EE045 Electronic Components 2 Th Inst

7/25/2019 EE045 Electronic Components 2 Th Inst

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SRI LANKA INSTITUTE of ADVANCED TECHNOLOGICAL EDUCATION

Training Unit

Electronic Components 2

Theory

No: EE 045

INDUSTRIETECHNIKINDUSTRIETECHNIK

ELECTRICAL and ELECTRONIC

ENGINEERING

Instructor Manual


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Training Unit


Theoretical Part

No.: EE 045

Edition: 2008Al l Rights Reserved

Editor: MCE Industrietechnik Linz GmbH & CoEducation and Training Systems, DM-1Lunzerst rasse 64 P.O.Box 36, A 4031 Linz / Aus triaTel. (+ 43 / 732) 6987 3475Fax (+ 43 / 732) 6980 4271Website: www.mcelinz.com


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ELECTRONIC COMPONENTS 2

CONTENTS Page

LEARNING OBJECTIVES...................................................................................................4

1 SEMICONDUCTOR MATERIALS................................................................................5

1.1 The structure of semiconductor crystals ..............................................................6

1.1.1 The silicon atom...............................................................................................6

1.2 Intrinsic conductivity.............................................................................................8

1.3

Doping of semiconductors ...................................................................................8

1.4 The construction of an N-type semiconductor .....................................................9

1.4.1

N-silicon .........................................................................................................10

1.5 The construction of a P-type semiconductor......................................................10

1.5.1 P-silicon..........................................................................................................11

1.6

PN-junction ........................................................................................................ 11

1.6.1 Reverse biased ..............................................................................................12

1.6.2 Forward biased ..............................................................................................13

2

SEMICONDUCTOR RECTIFIERS.............................................................................14

2.1 General ..............................................................................................................14

2.2

Types of semiconductor rectifiers ......................................................................14

3 SEMICONDUCTOR DIODES.....................................................................................16

3.1 General ..............................................................................................................16

3.2 Characteristic curve of a semiconductor diode .................................................. 18

3.3 Full-wave rectifier in centre-tap circuit ...............................................................20

3.4 Testing semiconductor diodes ...........................................................................21

3.5 Nominal values, limiting values and characteristic values for diodes.................22

3.5.1 Examples of limiting values for diodes...........................................................22

3.5.2 Examples of characteristics ........................................................................... 23


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4 SPECIAL DIODES......................................................................................................24

4.1

Zener diodes ......................................................................................................24

4.1.1 General ..........................................................................................................24

4.1.2 Characteristic curve and circuit symbols for a Zener diode ...........................24

4.1.3 The Zener effect.............................................................................................25

4.1.4 The avalanche effect......................................................................................25

4.1.5 Application of Zener diodes............................................................................25

4.2

Capacitance diodes or varactor diode ...............................................................27

4.2.1

General ..........................................................................................................27

4.2.2 Application......................................................................................................27


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LEARNING OBJECTIVES

The trainee should...

- name the three most important semiconductor materials.

- understand the terms "valence electron" and "hole" in a semiconductor.

- explain the term "doping" of a semiconductor.

- understand the meaning of P and N-type semiconductors.

- understand the principle of operation of a semiconductor junction (PN-junction).

- explain the method of Operation of a half-wave rectifier, a full-wave rectifier and a

bridge rectifier.- explain the principles of operation of a Zener diode.

- describe the method of Operation of a voltage stabilizer circuit using a Zener diode.


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1 SEMICONDUCTOR MATERIALS

Semiconductors are materials, the resistance of which is greater than that of electrical

conductors, but less than that of non-conducting materials (insulating materials).


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The following are the most important semiconductor materials

- Silicon

- Germanium

- Selenium

They have a mono-crystalline structure.

Gallium arsenide and indium phosphide are mixed crystals.

The semiconductor crystals have to have a very high degree of purity, 1010: 1, which

means that there is a maximum of 1 impurity atom in 10 10silicon atoms.

1.1 The structure of semiconductor crystals

1.1.1 The silicon atom

Each silicon atom has valence electrons (also referred to as free electrons) in the outer

ring.

Valency represents the ability to join with other atoms.


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NOTE:

Silicon has a valency of 4 because of its 4 valence electrons.

A simple representation of a crystal grid (2-dimensional)

A silicon atom can join with other silicon atoms due to its 4 valence electrons. This means

that valence electrons circulate around their own and adjacent atoms.


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1.2 Intrinsic conductivity

The electrical conductivity of a material depends on its free charge carriers (electrons in

the outer shell).

A very pure silicon crystal should, therefore, be an insulator, as all the electrons in the

outer ring are required for the crystalline bonds.

The fact that there is a small amount of conductivity (intrinsic conductivity) is due to the

following three causes

- Breaking of crystal bonds

- Remaining impurities

- Surface conductivity

NOTE:

The intrinsic conductivity is greatly dependent on temperature.

1.3 Doping of semiconductors

Impurity atoms are "introduced" into a pure semiconductor. This process is called

"doping".

If a pure semiconductor with acceptors * (indium, gallium, boron or aluminium) is doped,

then a P-conducting material is formed. If a pure semiconductor with donors ** (arsenic,

antimony or phosphorus) is doped, then an N-conducting material is formed.

* Derived from the Latin verb "acceptare" = to accept

** Derived from the Latin verb "donare" = to give


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1.4 The construction of an N-type semiconductor

Very pure silicon has impurities deliberately added to it. Atoms of a material with a valency

of 5 are introduced into the crystal.

Sb = antimony = valency 5

The donor atom requires 4 valence electrons for bonding with the silicon atom. One

valence electron is not needed to form a bond. It is, therefore, used as a free electron for

conducting electrical current.


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1.4.1 N-silicon

N-silicon is a semiconductor material with free, negative charge carriers. It is not charged

electrically, since there is the same number of electrons as there is of protons in the

material.

1.5 The construction of a P-type semiconductor

Very pure silicon is doped with a material of valency 3 (e g indium).

electron movement

hole movement

In = indium - valency 3

If one dopes with acceptors, then one crystalline bond remains free, due to the lack of

electrons. The free bond is called a hole.

hole movement

electron movement


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If, due to heating, an electron breaks away from a bond situated near a free bond (hole), it

is attracted by the hole (positive charged) and forced to complete the free bond. As a

result, the hole disappears. The hole now appears where the electron was previously

situated. The hole has moved from one place to another in the opposite direction as the

electron.

1.5.1 P-silicon

P-silicon is a doped semiconductor with free, positive charge carriers. It is not electrically

charged.

1.6 PN-junction

If a pure silicon crystal is doped on one side with acceptors and on the other side with

donors, then a thin layer is formed between these two zones. There are no charge carriers

(electrons or holes) inside this layer.


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This layer is called the PN-junction, blocking layer, boundary layer or depletion layer.

The charge carriers, free electrons and electron deficiencies (holes) evenly distributed

over the rest of the crystal.

1.6.1 Reverse biased

If a voltage is applied to these two layers, with the positive pole connected to the N-zone

and the negative pole to the P-zone (reverse biased) the following action takes place in a

crystal:

- Opposite charges attract

- The depletion layer expands

- The PN-junction develops a high resistance

- Only a very small current (reverse current) flows in the reverse direction


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1.6.2 Forward biased

If the voltage is now reversed, i.e. the positive pole is connected to the P-zone and the

negative pole is connected to the N-zone (forward biased) then the following action will

take place in the crystal:

- Opposite charges attract

- The charge carriers are pushed into the inside of the crystal

- They flood the depletion layer

- The PN-junction develops a low resistance

- An electrical current flows in the forward direction

With reference to (b) above

This process is called "diffusion" and starts at about 0.3 V for germanium and at about 0.6

V for silicon. This voltage is called the "threshold voltage".


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2 SEMICONDUCTOR RECTIFIERS

2.1 General

A semiconductor rectifier is constructed by installing a suitable, doped crystal in a case

and providing it with cooling fins.

2.2 Types of semiconductor rectifiers

- Silicon rectifiers

These are rectifiers which may have a reverse voltage of several thousand volts and

forward current over 1.5 kA.

They are the most frequently used rectifiers.

- Germanium rectifiers

Germanium rectifiers use a low reverse voltage.

They are rarely used in industrial electronics.

- Selenium rectifiers

Selenium rectifiers are used for small loads. They are insensitive to short overloads.- Cuprous oxide rectifiers (copper oxide)

These rectifiers have the lowest diffusion voltage and are suitable for small loads only.


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Table of comparison for rectifiers

Term Silicon Germanium selenium cuprous oxide

Threshold voltage (V) 0.7 V 0.3 V 0.6 V 0.2 V

Maximum reverse

voltage (V)

>3000 V 200 V 40 V 6 V

Maximum junction

temperature (C)

200 V 90C 85C 50C

Relative space

required for

same load 1 3 15 30


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3 SEMICONDUCTOR DIODES

3.1 General

Rectifier type semiconductor diodes cannot be used for high frequencies, as they have a

large depletion layer.

For high frequency semiconductor diodes the area of the PN-junction is small, and,

therefore, the depletion layer capacitance is reduced.

The capacitance of a parallel-plate capacitor depends on the following:

- The plate size

- The width of dielectric

- The material of the dielectric

The depletion layer of a diode acts as the dielectric of a parallel-plate capacitor.


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A comparison between a semiconductor rectifier diode and a detector diode (point contact

diode)

Semiconductor rectifier diode Semiconductor detector diode

(point contact diode)

For many years, point contact diodes were used exclusively as high frequency rectifiers

and switching diodes.

The latest developments are the so called "epitaxial planar diodes" (special manufacturing

process).

They have switching times of a few nanoseconds. The switching currents are a few

hundred mA.


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3.2 Characteristic curve of a semiconductor diode

UTh= 0.6 silicon

UTh= 0.3 germanium

If the source voltage, in a circuit containing a diode, is large compared with the threshold

voltage (UTh) of the diode, the diode characteristic can be represented as follows:

Ud= 0 for forward biased

Id = 0 for reverse biased

The above representation is very often used in electrical circuits. It can be seen that the

diode behaves as a perfect conductor for positive voltages and as an open circuit for

negative voltages.


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As an example consider the following rectifier circuit (half-wave rectifier).

UI= input (source) voltage

UO= output (lad) voltage

When UIis positive, the diode is forward biased (UD= 0).

When UIis negative, the diode is reverse biased (UO= 0).


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In drawing (c) above, the output waveform has only positive parts.

3.3 Full-wave rectifier in centre-tap circuit

Full-wave rectification in a centre-tapped circuit is a type of circuit which uses both half

cycles of an alternating voltage.


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voltage waveform

Although better use is made of the voltage than in half-wave rectification, this circuit is

used relatively rarely. One of the main reasons for this is the stringent requirements forthe transformer. It must have a centre-tap and each half of the winding must supply the

same voltage as for the half-wave rectification, which makes its manufacture relatively

expensive.

For this reason experiments with this type of rectification have been omitted.

3.4 Testing semiconductor diodes

The simplest method of testing whether a diode is defective or working correctly is to

measure the resistance with an ohmmeter, which is operated by a torch battery.

In one direction, the forward resistance lies between some tenths of an ohm and about

100 ohms, according to the type of diode. The reverse resistance lies in the megohm

range. For small diodes there is always a danger of overload. Therefore, the

measurement should not take too long.

The supply voltage for the ohmmeter must be above the threshold voltage.


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NOTE:

A diode is defective if the resistance is very low or very high in both forward and reverse

directions.

3.5 Nominal values, limiting values and characteristic values for diodes

Limiting values may never be exceeded otherwise the component will be destroyed.

3.5.1 Examples of limiting values for diodes

- Peak inverse voltage (VRM)

This is the highest voltage which may be applied to the diode in the reverse direction.

- Forward current (IF)

This is forward current for a specified crystal temperature (direct current value).

- Total loss (Ptot)

This is the maximum total loss.

- Depletion layer temperature (Tj)

This is the maximum temperature of the crystal in the area of the depletion layer.

NOTE:

Characteristics are properties or features of components which can be measured.


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3.5.2 Examples of characteristics

- Forward voltage (Vf)

This is at a specified forward current.

- Depletion layer capacitance

This is at a specified reverse voltage.

NOTE:

Nominal values are operating data recommended by the manufacturer and can be

exceeded as long as the limiting values are not reached.


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4 SPECIAL DIODES

4.1 Zener diodes

4.1.1 General

Zener diodes behave like semiconductor diodes in the forward direction. By suitable

doping, they develop a low resistance in the reverse direction after a certain voltage has

been exceeded.

NOTE:

Zener diodes are operated in the reverse direction and develop a low resistance when the

Zener voltage is reached.

4.1.2 Characteristic curve and circuit symbols for a Zener diode

The low-resistance condition is produced either by the Zener effect or by the avalanche

effect.


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4.1.3 The Zener effect

The Zener effect was investigated on insulating materials by Dr. Zener in 1920.

When the reverse voltage is increased, the depletion layer becomes wider.

The reverse voltage produces an electrical field in the depletion layer. If the electrical field

strength exceeds the value of 200 kV/cm then valence electrons are torn out of the atomic

bonds and are available to form an electrical current.

A critical voltage of between 2 and 600 V can be obtained according to the doping.

4.1.4 The avalanche effect

The charge carriers (electrons) which have been released due to the Zener effect are

accelerated by the electrical field to such an extent that they knock other electrons out of

their bonds.

The number of free charge carriers increases like an avalanche and swamps the depletion

layer.

Due to the sudden breakdown in the depletion layer there in an instantaneous increase of

reverse current.

NOTE:

After the Zener breakdown, the current must be limited by a resistor RV.

4.1.5 Application of Zener diodes

Zener diodes are used for voltage stabilization (stabilized mains equipment), limiter diodes

and for reference values in control technology etc.


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Stabilization circuit

In order to build the above circuit the value for resistor RV must be known:

There is a minimum value for the load resistance RL in order that the Zener diode

operates in the reverse breakdown region:


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4.2 Capacitance diodes or varactor diode

4.2.1 General

Each blocked PN-junction acts like the dielectric of a capacitor. In a capacitance diode,

the thickness of the depletion layer can be affected by the level of the voltage applied.

That is:

Low reverse voltage - thin depletion layer - high capacitance.

High reverse voltage - thick depletion layer - low capacitance.

Characteristic curve and circuit symbol

4.2.2 Application

Capacitance diodes are replacing rotating plate variable capacitors in radio and television

sets to an increasing extent.


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Example

Resonant circuit tuning in television sets.


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EE 045


Theoretical Test


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EE 045


TEST 1

1. State what is meant by semiconductor materials.

2. Name three semiconductor materials.

3. State the degree of purity of semiconductor crystals.

4. State another name for valence electrons.

5. State three reasons why high purity silicon still has a small conductivity.

6. Explain what is meant by the process of "doping".

7. State what is meant by an N-type semiconductor.

8. Is an N-silicon semiconductor material charged electrically?

9. State what is meant by a P-type semiconductor.

10. State what is meant by a PN-junction.


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EE 045


TEST 2

1. State what is meant by a reverse biased diode.

2. State what takes place in a semiconductor rectifier when it is reverse biased?

3. State the threshold voltages for the following diodes2

- Germanium diode

- Silicon diode.

4. Which is the most frequently used type of rectifier?

5. State the properties of a cuprous oxide rectifier.

6. Why can semiconductor rectifier diodes not be used for high frequency?

7. Name a recently developed diode for high frequency switching and rectification.

8. Draw the characteristic curve (EU) of a semiconductor diode for both the forward and

reverse direction.

9. Draw the output waveform for the following circuit, if the input is a sinusoidal

alternating voltage.


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Draw a circuit used for recording the static characteristic curve of diodes.


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EE 045


TEST 3

1. Draw the characteristic curve and the circuit symbol for a capacitance diode (varactor

diode).

2. State the effect of capacitor (C1) on the output voltage of the circuit shown below.

3. Draw the circuit diagram of a full-wave rectifier using a centre-tap transformer.

4. Why is a full-wave rectifier using a centre-tap transformer rarely used?

5. State the disadvantages and advantage of a bridge rectifier over a full-wave rectifier

using a centre-tap transformer.

6. Describe the method of testing semiconductor diodes.

7. State what is meant by the peak inverse voltage of a diode.


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8. Draw the characteristic curve and circuit symbols for a Zener diode.

9. State the application of Zener diodes.

10. State the application of capacitance diodes.


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EE 045


TEST 1

(Solution)

1. Semiconductors are materials, the resistance of which is greater than that of electrical

conductors, but less than that of non-conducting materials (insulating materials).

2. - Silicon

- Germanium

- Selenium

3. There is a maximum of one impurity atom in 1010silicon atoms.

4. Free electrons.

5. - Breaking of crystal bonds

- Remaining impurities

- Surface conductivity

6. Impurity atoms are "introduced" into a pure semiconductor. This is termed "doping".

7. Very pure silicon with impurities deliberately added to it. Atoms of a material with a

valence of 5 are introduced into the crystal.

8. No.

9. Very pure silicon doped with a material of valence 3 (e.g., indium).


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10. Pure silicon crystal doped on one side with acceptors and on the other side with

donors, a thin layer being formed between these two zones. There are no charge

carriers (electrons or holes) inside this layer.


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EE 045


TEST 2

(Solution)

1. If a voltage is applied to the two layers, so that the positive pole is connected to the N-

zone and the negative pole to the P-zone, then the diode is reversed biased.

2. - Opposite charges attract

- The depletion layer expands

- The PN-junction develops a high resistance

- Only a small current (reverse current) flows in the reverse direction

3. - 0.3 V

- 0.6 V

4. Silicon rectifier.

5. These rectifiers have the lowest diffusion voltage and are suitable for small loads only.

6. Because they have a large blocking layer (depletion layer capacity).

7. Epitaxial planar diodes.


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8.

UTh = 0.6 silicon

UTh= 0.3 germanium

9.


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10.


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EE 045


TEST 3

(Solution)

1.

2. The greater the capacitance of the capacitor, the nearer the output voltage will be to

the peak AC input voltage. The AC component of the DC voltage becomes less and,

therefore, better filtering is obtained


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3.

4. Because it must have a centre-tap transformer and each half of the winding must

supply the same voltage as for the half-wave rectification, which makes its

manufacture relatively expensive.

5. Advantage:

The centre-tap transformer is not needed.

Disadvantages:

- The rather expensive construction (four diodes)

- A slight disadvantage is the higher diode voltage drop (2 x forward voltage per

branch).

6. The simplest method of testing whether a diode is defective or working correctly is to

measure its forward and reverse resistance with an Ohmmeter, which is operated by a

torch battery. If the resistances in both directions are similar (high or low) the diode is

defective.

7. This is the maximum voltage which may be applied to the diode in the reverse

direction.


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8.

9. Zener diodes are used for voltage stabilization, limiter diodes and for reference values

in control technology etc.

10. Capacitance diodes are replacing rotating plate variable capacitors in radio and

television sets in an increasing extent.


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KEY TO EVALUATION

PER CENT MARK

88 100 1

75 87 2

62 74 3

50 61 4

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EE045 Electronic Components 2 Th Inst

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