flow vs. Conventional Current Flow

    Figure 4: Conventional Current Flow

    Figure 5: Electron Current Flow

    The direction of current movement is referred to as electron flow. Before the electron

    theory, many believed that current actually flowed from positive to negative. This is

    called conventional current flow. Figure 4 and Figure 5, illustrate this concept.

    (Circuits)

    14.2.2. Semiconductors and Solid State

    Solid-state electronics are those circuits or devices built entirely from solid materials

    and in which the electrons, or other charge carriers, are confined entirely within the

    solid material.

    Semiconductor devices are electronic components that exploit the electronic

    properties of semiconductor materials, principally silicon, germanium, and gallium

    arsenide, as well as organic semiconductors. Semiconductors are thus materials

    whose conductivity lies between a good conductor and a good insulator.

    Solid state is a term that refers to electronic circuitry that is built entirely out of

    semiconductors e.g. Solid state drives replacing magnetic drives.

  • 14.2.3. Silicon vs. Germanium

    As explained earlier semiconductors are materials whose conductivity lies between a

    good conductor and a good insulator. The diagram below, Figure 6, shows their

    crystal lattice. It can be seen that their atoms are orderly and have 4 valence

    electrons. They are thus referred to as tetravalent electrons.

    NB: Other examples of tetravalent electrons are cadmium sulphide and gallium

    arsenide.

    (Hyperphysics)

    Figure 6: Lattice structure of Silicon and Germanium

    14.2.4. Doping

    For semiconductors to be used in electronic components such as diodes, transistors,

    thyristors and integrated circuits, their conductivity has to be increased considerably.

    This can be achieved by adding controlled amounts of impurities to the

    semiconductor materials during the manufacturing process. This process of adding

    impurity materials to the semiconductors is called doping. The impurity material

    atoms must be of similar size as the silicon atoms to fit into the crystal structure

    without causing too much distortion. Because the impurity material now provides the

    negative charge carriers (free electrons) or the positive charge carriers (holes), the

    silicon now becomes an extrinsic semiconductor.

    When silicon is doped with phosphorous an n-type extrinsic semiconductor is

    formed. Phosphorous has five electrons in its outer valence shell and is therefore

    called a pentavalent material. When it is doped into the silicon crystalline structure,

    four of its valence electrons can form covalent bonds. The fifth valence electron now

    becomes a free electron that is only loosely held by the atom. The impurity atom

    (phosphorous) is therefore called the donor atom. The doped impure silicon is now

    an n-type semiconductor because the majority charge carriers are negative

    electrons. The few positive intrinsic holes due to broken covalent bonds are still

    present and are called minority carriers.

    When silicon is doped with boron a p-type extrinsic semiconductor is formed. Boron

    has three electrons in its outer valence shell and is therefore called a trivalent

    material. When it is doped into the silicon crystalline structure, three of its valence

    electrons can form covalent bonds. The fourth bond is incomplete because of a

  • missing electron. The hole that exists because of the missing electron acts like a

    positive charge that can attract an electron from any nearby silicon atom. The

    impurity atom (boron) is therefore called the acceptor atom. The doped impure

    silicon is now a p-type semiconductor because the majority charge carriers are

    positive holes. The few negative intrinsic electrons due to broken covalent bonds are

    still present and are called minority carriers. (swart, pp. 184-188)

    14.2.5. P and N Type Doping

    The figures below provide a graphical display of the crystalline structure of both P

    and N semiconductors.

    14.2.5.1. Current Flow in a P Type Semiconductor

    Figure 7: Current flow in a P-Type Semiconductor

    Figure 8: P Type Semiconductor

  • 14.2.5.2. Current Flow in an N Type Semiconductor

    Figure 9: Current flow in an N-Type Semiconductor

    Figure 10: N Type Semiconductor

    14.2.6. Majority and Minority Carriers

    The more abundant charge carriers are called majority carriers. In n-type

    semiconductors they are electrons, while in p-type semiconductors they are holes.

    The less abundant charge carriers are called minority carriers; in n-type

    semiconductors they are holes, while in p-type semiconductors they are electrons.

    The PN Junction Diode

    14.2.7. Construction of the PN Junction Diode

    The formation of a PN Junction diode is simply the bringing together of a p-type

    material and an n-type material. The p-type material forms the anode and the n-type

    material forms the cathode.

    On formation the n-region has an excess of free electrons, with the donor atoms

    fixed in position. The p-type material, on the other hand, has an excess of positive

  • holes, with the acceptor atoms fixed in position. Because of thermal agitation the n-

    region will also contain a few positive holes and the p-region a few free electrons.

    When the junction is formed, the free electrons in the n-region tend to diffuse into the

    p-region. Similarly, holes tend to diffuse into the n-region. The result of this diffusion

    is that the n-region assumes a net positive charge and the p-region a net negative

    charge.

    Because of this charge distribution a potential difference exists across the junction.

    This internal potential barrier tends to oppose the further flow of electrons and holes

    and may be represented by a virtual battery across the junction. Electrons diffusing

    across the junction recombine with existing free electrons, with the result that the

    region in the vicinity of the junction becomes devoid of charge carriers. This is known

    as the depletion region.

    Figure 11, below shows the circuit symbol and block diagram of the PN Junction

    Diode. (swart, pp. 186-193)

    Figure 11: Circuit symbol and block diagram of PN Junction diode

    14.2.8. The Depletion Region

    In semiconductor physics, the depletion region, also called depletion layer, depletion

    zone, junction region, space charge region or space charge layer, is an insulating

    region within a conductive, doped semiconductor material where the mobile charge

    carriers have diffused away, or have been forced away by an electric field. The only

    elements left in the depletion region are ionized donor or acceptor impurities.

    (Wikipedia)

    14.2.9. Forward and Reverse Biasing

    14.2.9.1. Forward Biasing

    When an external battery is connected with its positive terminal to the p-type material

    and its negative terminal to the n-type material, the junction is said to be forward

    biased. The threshold of voltage across the diode is about 0.6 V for silicon and about

    0.2 V for germanium.

  • The external battery opposes and overcomes the effect of the virtual battery and

    carriers are easily able to cross the junction. The depletion region disappears and

    there is a steady flow of positive holes and free electrons across the junction.

    Together, these constitute a relatively large flow of current from the external battery.

    This is termed the forward current.

    Figure 12, below illustrates the Forward Biased Circuit

    Figure 12: Forward Bias Circuit

    14.2.9.2. Reverse Biasing

    When the polarity of the battery is reversed, with the positive terminal connected to

    the n-type material and the negative terminal to the p-type material, the p-n junction

    is reversed biased. The battery now helps the action of the junction voltage, thereby

    widening the depletion layer. The negative terminal supplies more electrons to the p-

    type material and the positive terminal supplies more holes to the n-type material.

    The junction voltage increases, repelling electrons and holes further into the material

    at the junction. The resistance of the junction increases, blocking off the current.

    Only a small leakage current flows, due to minority carriers crossing the junction.

  • Figure 13: Reverse Bias Circuit (Swart)

    14.2.10. Characteristic Curve and Symbol

    Figure 14: Characteristic curve of a diode

    When connected in reverse bias (negative to the anode and positive to the cathode),

    the depletion region will increase. Only a small leakage current will flow. If the

    reverse voltage increases, the depletion region will increase as well. If this voltage

    increases continuously, a point will be met when the depletion region takes over the

    entire pn-junction and the diode will break down.

    When forward biased (positive on the anode and negative on the cathode), then the

    depletion region will decrease. If the forward bias voltage is reached (0.6 V for silicon

    and 0.3 V for germanium), then conduction will take place. The amount of current

    that flows will be directly proportional to the amount of applied voltage (ohm’s law).

    14.2.11. The Diode Load Line

    A load line is used in graphical analysis of nonlinear electronic circuits; representing

    the constraint other parts of the circuit place on a non-linear device, like a diode or

    transistor. It is usually drawn on a graph of the current vs the voltage in the nonlinear

    device, called the device's characteristic curve. A load line, usually a straight line,

  • represents the response of the linear part of the circuit, connected to the nonlinear

    device in question. The operating point(s) of the circuit are the points where the

    characteristic curve and the load line intersect; at these points the current and

    voltage parameters of both parts of the circuit match. (Ref Wikipedia)

    From the characteristic curve of the diode we see that this curve is not linear. It is

    therefore not easy to calculate at which point the diode is operating at, because we

    don’t have a formula for the curve. However, we can determine the operating point

    graphically. If the diode is connected to a DC energy source in series with a load

    resistor we can find a point on the curve at which the diode is operating. To find this

    out we need to plot the characteristic curve of the resistor on that same characteristic

    curve of the diode. The intersection of these two graphs thus results in the Q-Point

    which is the operating point of the diode.

    At the Q-Point 𝐈𝐝 = 𝐈𝐫

    𝐕𝐒=𝐕𝐝 + 𝐕𝐫

    = 𝐕𝐝 + 𝐈𝐫 × 𝐑𝐥𝐨𝐚𝐝

    Where:

    𝐕𝐒 = 𝐒𝐮𝐩𝐩𝐥𝐲 𝐕𝐨𝐥𝐭𝐚𝐠𝐞

    𝐕𝐝 = 𝐃𝐢𝐨𝐝𝐞 𝐕𝐨𝐥𝐭𝐚𝐠𝐞

    𝐕𝐫 = 𝐑𝐞𝐬𝐢𝐬𝐭𝐨𝐫 𝐕𝐨𝐥𝐭𝐚𝐠𝐞

    𝐈𝐝 = 𝐃𝐢𝐨𝐝𝐞 𝐂𝐮𝐫𝐫𝐞𝐧𝐭

    𝐈𝐫 = 𝐑𝐞𝐬𝐢𝐬𝐭𝐨𝐫 𝐂𝐮𝐫𝐫𝐞𝐧𝐭

    (swart)

    Figure 15: Diode DC Load line superimposed onto Diode Characteristic Curve

    Example 1

    A silicon PN junction diode is connected to a 10V supply in series with a 1.25kΩ load

    resistor. Use a dc load line to determine the Q-point at which the diode is operating.

    Solution

  • Resistor current 𝐼𝐿 =𝑉𝑠

    𝑅𝑙

    = 10𝑉

    1.25𝐾Ω

    =80𝑚𝐴

    From the Q-Point of the graph below we can see that the diode is operating at 0.8V

    and 70mA. At this stage the diode is dissipating 56 milli watts, which is the product of

    the voltage across it and the current through it. It is important to make sure that the

    diode does not have to dissipate more heat than what it is designed for (see diode

    datasheet). The voltage across the load resistor at this point will be 9.8V, which is

    obtained from the manufacturer’s datasheet as well.

    (swart)

    Figure 16

  • 14.3. Practical: The diode load line

    A silicon PN junction diode is connected to a 5V supply in series with a 1.2 kΩ load

    resistor. Use a DC load line to determine the Q-Point at which the diode is operating.

    Aim: To construct a simple diode circuit and determine its load line.

    Requirements:

    1.25KΩ or equivalent

    1N4007 diode or equivalent

    Power supply 10V – digital analogue trainer

    What you are required to do?

    1. Connect the circuit as shown below.

    2. Measure the current across the load resistor RL

    3. Research the applicable diode and find out its exact forward conduction

    voltage.

    4. Draw a graph clearly indicating the Q-point and dc load line

    5. Show all calculations.

    6. Record your observations.

    Circuit Diagram

    (swart)

    Measurements

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  • Redraw Circuit diagram with measurements

    Graph

    Calculations

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    Observations

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  • 14.4. The Zener Diode

    14.4.1. Construction

    A Zener diode is a silicon junction diode that is designed for voltage stabilization,

    since it has the ability to maintain a constant voltage across its terminals under

    reverse bias conditions. When it is forward biased, it behaves like an ordinary

    junction diode. It is constructed in the same way as a PN Junction diode, but the

    doping levels are much higher, in order to lower the reverse breakdown levels to a

    few volts. Zener diodes with reverse breakdown voltages of between 3 V and 30 V

    are available.

    14.4.2. Principle of operation

    A conventional solid-state diode allows significant current if it is reverse-biased

    above its reverse breakdown voltage. When the reverse bias breakdown voltage is

    exceeded, a conventional diode is subject to high current due to avalanche

    breakdown. Unless this current is limited by circuitry, the diode may be permanently

    damaged or destroyed due to overheating. A Zener diode exhibits almost the same

    properties, except the device is specially designed so as to have a reduced

    breakdown voltage, the so-called Zener voltage. By contrast with the conventional

    device, a reverse-biased Zener diode exhibits a controlled breakdown and allows the

    current to keep the voltage across the Zener diode close to the Zener breakdown

    voltage. For example, a diode with a Zener breakdown voltage of 3.2 V exhibits a

    voltage drop of very nearly 3.2 V across a wide range of reverse currents. The Zener

    diode is therefore ideal for applications such as the generation of a reference voltage

    (e.g. for an amplifier stage), or as a voltage stabilizer for low-current applications.

    14.4.3. Forward Biasing

    When forward-biased, zener diodes behave much the same as standard rectifying

    diodes: they have a forward voltage drop which follows the “diode equation” and is

    about 0.7 volts for silicon and 0.3 V for germanium.

    14.4.4. Reverse Biasing

    In reverse-bias mode, they do not conduct until the applied voltage reaches or

    exceeds the so-called zener voltage, at which point the diode is able to conduct

    substantial current, and in doing so will try to limit the voltage dropped across it to

    that zener voltage point. So long as the power dissipated by this reverse current

    does not exceed the diode's thermal limits, the diode will not be harmed.

    14.4.5. Avalanche Breakthrough vs Controlled Breakthrough

    The breakdown voltage of an insulator is the minimum voltage that causes a portion

    of an insulator to become electrically conductive. The breakdown voltage of a diode

    is the minimum reverse voltage that will make the diode conduct in reverse.

    Avalanche breakdown is a phenomenon that can occur in both insulating and

    semiconducting materials. It is a form of electric current multiplication that allows

  • very large currents within materials which are otherwise good insulators. It is a type

    of electron avalanche. The avalanche process occurs when the carriers in the

    transition region are accelerated by the electric field and energies sufficiently to free

    electron-hole pairs via collisions with bound electrons.

    The Zener effect is a type of electrical breakdown in a reverse biased p-n diode in

    which the electric field enables tunnelling of electrons from the valence to the

    conduction band of a semiconductor, leading to a large number of free minority

    carriers, which suddenly increase the reverse current. Zener breakdown is employed

    in a Zener diode.

    Either the Zener or the avalanche effect may occur independently, or both may occur

    simultaneously. In general, diode junctions which break down below 5 V are caused

    by the Zener effect, while junctions which experience breakdown above 5 V are

    caused by the avalanche effect. Figure 17 below demonstrates graphically the

    relationship between Avalanche Breakthrough and Controlled breakthrough.

    (Hyperphysics)

    Figure 17: Relationship between Avalanche Breakthrough and Controlled Breakthrough

    14.4.6. The Zener as a Voltage Regulator

    When the Zener diode is forward biased, it behaves as a normal diode. When a

    small reverse voltage is applied the current is negligible. As the reverse voltage is

    increased, a value is reached at which the current suddenly increases very rapidly.

    The voltage across the Zener Diode after breakdown is termed the “reference

    voltage” and its value for a given Zener diode remains practically constant over a

    wide range of current values, provided the maximum permissible junction

    temperature is not exceeded (see manufactures data sheet). The diagram above

  • shows how a Zener Diode can be used as a voltage stabiliser. The purpose of the

    series resistor is to limit the reverse current through the diode to a safe value.

    Figure 18: Zener Voltage Stabiliser

    14.4.7. The Zener Characteristic Curve and Symbol

    Figure 19: The Zener Symbol

    (Talkingelectronics.com)

    Figure 20: Zener Diode Characteristic Curve

    R1 1

    kZ

    1 1

    N2804

    +

    -

    +

    -

    Variable Voltage

    Constant Voltage

  • 14.4.8. Zener Diode Series Resistor Calculations

    When a Zener diode is used in a circuit, it must be protected by a series resistor

    which limits the current flow to below the diode’s maximum rated reverse current

    (Izmax). This current is derived from the power rating (Pz) of the Zener which is

    provided in component catalogues. The series resistor Rs also absorbs the

    difference in voltage between the supply voltage and Zener voltage Vz

    Example 2

    Use a Zener diode to provide a stable 9.1 V level from a D.C. supply that fluctuates

    between 10 V and 11 V. The Zener diode has a maximum power rating (Pz) of 182

    milli watts. Show through calculations how the series resistor Rs is chosen and draw

    the circuit to demonstrate the configuration in which the Zener diode is used.

    Solution

    E = Vrs + Vz ...................Kirchoffs Laws

    Therefore

    Vrs = E − Vz

    = 11V − 9.1V

    = 1.9V

    Zener Current Iz =PZ

    Vz

    =182mW

    9.1V

    = 20mA

    Since this is the same current that flows through the resistor we can calculate the

    current through the resistor.

    Series Resistor Rs =Vrs

    Iz

    =1.9V

    20mA

    = 95Ω

    (Swart, pp. 105-106)

    Figure 21

  • 14.5. Practical: Determine the value of the series

    resistor for a Zener Diode

    Aim: To construct a zener voltage regulator.

    Requirements:

    Multi meter

    variable DC voltage source

    4Ω resistor

    Zener Diode IN5347(or equivalent) and

    ammeter

    What is required from you?

    Table 1 below shows data for a list of Zener Diodes. Perform the necessary

    calculations and choose the right Zener from the list to provide a stable regulated

    10V supply to a 50Ω load from an unregulated 12V supply. Calculate the value of the

    series resistor.

    Table 1

    Type No Voltage Power Rating

    IN 5240 10 0.4 IN 5242 12 0.4 IN 4740A 10 1 IN 4742A 12 1 IN 5347 10 5 IN 5349 12 5

    Build the appropriate circuit once you have conducted all calculations.

    Calculations

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  • Draw the circuit diagram

    Build your circuit and let your teacher check it before you turn on the power.

  • 14.6. The NPN Transistor

    The transistor is a 3 element device made up of semiconductor materials. One of the

    popular methods of manufacturing a transistor is by growing a crystal. Transistors

    are made, and the junctions between elements are formed, by methods similar to

    those used to make a PN Junction Diode.

    Transistors are used to control current (switch) or to amplify an input voltage or

    current. They are used to amplify an extremely small radio signal to a much larger

    signal, strong enough to operate the radio’s loudspeaker. Transistors are also used

    as electronic switches in logic and computer circuits.

    14.6.1. Construction

    NPN is one of the two types of bipolar transistors, consisting of a layer of P-doped

    semiconductor (the "base") between two N-doped layers. A small current entering

    the base is amplified to produce a large collector and emitter current. That is, when

    there is a positive potential difference measured from the emitter of an NPN

    transistor to its base (i.e., when the base is high relative to the emitter) as well as

    positive potential difference measured from the base to the collector, the transistor

    becomes active. In this "on" state, current flows between the collector and emitter of

    the transistor. Most of the current is carried by electrons moving from emitter to

    collector as minority carriers in the P-type base region. To allow for greater current

    and faster operation, most bipolar transistors used today are NPN because electron

    mobility is higher than hole mobility.

    (citycollegiate)

    Figure 22: Construction and Symbol NPN Transistor

    14.6.2. Principle of Operation

    The emitter is connected to the negative. The collector is connected to the positive.

    When a positive voltage is connected to the base and it is sufficient in size to forward

    bias the base-emitter pn-junction (Si = 0.6 V and Ge = 0.3 V), the transistor will

    become conductive. Electrons moves from the emitter to the base. The influx of

    minority carriers into the base will forward bias the base-collector junction. When this

    happens, current will flow from the base to the collector and to the positive supply.

    General Transistor equation: 𝐼𝐸=𝐼𝐵 + 𝐼𝐶 (Thomson, 2013)

  • 14.6.3. The Purpose of Biasing and Thermal Runaway

    14.6.3.1. Biasing

    Transistor Biasing is the process of setting a transistors DC operating voltage or

    current conditions to the correct level so that any AC input signal can be amplified

    correctly by the transistor. A transistors steady state of operation depends a great

    deal on its base current, collector voltage, and collector current and therefore, if a

    transistor is to operate as a linear amplifier, it must be properly biased to have a

    suitable operating point.

    (Talkingelectronics.com)

    Figure 23: Transistor Biasing Circuits

    14.6.3.2. Thermal Runaway

    Thermal runaway refers to a situation where an increase in temperature changes the

    conditions in a way that causes a further increase in temperature, often leading to a

    destructive result.

    If this situation is not rectified in a transistor it could lead to destruction of the device.

    To prevent thermal runaway a series resistor is often connected to the emitter of the

    transistor. (circuits)

    14.6.4. Forward Biasing

  • Figure 24: NPN Forward Biased Circuit

    (Talkingelectronics.com)

    To forward bias an NPN transistor it is connected as shown in the circuit above.

    1. The collector is connected to high positive voltage with respect to base i.e. Vbc

    is very high. So c-b junction is reverse biased. Vbc >> Vbe.

    2. The base is connected to low positive voltage with respect to the emitter i.e.

    Vbe is low.

    3. When we increase Vbe ≥ 0.7 V (the value 0.7 V is a typical value of potential

    barrier voltage for silicon transistors) the transistor is forward biased.

    4. Now large numbers of electrons in the emitter layer is repelled by the negative

    terminal of Vbe and they flow towards the b-e junction.

    5. They cross the junction and enter into the small base layer. Here some

    electrons combine with holes. Also, some of them are attracted by positive

    terminal of Vbe and the remaining maximum number of electrons flow into

    collector layer, crossing the second junction i.e. c-b junction.

    6. The resident electrons of the collector are repelled by these (guest) electrons

    and thus, all the electrons present in the collector layer are attracted by the

    positive terminal of Vcb.

    7. Thus, all these electrons complete their journey back into emitter layer and

    produce conventional currents in the transistor as shown in the above circuit.

    8. Thus, as per Kirchhoff Current Law, we can write, Ic + Ib = Ie.

    9. Now when Vbe is still increased, more electrons are repelled by the negative

    terminal of Vbe. So the base-emitter junction is more and more forward biased.

    Thus the base current (Ib) increases, which in turn increases Ic.

    10. Hence, we can say that collector current (Ic) is the function of base current (Ib).

    11. But there is a typical value of Vbe for each transistor, at which the collector

    current Ic no longer remains the function of base current Ib.

    12. Also, the collector current is directly proportional to the base current.

    13. In all this process, the maximum number of electrons from the emitter layer

    flow into collector layer. So the collector current is ALMOST EQUAL to the

    emitter current. Hence we say that, collector current is proportional to the

    emitter current. (yashplushtt5) (Unknown, n.d.)

  • 14.6.5. Reverse Biasing

    Figure 25: NPN Reverse Biased Circuit

    (Talkingelectronics.com)

    In this method both the junctions are reverse biased as the batteries are connected

    in opposite direction as shown in the diagram above. Due to the Vcb battery, the

    collector-base junction is reverse biased. Similarly, due to the Vcb battery, the base-

    emitter junction is also reverse biased. So charges cannot flow and current in the

    transistor is practically zero. This method is not useful as the transistor is in “cut-off”

    state since the base current is zero.

    14.6.6. NPN Input Curve

    (Talkingelectronics.com)

    Figure 26: NPN Input Curve

    Figure 26, shows the base emitter voltage against the base emitter current. IBE/VBE

    shows the input CONDUCTANCE of the transistor. As conductance I / V is the

    reciprocal of RESISTANCE (V / I) this curve can be used to determine the input

    resistance of the transistor. The steepness of this particular curve when the VBE is

  • above 1 volt shows that the input conductance is very high, and there is a large

    increase in current (in practice, usually enough to destroy the transistor!) for a very

    small increase in VBE. Therefore the input RESISTANCE must be low. Around 0.6 to

    0.7 volts the graph curves shows that the input resistance of a transistor varies,

    according to the amount of base current flowing, and below about 0.5 volts base

    current ceases.

    14.6.7. NPN Output Curve

    (Talkingelectronics.com)

    Figure 27: NPN Output Curve

    Figure 27, shows the output characteristic curve of the NPN Transistor. The near

    horizontal parts of the graph lines show that a change in collector emitter voltage VCE

    has almost no effect on collector current in this region. Therefore the graph shows

    that the output resistance of the transistor is high.

    14.6.7.1. The Transistor Characteristic Curve

    Figure 28, below provides us with a graphical representation of the comparison

    between a simple transistor circuit and its characteristic curve.

    Figure 28: Characteristic Curve

    14.6.8. Regions of Operation

    The transistor is divided into 3 distinct regions. The active region is where the

    transistor operates as an amplifier.

  • In the saturation region the transistor is fully switched on and Ib has no effect on the

    magnitude of Ic. In the cut-off region the transistor is switched off. When the

    transistor is used as a switch it operates in these two regions.

    (E-Learning)

    Figure 29: Regions of Operation

    14.6.9. The transistor DC Load Line

    (E-Learning)

    Figure 30: DC Load Line

    From the characteristic curve we can see that transistor can safely handle 12 V

    across its collector emitter terminals, and a collector current of 70 mA. When these

    two points are joined by a straight line it is termed the DC Load Line of the transistor.

    The load line is an imaginary straight line along which the operating point moves up

    and down. The Q-point (quiescent point) is the steady state operating point of the

    transistor and it is chosen at the middle of the load line.

  • From this diagram it is evident that the DC Load line provides us with a tool to

    determine the characteristics of the output signal of the amplifier. Below are just a

    few characteristics of a transistor that can be determined from the load line:

    1. The load line can be used to determine the parameters within which the

    transistor can be operated.

    2. The Q-Point is determined by the DC Biasing circuit of the transistor.

    3. By changing the supply voltage the load line can be shifted in parallel.

    4. The load line can be used to determine the AC current, voltage and power

    gain of the signal that is applied to the base of the transistor.

    14.6.10. Transistor Power related to the load line

    (E-Learning)

    Figure 31: Transistor DC Load line with Q-Point

    From the diagram above it can be seen that the transistor operates optimally at

    40𝑚𝐴 and 6V. These values are taken at the center of the characteristic curve at the point the Q-Point intersects the load line. Apart from helping us obtain the optimal

    operating point of the transistor, these two value can also help us calculate the

    Power dissipated by the transistor. Since Power is the product of current and

    voltage, similarly if these two values are multiplied they help us obtain the Power of

    the Transistor. For the transistor represented in Figure 31 above, its Power can be

    calculated as follows:

    Transisitor Power (P) = ICQ × VCQ

    = 40mA × 6V

    ICQ and VCQ are the transistors respective current and voltage values at the Q − Point

    14.6.11. Influence of the DC Load Line on the Characteristics

    of a Transistor

  • Figure 32: DC Load line with Q-Point of Transistor.

    The slope of the load line invariably has an effect on the transistor. Similarly the

    position of the Q-Point has an effect on the characteristics of the transistor. The

    position of the Q Point on the DC load line determines the maximum signal that we

    can get from the circuit before clipping can occur. Figure 32 (a) shows the Q-point

    near the cut off region. Here the signal starts to clip at point A. This is called cut off

    clipping. In Figure 32 (b) the Q Point is located near the saturation region. Clipping

    now starts at point B. This is caused by the close proximity to the saturation region.

    Figure 32 (c) has the Q point at the centre of the load line. In this condition we get

    the maximum possible output signal. The signal at this point is undistorted.

    The position of the Q point and slope of the load line play an important part in the

    use of a transistor for amplification purposes as it determines what type of amplifier

    the transistor can be used for.

    14.6.12. Transistor Symbols

    Figure 33: NPN Transistor Symbol

    Figure 34: PNP Transistor Symbol

  • 14.7. Transistor Applications

    14.7.1. The Transistor as a Switch

    The transistor can be used as an electronic switch. Such switches are used in

    computer circuits and logic circuits to supply low and high voltage levels.

    In Figure 35, when the switch is open there is no current to the base, so the

    transistor is in the cut off condition with no collector current. The entire voltage drop

    is across the transistor.

    The base resistor is chosen small enough so that the base current drives the

    transistor into saturation.

    In this example the mechanical switch is used to produce the base current to close

    the transistor switch to show the principles. In practice, any voltage on the base

    sufficient to drive the transistor to saturation will close the switch and light the bulb.

    (Hyperphysics)

    Figure 35: The Transistor as a switch

    14.7.2. Transistor as an Amplifier

    The common-emitter amplifier is designed so that a small change in voltage (Vin)

    changes the small current through the base of the transistor; the transistor's current

    amplification combined with the properties of the circuit mean that small alterations in

    Vin produce large changes in Vout.

    Various configurations of single transistor amplifier are possible, with some providing

    current gain, some voltage gain, and some both.

    From mobile phones to televisions, vast numbers of products include amplifiers for

    sound reproduction, radio transmission, and signal processing. The first discreet-

    transistor audio amplifiers barely supplied a few hundred mill watts, but power and

    audio fidelity gradually increased as better transistors became available and

    amplifier architecture evolved.

    Modern transistor audio amplifiers of up to a few hundred watts are common and

    relatively inexpensive.

  • (Wikipedia)

    Figure 36: Common emitter amplifier

    14.7.3. Transistor Gain

    The number of times an input signal is amplified is called Gain. If only voltage has

    been amplified it is called voltage gain and if only current has been amplified it is

    called current gain. It is often expressed using the logarithmic decibel (dB) units ("dB

    gain"). The symbol “A” is usually associated with gain.

    AP= Power gain

    AV= Voltage gain

    AI= Current gain

    Usually we are interested in power gain (AP). Since gain is only a ratio between

    similar parameters, it has no units.

    𝐏𝐨𝐰𝐞𝐫 𝐆𝐚𝐢𝐧 =𝐎𝐮𝐭𝐩𝐮𝐭 𝐏𝐨𝐰𝐞𝐫

    𝐈𝐧𝐩𝐮𝐭 𝐏𝐨𝐰𝐞𝐫

    𝐕𝐨𝐥𝐭𝐚𝐠𝐞 𝐆𝐚𝐢𝐧 =𝐎𝐮𝐭𝐩𝐮𝐭 𝐕𝐨𝐥𝐭𝐚𝐠𝐞

    𝐈𝐧𝐩𝐮𝐭 𝐕𝐨𝐥𝐭𝐚𝐠𝐞

    𝐂𝐮𝐫𝐫𝐞𝐧𝐭 𝐆𝐚𝐢𝐧 =𝐎𝐮𝐭𝐩𝐮𝐭 𝐂𝐮𝐫𝐫𝐞𝐧𝐭

    𝐈𝐧𝐩𝐮𝐭 𝐂𝐮𝐫𝐫𝐞𝐧𝐭

    14.7.4. Current Gain

    The current flowing between emitter and collector of a transistor is much greater than

    that flowing between base and emitter. So a small base current is controlling the

    much larger collector current. The ratio of the two currents, IC / IB is constant,

    provided that the collector emitter voltage VCE is constant. Therefore, if the base

    current rises, so does collector current.

  • This ratio is the current gain of the transistor and is given the symbol hfe. A fairly low

    gain transistor might have a current gain of 20 to 50, while a high gain type may

    have a gain of 300 to 800 or more. The spread of values of hfe for any given

    transistor is quite large, even in transistors of the same type and batch.

    14.7.5. Voltage Gain

    As mentioned earlier, gain is the amplification factor of the transistor. The number of

    times an input signal is amplified is called the gain of the amplifier. Voltage gain is

    thus the number of times the voltage is amplified in an amplifier. It is given by the

    formula:

    𝐴𝑉 =𝑂𝑢𝑡𝑝𝑢𝑡 𝑉𝑜𝑙𝑡𝑎𝑔𝑒

    𝐼𝑛𝑝𝑢𝑡 𝑉𝑜𝑙𝑡𝑎𝑔𝑒

    𝐴𝑉 =𝑉𝑜𝑢𝑡𝑉𝑖𝑛

  • 14.8. Practical: Determine the DC Load Line of a

    Transistor

    Aim: To analyse the load line of an NPN transistor

    Requirements: BC 141 transistor, 10KΩ, 100Ω and 10V power supply

    What is required from you?

    Construct the circuit below and determine the load line of the transistor from its

    characteristic curve.

    (technologia)

    Draw the Characteristic Curve of the Transistor here

  • 14.9. Practical: Build a circuit using the transistor as a

    switch

    Aim: To use a transistor as a switch

    Requirements:

    5V power supply

    Breadboard

    Digital Analogue Trainer

    Pushbutton switch

    Resistors - 220Ω, 2700Ω

    Transistor BC 548

    Leads

    Red L.E.D

    What is required from you?

    1. On your breadboard construct the circuit below.

    2. Measure the voltage across the LED when the switch is “on” and when “off”.

    Record your answers.

    3. Record the voltages across Vbe and Vce when the LED is on. Record your

    answers.

    4. Record your observations

    Circuit Diagram

  • Measurements

    Voltage across LED – ON

    Voltage across LED – OFF

    Vbe when LED ON

    Vbe when LED OFF

    Vce when LED ON

    Vce when LED OFF

    Observations

    ____________________________________

    ____________________________________

    ____________________________________

    ____________________________________

    ____________________________________

    ____________________________________

    ____________________________________

    ____________________________________

    ____________________________________

  • 14.10. The PNP Transistor

    14.10.1. Construction

    The only way that the PNP-transistor is different from the NPN-transistor is that the

    base is connected to the n-type material and the collector and emitter are the p-type

    materials.

    The emitter is connected to the positive. The collector is connected to the negative.

    When a negative voltage is connected to the base and it is sufficient in size to the

    forward bias the base-emitter pn-junction (Si = 0.6 V and Ge = 0.3 V), the transistor

    will become conductive. Electrons moves from the base to the emitter. The influx of

    minority carriers into the base will forward bias the base-collector junction. When this

    happens, current will flow from the collector to the base and through the emitter to

    the positive supply.

    Symbol

    Figure 37: Composition

    14.10.2. Principle of Operation

    The PNP transistor works essentially the same as the NPN transistor. However,

    since the emitter, base, and collector in the PNP transistor are made of materials

    that are different from those used in the NPN transistor, different current carriers flow

    in the PNP unit. The majority current carriers in the PNP transistor are holes. This is

    in contrast to the NPN transistor where the majority current carriers are electrons. To

    support this different type of current (hole flow), the bias batteries are reversed for

    the PNP transistor. A typical bias setup for the PNP transistor is shown in Figure 38.

  • Notice that the procedure used to properly bias the NPN transistor also applies here

    to the PNP transistor. The first letter (p) in the PNP sequence indicates the polarity of

    the voltage required for the emitter (positive), and the second letter (n) indicates the

    polarity of the base voltage (negative). Since the base-collector junction is always

    reverse biased, then the opposite polarity voltage (negative) must be used for the

    collector. Thus, the base of the PNP transistor must be negative with respect to the

    emitter, and the collector must be more negative than the base. Remember, just as

    in the case of the NPN transistor, this difference in supply voltage is necessary to

    have current flow (hole flow in the case of the PNP transistor) from the emitter to the

    collector. Although hole flow is the predominant type of current flow in the PNP

    transistor, hole flow only takes place within the transistor itself, while electrons flow in

    the external circuit. However, it is the internal hole flow that leads to electron flow in

    the external wires connected to the transistor.

    Figure 38: PNP Biasing

    Now let us consider what happens when the emitter-base junction is forward

    biased.

    1. With the bias setup as shown in Figure 38 above, the positive terminal of the

    battery repels the emitter holes toward the base, while the negative terminal drives

    the base electrons toward the emitter.

    2. When an emitter hole and a base electron meet, they combine. For each electron

    that combines with a hole, another electron leaves the negative terminal of the

    battery, and enters the base.

    3. At the same time, an electron leaves the emitter, creating a new hole and enters

    the positive terminal of the battery. This movement of electrons into the base and out

  • of the emitter constitutes base current flow (IB), and the path these electrons take is

    referred to as the emitter-base circuit.

    4. In the reverse-biased junction, the negative voltage on the collector and the

    positive voltage on the base block the majority current carriers from crossing the

    junction.

    5. This same negative collector voltage acts as forward bias for the minority current

    holes in the base, which cross the junction and enter the collector. The minority

    current electrons in the collector also sense forward bias-the positive base voltage-

    and move into the base.

    6. The holes in the collector are filled by electrons that flow from the negative

    terminal of the battery. At the same time the electrons leave the negative terminal of

    the battery, other electrons in the base break their covalent bonds and enter the

    positive terminal of the battery. Although there is only minority current flow in the

    reverse-biased junction, it is still very small because of the limited number of minority

    current carriers.

    7. The interaction between the forward- and reverse-biased junctions in a PNP transistor is very similar to that in an NPN transistor, except that in the PNP transistor, the majority current carriers are holes.

    8. In the PNP transistor, the positive voltage on the emitter repels the holes toward the base. Once in the base, the holes combine with base electrons. But again, remember that the base region is made very thin to prevent the recombination of holes with electrons. Therefore, well over 90 percent of the holes that enter the base become attracted to the large negative collector voltage and pass right through the base. However, for each electron and hole that combine in the base region, another electron leaves the negative terminal of the base battery (V BB) and enters the base as base current (IB). At the same time an electron leaves the negative terminal of the battery, another electron leaves the emitter as IE (creating a new hole) and enters the positive terminal of VBB. Meanwhile, in the collector circuit, electrons from the collector battery (VCC) enter the collector as Ic and combine with the excess holes from the base. For each hole that is neutralized in the collector by an electron, another electron leaves the emitter and starts its way back to the positive terminal of VCC.

  • Figure 39: Total current flow in the PNP transistor

    Although current flow in the external circuit of the PNP transistor is opposite in direction to that of the NPN transistor, the majority carriers always flow from the emitter to the collector. This flow of majority carriers also results in the formation of two individual current loops within each transistor. One loop is the base-current path, and the other loop is the collector-current path. The combination of the current in both of these loops (IB + IC) results in total transistor current (IE). (radartut)

    14.10.3. Relation to NPN Transistor

    The relation between the forward and reverse bias circuits are the same for both NPN and PNP transistors. The only difference between these transistors is the polarities of the voltages.

    The majority carriers in the PNP transistor are holes whilst the majority carriers in an NPN transistor are electrons.

    The most important thing to remember about the two different types of transistors is that the emitter-base voltage of the PNP transistor has the same controlling effect on collector current as that of the NPN transistor.

    14.10.4. Symbol

    Figure 40: PNP Transistor Symbol

  • 14.10.5. Applications

    The PNP transistor has the same applications as the NPN transistor. Some of its

    applications include:

    1. Switching circuits

    2. Amplifiers.

    14.10.5.1. Sample Circuits

    Figure 41: The PNP Transistor being used as a switch

    Figure 42: The PNP Transistor being used as an amplifier

  • 14.11. Practical: Build a circuit using a PNP Transistor

    as a switch.

    Aim: To use a PNP transistor as a switch

    Requirements:

    12V power supply

    Breadboard

    Digital Analogue Trainer

    Pushbutton switch

    Resistors -2k and 1K

    Transistor BC 557

    Leads

    Red L.E.D

    What you are required to do?

    1. Connect the circuit below

    1. Draw your circuit with the switch.

    2. Record all observations.

    Hint: Take note of the polarity of an NPN Transistor

  • Circuit Diagram

    Observations

    ________________________________________

    ________________________________________

    ________________________________________

    ________________________________________

    ________________________________________

  • 14.12. Thyristors – The Silicon Controlled Rectifier

    (SCR)

    14.13. Thyristors have at least four layers of alternating n- and p-type

    materials. They act as bi-stable switches and are activated by applying

    a pulse to the gate. Typical examples of Thyristors are The Silicon

    Controlled Rectifier (SCR) and The Triac. Thyristors have no

    amplification ability; therefore they are used in switching circuits only.

    Their characteristics make them particularly useful in power control

    circuits such as motor speed control and light dimmers.

    14.13.1. Construction

    The silicon controlled rectifier (SCR) is made of four alternating layers on n- and p-

    type material. The top p-type material is the anode and the bottom p-type material is

    the cathode. The bottom p-type material is the gate and is used to trigger the SCR

    (turn it on). A depletion region will form around each pn-junction. Figure 43 below

    shows the symbol and composition of the SCR.

    Figure 43: The SCR

    14.13.2. Principle of Operation

    In reverse bias, the SCR acts as a normal diode. When the anode is connected to a

    positive and the cathode is connected to a negative, then the SCR starts to become

    forward biased. When the gate receives a positive pulse, then the bottom pn-junction

    becomes forward biased and conduction takes place. The large amount of electrons

  • flowing into the bottom p-type material is seen on the top n-type material as a

    negative supply. This allows the top pn-junction to become forward biased and

    conduction takes place through the cathode and anode of the SCR. The gate pulse

    can now be removed.

    The SCR will remain active until the supply current drops below the holding current,

    the voltage across the anode and cathode drops below the holding voltage or the

    supply is reversed. (Thomson, 2013)

    Figure 44 below illustrates the SCR’S operation.

    p

    n

    p

    n

    anode

    cathode

    gate

    1

    +

    +

    -

    ---3

    2

    Figure 44: Operation of the SCR

    14.13.3. Biasing

    Figure 45: SCR Biasing

    (Talkingelectronics.com)

  • Figure 45 above it is evident that in order to switch an SCR on a gate pulse is

    needed.

    14.13.4. Symbol

    (Wikipedia)

    Figure 46: SCR Symbol

    14.13.5. Characteristic Curves

    Figure 47 shows the typical characteristic curve of a SCR (Panel, 2005)

    Figure 47: SCR Characteristic Curve

    Break over voltage –minimum voltage required to turn SCR on.

  • Holding current – minimum anode current required to keep the SCR in the on

    state.

    Latching current – Minimum anode current required to maintain the SCR in

    the on state immediately after the SCR has been turned on and the gate

    current has been removed

    Peak reverse voltage – This is the maximum reverse voltage that can be

    applied to the SCR without conducting in the reverse direction.

    Forward current rating – maximum anode current that the SCR is capable of

    passing without destruction

    14.13.6. Applications

    Silicon Controlled Rectifiers have no amplification ability; therefore they are used in

    switching circuits only. Their characteristics make them particularly useful in power

    control circuits such as motor speed control and light dimmers.

    SCR Phase Control

    Figure 48: Phase Control

    14.13.6.1. Operation

    At the start of each positive half-cycle the SCR is “OFF”. On the application of the

    gate pulse, it triggers the SCR into conduction and remains fully latched “ON” for the

    duration of the positive cycle. If the thyristor is triggered at the beginning of the half-

    cycle (θ = 0º), the load (a lamp) will be “ON” for the full positive cycle of the AC

    waveform (half-wave rectified AC) at a high average voltage of 0.318 × Vp

  • As the application of the trigger pulse increases along the half-cycle (θ = 0º to 90º),

    the lamp is illuminated for less time and the average voltage delivered to the lamp

    will also be proportionally less reducing its brightness.

    We can therefore use a silicon controlled rectifier as an AC light dimmer as well as in

    a variety of other AC power applications such as: AC motor-speed control,

    temperature control systems and power regulator circuits, etc.

    Thus far we have seen that a thyristor is essentially a half-wave device that conducts

    in only the positive half of the cycle when the Anode is positive and blocks current

    flow like a diode when the Anode is negative, irrespective of the Gate signal.

    14.13.6.2. The SCR as a Relaxation Oscillator

    In electronics a relaxation oscillator is a nonlinear electronic oscillator circuit that

    produces a non-sinusoidal repetitive output signal, such as a triangle wave or square

    wave.

    Circuit Diagram

    Figure 49: Relaxation oscillator Circuit

    From the circuit, when power is applied, capacitor Ct charges and the voltage across

    resistor RT decreases. When the voltage across RT drops to 0.6 V less the gate

    voltage, the SCR turns on. This is the cathode voltage of the SCR. This turn-on

    produces current flow through CT and a voltage spike across R3.

    Since the large value of RT prevents there being sufficient current to maintain

    conduction, the SCR immediately turns off, and RT begins its charge cycle again.

    14.13.6.3. Phase Control

    Phase control is the most common form of Thyristor power control. The Thyristor is

    held in the off condition, that is, all current flow in the circuit is blocked by the

    Thyristor except a minute leakage current. Then the Thyristor is triggered into an

  • “on” condition by the control circuitry. For full-wave AC control, a single Triac or two

    SCRs connected in inverse parallel may be used. One of two methods may be used

    for full-wave DC control, a bridge rectifier formed by two SCRs or an SCR placed in

    series with a diode bridge.

    (littlefuse)

    Figure 50: SCR Phase Control

    14.13.6.4. Switch mode applications

    The SCR is a bi-stable device. This means that it has a conducting state and a non-

    conducting state. This characteristic allows for the SCR to be used for switching of

    power signals owing to their long life, high operation speed and freedom from other

    defects associated with mechanical and electro-mechanical switches. (circuitstoday)

    Figure 51: Static AC Circuit Breaker

  • Figure 51 shows a circuit in which two SCRs are used for making and breaking an

    AC circuit. The input voltage is alternating and the trigger pulses are applied to the

    gates of SCRs through the control switch S. Resistance R is provided in the gate

    circuit to limit the gate current while resistors R1 and R2 are to protect the diodes D1 and D2 respectively.

    When starting the circuit, switch S is closed, SCR1 will fire at the beginning of the

    positive half-cycle because it is forward biased. It will turn-off when the current goes

    through the zero value. As soon as SCR1 is turned-off, SCR2 will fire since the

    voltage polarity is already reversed and it gets the proper gate current. The circuit

    can be broken by opening the switch S. Opening of the gate circuit poses no

    problem, as current through this switch is small. As no further gate signal will be

    applied to the SCRs when switch S is open, the SCRs will not be triggered and the

    load current will be zero. The maximum time delay for breaking the circuit is one half-

    cycle.

    14.13.6.5. DC-DC Converter [buck/boost]

    A buck converter is a voltage step down and current step up converter. Figure 52

    below bests demonstrates its basic operation

    (Wikipedia)

    Figure 52: Circuit Configurations of a Buck Converter

    The buck converter is best understood in terms of the relation between current and

    voltage of the inductor. Beginning with the switch open (in the "off" position), the

    current in the circuit is 0. When the switch is first closed, the current will begin to

    increase, and the inductor will produce an opposing voltage across its terminals in

    response to the changing current. This voltage drop counteracts the voltage of the

    source and therefore reduces the net voltage across the load.

    Over time, the rate of change of current decreases, and the voltage across the

    inductor also then decreases, increasing the voltage at the load. During this time, the

    inductor is storing energy in the form of a magnetic field. If the switch is opened while

  • the current is still changing, then there will always be a voltage drop across the

    inductor, so the net voltage at the load will always be less than the input voltage

    source.

    When the switch is opened again, the voltage source will be removed from the

    circuit, and the current will decrease. The changing current will produce a change in

    voltage across the inductor, now aiding the source voltage. The stored energy in the

    inductor's magnetic field supports current flow through the load. During this time, the

    inductor is discharging its stored energy into the rest of the circuit. If the switch is

    closed again before the inductor fully discharges, the voltage at the load will always

    be greater than zero.

    The switch in a Buck Converter is replaced by a SCR due to its ability to have

    switching applications.

    Figure 53: Buck Converter using a SCR

    (NPTEL)

  • 14.14. Practical: Construct a Relaxation Oscillator and

    analyse its waveform using an oscilloscope.

    Aim: To construct and analyse a Relaxation Oscillator

    Requirements:

    Capacitor – 1.5uF

    Resistors – 86K, and 1MΩ Pot

    SCR – ECG 540

    Oscilloscope

    Digital Analogue Trainer What is required from you?

    1. Construct a relaxation Oscillator from the circuit below. 2. Using the oscilloscope analyse its output 3. Draw the output waveform on the oscillogram provided. 4. Record you observations

    Circuit Diagram

  • Oscillogram

    Observations

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  • 14.15. Practical: Construct a Light Dimmer Circuit using

    an SCR

    Aim: To connect an SCR in light dimming circuit

    Requirements:

    Power supply 6V AC

    Bread Board

    47K Pre-set resistor / POT

    1𝑢𝐹 Capacitor

    Diode 1N4007

    6 V Lamp

    SCR – CMR 106-4

    Hook up Leads

    Circuit Diagram

    Connect the circuit as shown above.

    Observation

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  • 14.16. The Triac

    The triac is an AC version of the Silicon Controlled Rectifier. Its name originates from

    “triode for alternating current” or “triode for AC”. The major drawback of an SCR is

    that it can conduct current in one direction only. Therefore, an SCR can only control

    DC power or forward biased half cycles of AC in a load. However, in an AC system,

    it is often desirable and necessary to exercise control over both positive and

    negative half cycles. For this purpose a Triac is used. (Talkingelectronics.com)

    Figure 54: Triac Composition

    14.16.1. Construction

    The triac has four possible conduction paths, depending on the biasing and the

    polarity of the connections. The main terminals will always have opposite polarities.

    The gate’s polarity (with reference to main terminal 2) will be such that current will

    flow from the more negative to the more positive. When this happens, the breakdown

    occurs and then conduction takes place.

    Figure 55 represents the four conduction paths of the Triac.

    (Thomson, 2013)

    Figure 55: Conduction Paths of the Triac

  • 14.16.2. Principle of Operation

    Figure 56: Four Quadrants of a Triac

    (Hyperphysics)

    To explain how TRIACs work, one has to individually analyse the triggering in each

    one of the four quadrants. The four quadrants are illustrated in Figure 56 according

    to the voltage on the gate and the MT2 terminals with respect to the MT1 terminal.

    The MT1 and MT2 terminals are also commonly referred to as A1 and A2,

    respectively.

    The relative sensitivity depends on the physical structure of a particular triac, but as

    a rule, quadrant I is the most sensitive (least gate current required) and quadrant IV

    is the least sensitive (most gate current required).

    In quadrants 1 and 2, MT2 is positive and current flows from MT2 to MT1 through P,

    N, P and N layers. The N region attached to MT2 does not participate significantly. In

    quadrants 3 and 4, MT2 is negative and current flows from MT1 to MT2, also through

    P, N, P and N layers. The N region attached to MT2 is active, but the N region

    attached to MT1 only participates in the initial triggering, not the bulk current flow.

    In most applications, the gate current comes from MT2, so quadrants 1 and 3 are the

    only operating modes.

  • (Hyperphysics)

    Figure 57: Quadrant 1 Operation

    Quadrant I operation occurs when the gate and MT2 are positive with respect to

    MT1. The precise mechanism is illustrated in Figure 57. The gate current makes an

    equivalent NPN transistor switch on, which in turn draws current from the base of an

    equivalent PNP transistor, turning it on also. Part of the gate current (dotted line) is

    lost through the ohmic path across the p-silicon, flowing directly into MT1 without

    passing through the NPN transistor base. In this case, the injection of holes in the p-

    silicon makes the stacked n, p and n layers beneath MT1 behave like a NPN

    transistor, which turns on due to the presence of a current in its base. This, in turn,

    causes the p, n and p layers over MT2 to behave like a PNP transistor, which turns

    on because its n-type base becomes forward-biased with respect to its emitter,

    (MT2). Thus, the triggering scheme is the same as an SCR.

    However, the structure is different from SCRs. In particular, TRIAC always has a

    small current flowing directly from the gate to MT1 through the p-silicon without

    passing through the p-n junction between the base and the emitter of the equivalent

    NPN transistor. This current is indicated in Figure 57 by a dotted red line and it is the

    reason why a TRIAC needs more gate current to turn on than a comparably rated

    SCR.

    Generally, this quadrant is the most sensitive of the four. This is because it is the

    only quadrant where gate current is injected directly into the base of one of the main

    device transistors.

  • (Hyperphysics)

    Figure 58: Quadrant 2 Operation

    Quadrant II operation occurs when the gate is negative and MT2 is positive with

    respect to MT1.

    Figure 58 gives a graphical explanation of the triggering process. The turn-on of the

    device is three-fold and starts when the current from MT1 flows into the gate through

    the p-n junction under the gate. This switches on a structure composed by an NPN

    transistor and a PNP transistor, which has the gate as cathode. As current into the

    gate increases, the potential of the left side of the p-silicon under the gate rises

    towards MT1, since the difference in potential between the gate and MT2 tends to

    lower: this establishes a current between the left side and the right side of the p-

    silicon, which in turn switches on the NPN transistor under the MT1 terminal and as

    a consequence also the PNP transistor between MT2 and the right side of the upper

    p-silicon. So, in the end, the structure which is crossed by the major portion of the

    current is the same as Quadrant I operation.

    (Hyperphysics)

    Figure 59: Quadrant 3 Operation

    Quadrant III operation occurs when the gate and MT2 are negative with respect to

    MT1. The whole process is outlined in Figure 59. The process happens in different

    steps here too. In the first phase, the pn junction between the MT1 terminal and the

  • gate becomes forward-biased (step 1). As forward-biasing implies the injection of

    minority carriers in the two layers joining the junction, electrons are injected in the p-

    layer under the gate. Some of these electrons do not recombine and escape to the

    underlying n-region (step 2). This in turn lowers the potential of the n-region, acting

    as the base of a PNP transistor which switches on (turning the transistor on without

    directly lowering the base potential is called remote gate control). The lower p-layer

    works as the collector of this PNP transistor and has its voltage heightened: actually,

    this p-layer also acts as the base of an NPN transistor made up by the last three

    layers just over the MT2 terminal, which, in turn, gets activated. Therefore, the red

    arrow labelled with a "3" in Figure 59 shows the final conduction path of the current.

    (Hyperphysics)

    Figure 60: Quadrant 4 Operation

    Quadrant IV operation occurs when the gate is positive and MT2 is negative with

    respect to MT1. Triggering in this quadrant is similar to triggering in Quadrant III. The

    process uses a remote gate control and is illustrated in Figure 60. As current flows

    from the p-layer under the gate into the n-layer under MT1, minority carriers in the

    form of free electrons are injected into the p-region and some of them are collected

    by the underlying np-junction and pass into the adjoining n-region without

    recombining. As in the case of a triggering in Quadrant III, this lowers the potential of

    the n-layer and turns on the PNP transistor formed by the n-layer and the two p-

    layers next to it. The lower p-layer works as the collector of this PNP transistor and

    has its voltage heightened: actually, this p-layer also acts as the base of an NPN

    transistor made up by the last three layers just over the MT2 terminal, which, in turn,

    gets activated. Therefore, the red arrow labelled with a "3" in Figure 60 shows the

    final conduction path of the current.

    Generally, this quadrant is the least sensitive of the four. In addition, some models of

    TRIACs cannot be triggered in this quadrant but only in the other three.

    14.16.3. Purpose of Biasing

    TRIACs are part of the thyristor family and are closely related to silicon-controlled

    rectifiers (SCR). However, unlike SCRs, which are unidirectional devices (that is,

  • they can conduct current only in one direction), TRIACs are bidirectional and so

    current can flow in either direction. Another difference from SCRs is that TRIAC

    current flow can be enabled by either a positive or negative current applied to its

    gate electrode, whereas SCRs can be triggered only by a positive gate pulse. To

    create a triggering current, a positive or negative voltage has to be applied to the

    gate with respect to the MT1 terminal (otherwise known as A1).

    Once triggered, the device continues to conduct until the current drops below a

    certain threshold called the holding current. It is for the reasons outlined above that

    biasing plays an integral part in the operation of the Triac.

    14.16.4. Symbol

    Figure 61; Triac Circuit Symbol

    14.16.5. The Triac Characteristic Curve

    Figure 62 shows the typical characteristic curve of a triac

    (Swart)

    Figure 62: Triac Characteristic Curve

  • Since a triac consists of essentially two SCR’S of opposite orientation fabricated in

    the same crystal, its operating characteristics in the first and third quadrants of the

    characteristic curve are the same except for the direction of applied voltage and

    current flow. The following points can be noted from the characteristic curve:

    1. The V-I characteristics for the triac in the 1st and 3rd quadrants are essentially

    identical to those of an SCR in the 1st quadrant.

    2. The triac can be operated with either a positive or negative gate control

    voltage but under normal operation the gate voltage is positive in quadrant 1

    and negative in quadrant 3.

    3. The supply voltage at which the triac is turned on depends upon the gate

    current. The greater the gate current, the smaller the supply voltage at which

    the triac is turned on. This allows us to use a triac to control AC power in a

    load from 0 to full load power in a smooth and continuous manner with no loss

    in the controlling device. (Talkingelectronics.com)

    14.16.6. Application

    Triacs have no amplification ability; therefore they are used in switching circuits only.

    Their characteristics make them particularly useful in power control circuits such as

    motor speed control and light dimmers. Ref: (Thomson) Ref: (Swart)

    Figure 63: Typical Light Dimmer Circuit using Triac

  • (Talkingelectronics.com)

    Figure 64: A typical Triac electronic changeover circuit

    Two triacs are used in this circuit viz. TR 1 and TR2. When TR1 is turned on TR2 is

    turned off, the line input is connected across the full transformer primary AC.

    However, if it is desired to change the tapping so that the input appears across part

    AB of the primary, then TR2 is turned on and TR1 turned off. The gate control

    signals are so controlled that both triacs are never switched on together. This avoids

    a dangerous short circuit on the section BC of the transformer.

    Apart from circuits like the ones mentioned above triacs are used in Relaxation

    Oscillators, DC-DC Converters and for other switch mode applications.

    (Hyperphysics)

    Figure 65: Triac used as an AC Switch

    When switch S is thrown to position 1, the triac is cut off and the output power of

    lamp is zero. As the switch is thrown to position 2, a small gate current (a few mA)

  • flowing through the gate turns the triac on. Consequently, the lamp is switched on to

    give full output of 1000 watts.

    14.16.7. Circuit Diagrams

    (schematicdiagram) Figure 66: A Triac used for Motor Speed Control

    (Talkingelectronics.com)

    Figure 67: The Triac used in a Simple AC Power Control Circuit

  • 14.17. Practical: Construct a light dimmer circuit

    Aim: To construct and examine a light dimmer circuit using a Triac

    Requirements:

    • Resistors -1K, 220K Pot,

    • TIC226- Triac

    • D1 – Diac 32 V

    • Capacitors -100𝑛𝐹 X 2 • 220V power supply

    What are you required to do?

    Construct the circuit below.

    Observation

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  • 14.18. The Diac

    The diac is a two terminal, three layer bidirectional device which can be switched

    from its off state to on state for either polarity of the applied voltage.

    14.18.1. Construction

    The diac can be constructed in either NPN or PNP form. The two leads are

    connected to p-regions of silicon separated by an n-region. The structure of the diac

    is very similar to that of a transistor. However, there are several important

    differences:

    There is no terminal attached to the base layer.

    The three regions are identical in size.

    The doping concentrations are identical.

    Figure 68 shows the construction and symbol of the diac.

    Figure 68: Diac Construction and Circuit Symbol

    14.18.2. Operation

    The DIAC, or "diode for alternating current", is a diode that conducts current only

    after its break over voltage, VBO, has been reached momentarily.

    When this occurs, the diode enters the region of negative dynamic resistance,

    leading to a decrease in the voltage drop across the diode and, usually, a sharp

    increase in current through the diode. The diode remains "in conduction" until the

    current through it drops below a value characteristic for the device, called the holding

    current, IH. Below this value, the diode switches back to its high-resistance (non-

    conducting) state. This behaviour is bidirectional, meaning typically the same for

    both directions of current.

  • Most DIACs have a three-layer structure with break over voltage around 30 V. Their

    behaviour is somewhat similar to that of a neon lamp, but it is much more precisely

    controlled and takes place at a lower voltage.

    DIACs have no gate electrode, unlike some other thyristors that they are commonly

    used to trigger, such as TRIACs. Some TRIACs, like Quadrac, contain a built-in

    DIAC in series with the TRIAC's "gate" terminal for this purpose.

    DIACs are also called symmetrical trigger diodes due to the symmetry of their

    characteristic curve. Because DIACs are bidirectional devices, their terminals are not

    labelled as anode and cathode but as A1 and A2 or MT1 ("Main Terminal") and MT2.

    14.18.3. Purpose of Biasing

    Conduction occurs in a diac when the break over voltage is reached with either

    polarity across the two terminals.

    Once break over occurs, current is in a direction depending on the polarity of the

    voltage across the terminals. The device turns off when the current drops below the

    holding value IH.

    The equivalent circuit of a diac consists of four transistors arranged as shown below:

    (Talkingelectronics.com)

    Figure 69: The Equivalent Circuit of a Diac

    In the equivalent circuit Q1 and Q2 are forward-biased, and Q3 and Q4 are

    reversed-biased. The device operates on the upper right portion of the characteristic

    curve under this bias condition.

    When the diac is biased as in part (c), the PNPN structure from A2 to A1 is used. In

    the equivalent circuit, Q3 and Q4 are forward-biased, and Q1 and Q2 are reverse-

    biased. Under this bias condition the device operates on the lower left portion of the

    characteristic curve.

  • 14.18.4. Symbol

    Figure 70: Basic construction and Symbol of the Diac

    14.18.5. Characteristic Curve

    (Talkingelectronics.com)

    Figure 71: Diac Characteristic Curve

    For applied voltage less than +VBO and negative voltages less than –VBO, a small

    leakage current (IBO) flows through the device. Under such conditions, the diac

    blocks the flow of current and effectively behaves as an open circuit.

    When the positive or negative applied voltage is equal to or greater than the

    breakdown voltage the diac begins to conduct and the voltage drop across it

    becomes a few volts. Conduction then continues until the device current drops below

    its holding current.

  • 14.18.6. Application

    Diacs are primarily used for triggering of triacs. Examples of diac applications are:

    light dimmer circuits, relaxation oscillators and switch mode applications.

    14.18.7. Circuit Diagram Application

    A diac used in a light dimmer circuit to trigger a triac

    (Talkingelectronics.com)

    Figure 72: Diac used in Light Dimmer Circuit

    14.19. Homework Exercises

    1. What is a datasheet and what is its purpose?

    2. Explain the term “Dual in Line Package”

    3. Draw diagrams that best represent the differences between Electron Current

    Flow and Conventional Current Flow.

    4. Define the term “Solid State Device”

    5. What is doping and why is it important?

    6. Draw a diagram that best represents the PN Junction. Show all majority and

    minority carriers.

    7. Draw the characteristic curve of a diode.

    8. Why are Zener Diodes commonly used in Power Supplies?

    9. Define the following terminology

    a) Avalanche Effect

    b) Zener Effect

    10. Draw the circuit symbol for a NPN and PNP transistor.

    11. Using the characteristic curve and DC load line of a transistor, explain the

    term “Regions of a transistor”

    12. Show diagrammatically how you would use a transistor as a switch.

    13. What does the acronym SCR stand for?

    14. Explain the operation of the SCR.

    15. List 3 disadvantages of an SCR.

    16. Show how you would use an SCR as a Relaxation Oscillator.

    17. How have Triacs overcome the disadvantages of the SCR?

    18. Differentiate between a Diac and a Transistor.

  • 19. Use a Triac to control the brightness of a bulb that is connected to household

    power. Draw the circuit to show how the Triac must be connected. Draw one

    cycle of the input and load voltages if the firing angle is set at 135⁰.

    20. How is a Diac turned on?

    21. How is a Diac turned off?

    22. Explain the function of a Diac.

    23. Draw the equivalent circuit of a Diac using Transistors.

    24. Draw a circuit showing how a Triac can be used to control AC.

    25. Draw a circuit representing a Relaxation Oscillator.

    Class Activity 1

    1. Conduct research on the following transistor:

    BC557B

    From your research answer the following questions on the transistor:

    1.1. Within what temperature parameters is it advisable to store this transistor?

    1.2. List at least 3 uses of this type of transistor.

    1.3. Draw a diagram clearly showing the PIN CONFIGURATION of this transistor. 1.4. What is the typical value of DC Current gain of this transistor? 2. List 3 advantages of Surface Mounted Components 3. Define the term “Dual in Line Package”

    Class Activity 2

    1. Show graphically the difference between electron current flow and

    conventional current flow.

    2. Define the following terms :

    a) Semiconductor

    b) Solid State Device

    c) Doping

    3. Fill in the missing words from the statement below:

    N-Type Doping is a process where a tetravalent atom combines with a

    __________________. P-Type Doping is a process where a tetravalent atom

    combines with a __________________________.

    4. Draw a clearly labelled diagram showing a PN Junction connected in Forward

    Bias.

  • 5. With reference to PN Junction theory, explain the terms “Majority and Minority

    Carriers”

    Class Activity 3

    A silicon PN junction diode is connected to a 10V supply in series with a 1.2kΩ load

    resistor. Use a DC load line to determine the Q-Point at which the diode is operating.

    Class Activity 4

    1. Explain the term “Breakdown Voltage” and “Avalanche Breakdown” with respect to the Zener Diode.

    2. Draw the characteristic curve of a Zener Diode. 3. With the aid of a suitable diagram explain how you would use a Zener Diode

    for voltage stabilization.

    Class Activity 5

    1. Draw a block diagram and circuit symbol showing all the relevant details pertaining to a NPN transistor.

    2. Define the term “transistor biasing” 3. Draw 4 types of biasing circuits you are aware of. 4. Explain the term “Thermal Runaway”

    Class Activity 6

    1. Draw block diagrams to represent the difference between a NPN and PNP

    Transistor.

    2. Explain the basic operation of a PNP Transistor.

    3. List 2 applications of a PNP Transistor.

    4. What is the Mathematical relationship betwee