1. Photodiodes

1.1 Introduction Photodiodes are semiconductor light sensors that generate a current or voltage when

the P-N junction in the semiconductor is illuminated by light. Photodiodes incorporate a P and an N type layer. The system has the electrical characteristics of a rectifier. Radiation directed in the vicinity of the PN junction cause a flow of current. Photodiodes are very useful for applications where space is restricted. The effective area of a photodiode is about 0.2 mm2 and it has a pinhead of a diameter of 0.5 mm.1

The P-layer material at the active surface and the N-layer material at the substrate form a PN junction which operates as a photoelectric converter. The usual P-layer for a Si photodiode is formed by the selective diffusion of boron, to a thickness of approximately 1 µm or less and the neutral region at the junction between the P and N layers is known as the depletion layer.

By controlling the thickness of the outer P-layer, substrate N-layer and bottom N+-layer as well as the doping concentration, the spectral response and frequency response can be controlled.

When a reverse-biased p-n junction diode is illuminated with light, extra electron-hole pairs are generated in p and n regions. As a result, the minority carrier concentration changes significantly but the majority carrier concentration does not change much. The generated minority carriers increase the reverse current. The diode current is found to vary almost linearly with the light flux. The diodes so operated are referred to as photo diodes. These diodes are used in fast reading of tapes and film sound tracks, light-operated switches, and light-detection systems.

Photodiodes made from gallium arsenide are preferable to silicon photodiodes. Because silicon is an indirect-gap semiconductor, the photon absorption init requires the assistance of phonons2 to create electron-hole pairs. Gallium arsenide, on the other hand, is a direct-gap semiconductor, and so can absorb photons without phonons. Hence, GaAs photodiodes have greater speed of operation, and are more sensitive to the intensity of the incident light than Si photodiodes.3

1.2 Photodiode Typesa) PN Photodiode b) PIN Photodiode c) Schottky Type Photodiode d) APD (Avalanche Photodiode) All of these types provide the following features and are widely used for the detection

of the intensity, position, color and presence of light.

1.3 Principle of OperationWhen light strikes a photodiode, the electron within the crystal structure becomes

stimulated. If the light energy is greater than the band gap energy Eg, the electrons are pulled up into the conduction band, leaving holes in their place in the valence band. These electron-1 A.K. Sawhney, A Course in Electrical and Electronic Measurements and Instrumentation, (Delhi: Dhanpat Rai and Co, 2008) p 718.2 Phonons are a quantum mechanical version of a special type of vibrational motion, known as normal modes in classical mechanics, in which each part of a lattice oscillates with the same frequency.3 D. Chattopadhyay and P.C. Rakshit, Electronics: Fundamentals and Applications, (New Delhi: New Age International Limited, 2008) p 77.

Circuit Diagram of a Photodiode

hole pairs occur throughout the P-layer, depletion layer and N-layer materials. In the depletion layer the electric field accelerates these electrons toward the N-layer and the holes toward the P-layer. Of the electron-hole pairs generated in the N-layer, the electrons, along with electrons that have arrived from the P-layer, are left in the N-layer conduction band. The holes at this time are being diffused through the N-layer up to the depletion layer while being accelerated, and collected in the P-layer valence band.

In this manner, electron-hole pairs which are generated in proportion to the amount of incident light are collected in the N- and P-layers. This results in a positive charge in the P-layer and a negative charge in the N-layer.

If an external circuit is connected between the P- and N-layers, electrons will flow away from the N-layer, and holes will flow away from the P-layer toward the opposite respective electrodes. These electrons and holes generating a current flow in a semiconductor are called the carriers.

The more light striking the junction, the larger is the reverse current in a photo diode.In this diode, a window lets light pass through the package to the junction. With

typical photodiodes the reverse current is in microamperes.

1.4 Circuit and Characteristics of a Photodiode

From the figure it is very clear that the output resistance R= V/is very high and is of the order of tens of MThe d.c. resistance, V/I is the diode leakage resistance and that too is very high. This resistance depends upon the light intensity. The frequency response of a photodiode is largely dependent upon the intrinsic capacity which is typically 2 pF fro a reverse bias of -10 V.

The cut off frequency is given by fc=1/2RLC where C is capacitance of photodiode and RL is the load resistance. The cut off frequency is of the order of MHz. Even in dark there is always leakage current of the photodiode and this current is known as dark current. The dark current double about every 10o C increase in temperature for silicon and this decreases the signal to noise ratio.

1.5 Temperature Characteristics Ambient temperature variations greatly affect photodiode sensitivity and dark current.

The cause of this is variation in the light absorption coefficient which is temperature related. For long wavelengths, sensitivity increases with increasing temperature and this increase becomes prominent at wavelengths longer than the peak wavelength. For short wavelengths, it decreases. Since ultraviolet enhanced photodiodes are designed to have low absorption in the short wavelength region, the temperature coefficient is extremely small at wavelengths shorter than the peak wavelength.

The variation in dark current with respect to temperature occurs as a result of increasing temperatures causing electrons in the valence band to become excited, pulling them into the conduction band. A constant increase in dark current is observed with increasing temperature. There is a twofold increase in dark current for a temperature rise from 5°C to 10°C.

1.6 Applications1. Because of their fast response time, photodiodes are sued a scene film sound track

readers.2. They can be used as detectors of modulated light in optical communication

systems and also in switching circuits.

1.7 Features of photodiodes 1. Photodiodes show excellent linearity with respect to incident light 2. They are preferable because of low noise 3. They have wide spectral response 4. Photodiodes are mechanically rugged 5. Photodiodes are compact and lightweight 6. They are durable because of their long life.

Light Emitting Diodes

2. Light Emitting Diode

2.1 IntroductionLight emitting diode is primarily a PN junction semiconductor device. The basic

mechanism of electro-magnetic radiation from the device is injection luminescence (IL). When a p-n diode is forward biased, the carriers injected across the junction enhance

the carrier concentrations above the thermal equilibrium values. These excess carriers recombine and emit electromagnetic energy corresponding to the band gap.4

2.2 ConstructionOne way to construct an LED is to deposit three semiconductor layers on a substrate.

Between p-type and n-type semiconductor layers, an active region emits light when an electron and hole recombine. Considering the p-n combination to be a diode, then when the diode is forward biased, holes from the p-type material and electrons from the n-type material are both driven into the active region. The light is produced by a solid state process called electroluminescence.

In this particular design, the layers of the LED emit light all the way around the layered structure, and the LED structure is placed in a tiny reflective cup so that the light from the active layer will be reflected toward the desired exit direction.

2.3 Principle of Operation of LEDsThe electrons from the n-side cross the junction and enter the p-type. Immediately on

entering the p-type each electron falls into a hole and recombination takes place. Also some holes may cross the junction fro the p-type to n-type. Then recombination takes place around the junction. Each recombination radiates energy. In an ordinary diode, the radiated energy is

4 D. Chattopadhyay and P.C. Rakshit, Electronics: Fundamentals and Applications, (New Delhi: New Age International Limited, 2008) p 75

in the form of heat. In LED the radiated energy is in the form of light. This whole process occurs in two steps namely:

a) Injection of minority carriers across the junction and b) The radiative recombination of minority carriers.

When the diode has zero bias under thermal equilibrium conditions the space charge layer potential prevents the cross-over of large concentrations of mobile conduction band electrons and the valence band holes across the junction. When a forward bias is applied to the device the magnitude of this potential barrier is reduced making it possible for diffusion of electrons and holes across the junction. The minority carrier concentrations on both sides of the junction are considerably increased which increases their rate of recombination. This is responsible for electromagnetic radiation.

Two types of radiative recombination mechanism are commonly encountered in LEDs depending upon the band gap characteristics of the semi-conductor material used. These are:

a) Direct Recombinationb) Indirect Recombination

When an excess minority carrier electron concentration V is injected into the P side of a PN junction diode, the probability of recombination with the available holes is proportional to the concentration of V and P. For such a process the radiative recombination rate is,

R = Rc(/V) PRc = Recombination co-efficient.The emission of photons as a result of recombination of electrons and holes is

possible only when both energy and momentum are conserved. A photon can have considerable energy but its momentum is very small. Therefore, the simplest and most probable recombination process will be that where the electrons and holes have the same value of momentum. This situation exists in many semiconductors which are of the direct band-gap type with the conduction band minimum and valence band maximum being at zero momentum position. The recombination process in this is direct type.

In indirect band gap semiconductors, the conduction band minimum and the valence band maximum occur at different values of momentum and therefore, in this case it is necessary to involve a third particle to conserve the momentum. Therefore, this process is known as indirect type recombination. Phonons and chemical impurities are used for this operation. The probability of recombination in this is obviously much lower than that in the direct band-gap semiconductors.5

2.4 Colours of LEDs The colour of an LED is determined by the semiconductor material, not by the

colouring of the 'package' (the plastic body). LEDs of all colours are available in uncoloured packages which may be diffused (milky) or clear (often described as 'water clear'). The coloured packages are also available as diffused (the standard type) or transparent.

LEDs are available in red, orange, amber, yellow, green, blue and white. Blue and white LEDs are much more expensive than the other colours. Based on the number of combination of colours an LED could either be bi-colour or tri-colour.

A bi-colour LED has two LEDs wired in 'inverse parallel' (one forwards, one backwards) combined in one package with two leads. Only one of the LEDs can be lit at one time and they are less useful than the tri-colour LEDs described above.

5 A.K. Sawhney, A Course in Electrical and Electronic Measurements and Instrumentation, (Delhi: Dhanpat Rai and Co, 2008) p 710 - 711

The most popular type of tri-colour LED has a red and a green LED combined in one package with three leads. They are called tri-colour because mixed red and green light appears to be yellow and this is produced when both the red and green LEDs are on.

2.5 Connecting LEDs in SeriesTo have several LEDs on at the same time it may be possible to connect LEDs in

series. This prolongs battery life by lighting several LEDs with the same current as just one LED. All the LEDs connected in series pass the same current so it is best if they are all the same type. The power supply must have sufficient voltage to provide about 2V for each LED (4V for blue and white) plus at least another 2V for the resistor. To work out a value for the resistor you must add up all the LED voltages and use this for VL.

2.6 Connecting LEDs in ParallelConnecting several LEDs in parallel with just one resistor shared between them is

generally not a good idea. If the LEDs require slightly different voltages only the lowest voltage LED will light and it may be destroyed by the larger current flowing through it. Although identical LEDs can be successfully connected in parallel with one resistor this rarely offers any useful benefit because resistors are very cheap and the current used is the same as connecting the LEDs individually. If LEDs are in parallel each one should have its own resistor.

2.7 Advantages of LEDs1. First and foremost is efficiency: LEDs output far more light power per watt of

electrical input than an incandescent lamp. This is a significant advantage if the circuit in question is battery-powered, efficiency translating to longer battery life.

2. Second is the fact that LEDs are far more reliable, having a much greater service life than incandescent lamps. This is because LEDs are “cold” devices: they operate at much cooler temperatures than an incandescent lamp with a white-hot metal filament, susceptible to breakage from mechanical and thermal shock.

3. Third is the high speed at which LEDs may be turned on and off. This advantage is also due to the “cold” operation of LEDs: they don't have to overcome thermal inertia in transitioning from off to on or vice versa. For this reason, LEDs are used to transmit digital (on/off) information as pulses of light, conducted in empty space or through fiber-optic cable, at very high rates of speed (millions of pulses per second).

4. LEDs excel in monochromatic lighting applications like traffic signals and automotive tail lights. Incandescents are abysmal in this application since they require filtering, decreasing efficiency. LEDs do not require filtering.

2.8 DisadvantagesOne major disadvantage of using LEDs as sources of illumination is their

monochromatic (single-color) emission. No one wants to read a book under the light of a red, green, or blue LED. However, if used in combination, LED colors may be mixed for a more broad-spectrum glow. A new broad spectrum light source is the white LED. While small white panel indicators have been available for many years, illumination grade devices are still in development.

3. Zener Diodes

3.1 IntroductionZener diodes are p-n diodes operated in the breakdown region of the reverse voltage

characteristic. These diodes are designed with sufficient power-dissipation capacities to work in the breakdown region. The device was named after Clarence Zener, who discovered this electrical property.

Zener diode is a special purpose diode that operates under reverse bias condition. Such type of diodes is normally fabricated using a silicon semiconductor because it has a higher temperature and current rating than Germanium. In the reverse bias condition, the breakdown of the diode occurs when its crystal structure is broken. This happens at the breakdown or Zener voltage. Due to the breakdown a very large current flows through the diode with a very small change in the voltage.

3.2 ConstructionA Zener diode is fabricated by controlling the doping level of impurities during

manufacture. The level of doping in Zener diodes is generally high. The Zener voltage is reduced with the increase in the levels of doping.

Zener diodes are commercially available with the Zener voltage of 2 volts - 200 volts.

3.3 Principle of OperationA Zener diode permits current in the forward direction like a normal diode, but also

in the reverse direction if the voltage is larger than the breakdown voltage known as "Zener knee voltage" or "Zener voltage".

A conventional solid-state diode will not allow significant current if it is reverse-biased below 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 external circuitry, the diode will be permanently damaged. In case of large forward bias (current in the direction of the arrow), the diode exhibits a voltage drop due to its junction built-in voltage and internal resistance. The amount of the voltage drop depends on the semiconductor material and the doping concentrations.

A Zener diode exhibits almost the same properties, except the device is specially designed so as to have a greatly reduced breakdown voltage, the so-called Zener voltage.

A Zener diode contains a heavily doped p-n junction allowing electrons to tunnel from the valence band of the p-type material to the conduction band of the n-type material. In the atomic scale, this tunneling corresponds to the transport of valence band electrons into the empty conduction band states; as a result of the reduced barrier between these bands and high electric fields that are induced due to the relatively high levels of dopings on both sides.

A reverse-biased Zener diode will exhibit a controlled breakdown and allow the current to keep the voltage across the Zener diode at the Zener voltage. However, the current is not unlimited, so the Zener diode is typically used to generate a reference voltage for an amplifier stage, or as a voltage stabilizer for low-current applications.

3.4 V/I Characteristics

The forward characteristics of a Zener diode are similar to those of a p-n junction diode. The Zener diode is operated over the breakdown region. The breakdown voltage depends upon the amount of doping. By controlling the thickness of depletion layer and doping concentration, it is possible to make a Zener diode to breakdown at a specified voltage.

During the operation in the breakdown region it does not burn out immediately. As long as the current through the diode is limited by the external circuit with the permissible values it does not burn out. The breakdown voltage can be controlled quite accurately in the doping process. While tolerances within 0.05% are available, the most widely used tolerances are 5% and 10%. 3.5 Uses of a Zener Diode

Zener diodes are widely used as a voltage regulator6 across a circuit. When connected in parallel with a variable voltage source so that it is reverse biased, a Zener diode conducts when the voltage reaches the diode's reverse breakdown voltage. From that point it keeps the voltage at that value.In the circuit shown, resistor R provides the voltage drop between U IN and UOUT. The value of R must satisfy two conditions:

1. R must be small enough that the current through D keeps D in reverse breakdown. If insufficient current exists through D, then UOUT will be unregulated, and less than the nominal breakdown voltage (this differs to voltage regulator tubes where the output voltage will be higher than nominal and could rise as high as UIN).

2. R must be large enough that the current through D does not destroy the device. If the current through D is ID, its breakdown voltage VB and its maximum power dissipation is PMAX, then IDVB < PMAX.

A Zener diode used in this way is known as a shunt voltage regulator. In a sense, a portion of the current through the resistor is shunted through the Zener diode, and the rest is through the load. Thus the voltage that the load sees is controlled by causing some fraction of the current from the power source to bypass it—hence the name, by analogy with locomotive switching points.

These devices are also encountered, typically in series with a base-emitter junction, in transistor stages where selective choice of a device centered on the avalanche/Zener point can be used to introduce compensating temperature co-efficient balancing of the transistor PN junction. An example of this kind of use would be a DC error amplifier used in a stabilized power supply circuit feedback loop system.

6 A voltage regulator is a circuit which maintains a constant voltage even when either input voltage or load current varies.

a) Cover Glass d) N-type Siliconb) Antireflective Coating e) P-type Siliconc) Contact Grid f) Back Contact

Structure and Symbolic Representation of a Solar Cell

4. Solar Cells

4.1 IntroductionThe solar cell is a p-n junction diode that transforms sunlight to electricity with large

conversion efficiency. When is light is allowed to fall on the diode, photons are absorbed and electron-hole pairs are produced in p and n regions. The electrons and the holes that are generated over a short distance from the junction can reach the depletion region of width W by diffusion.

A photodiode optimized for efficiently delivering power to a load is the solar cell. It operates in photovoltaic mode (PV) because it is forward biased by the voltage developed across the load resistance.

Solar cells are in fact large area semiconductor diodes. Due to photovoltaic effect energy of light (energy of photons) converts into electrical current. At p-n junction, an electric field is built up which leads to the separation of the charge carriers (electrons and holes). At incidence of photon stream onto semiconductor material the electrons are released, if the energy of photons is sufficient. Contact to a solar cell is realised due to metal contacts. If the circuit is closed, meaning an electrical load is connected, then direct current flows. The energy of photons comes in "packages" which are called quants. The energy of each quantum depends on the wavelength of the visible light or electromagnetic waves. The electrons are released; however, the electric current flows only if the energy of each quantum is greater than WL - WV (boundaries of valence and conductive bands).

4.2 Construction

The most commonly known solar cell is configured as a large-area p-n junction made from silicon. As a simplification, one can imagine bringing a layer of n-type silicon into

direct contact with a layer of p-type silicon. In practice, p-n junctions of silicon solar cells are not made in this way, but rather, by diffusing an n-type dopant into one side of a p-type wafer (or vice versa).

If a piece of p-type silicon is placed in intimate contact with a piece of n-type silicon, then a diffusion of electrons occurs from the region of high electron concentration (the n-type side of the junction) into the region of low electron concentration (p-type side of the

junction). When the electrons diffuse across the p-n junction, they recombine with holes on the p-type side. The diffusion of carriers does not happen indefinitely however, because of an electric field which is created by the imbalance of charge immediately on either side of the junction which this diffusion creates.

The electric field established across the p-n junction creates a diode that promotes charge flow, known as drift current, that opposes and eventually balances out the diffusion of electron and holes. This region where electrons and holes have diffused across the junction is called the depletion region because it no longer contains any mobile charge carriers. It is also known as the "space charge region".

4.3 Connection to an External LoadOhmic metal-semiconductor contacts are made to both the n-type and p-type sides of

the solar cell, and the electrodes connected to an external load. Electrons that are created on the n-type side, or have been "collected" by the junction and swept onto the n-type side, may travel through the wire, power the load, and continue through the wire until they reach the p-type semiconductor-metal contact. Here, they recombine with a hole that was either created as an electron-hole pair on the p-type side of the solar cell, or are swept across the junction from the n-type side after being created there.The voltage measured is equal to the difference in the quasi Fermi levels of the minority carriers i.e., electrons in the p-type portion, and holes in the n-type portion.

4.4 Mechanism of OperationPhotons in sunlight hit the solar panel and are absorbed by semi-conducting materials,

such as silicon. Electrons (negatively charged) are knocked loose from their atoms, allowing them to

flow through the material to produce electricity. Due to the special composition of solar cells, the electrons are only allowed to move in a single direction. The complementary positive charges that are also created (like bubbles) are called holes and flow in the direction opposite of the electrons in a silicon solar panel.

An array of solar cells converts solar energy into a usable amount of direct current (DC) electricity. When a photon hits a piece of silicon, one of three things can happen:

a) The photon can pass straight through the silicon — this (generally) happens for lower energy photons,

b) The photon can reflect off the surface, c) The photon can be absorbed by the silicon, if the photon energy is higher than the

silicon band gap value. This generates an electron-hole pair and sometimes heat, depending on the band structure.

When a photon is absorbed, its energy is given to an electron in the crystal lattice. Usually this electron is in the valence band, and is tightly bound in covalent bonds between neighboring atoms, and hence unable to move far. The energy given to it by the photon "excites" it into the conduction band, where it is free to move around within the semiconductor. The covalent bond that the electron was previously a part of now has one fewer electron — this is known as a hole. The presence of a missing covalent bond allows the bonded electrons of neighboring atoms to move into the "hole," leaving another hole behind, and in this way a hole can move through the lattice. Thus, it can be said that photons absorbed in the semiconductor create mobile electron-hole pairs.

A photon need only have greater energy than that of the band gap in order to excite an electron from the valence band into the conduction band. However, the solar frequency spectrum approximates a black body spectrum at ~6000 K, and as such, much of the solar radiation reaching the Earth is composed of photons with energies greater than the band gap

of silicon. These higher energy photons will be absorbed by the solar cell, but the difference in energy between these photons and the silicon band gap is converted into heat (via lattice vibrations — called phonons) rather than into usable electrical energy.

There are two main modes for charge carrier separation in a solar cell namely drift of carriers, driven by an electrostatic field established across the device diffusion of carriers from zones of high carrier concentration to zones of low carrier concentration.

In the widely used p-n junction solar cells, the dominant mode of charge carrier separation is by drift. However, in non-p-n-junction solar cells, a general electrostatic field has been confirmed to be absent, and the dominant mode of separation is via charge carrier diffusion.

4.5 Effect of Temperature on the Current-Voltage Characteristics of a Solar Cell

Temperature affects the characteristic equation in two ways: directly, via T in the exponential term, and indirectly via its effect on I0. (Strictly speaking, temperature affects all of the terms, but these two far more significantly than the others.) While increasing T reduces the magnitude of the exponent in the characteristic equation, the value of I0 increases in proportion to eT. The net effect is to reduce VOC linearly with increasing temperature. The magnitude of this reduction is inversely proportional to VOC; that is, cells with higher values of VOC suffer smaller reductions in voltage with increasing temperature. For most crystalline silicon solar cells the reduction is about 0.50%/°C, though the rate for the highest-efficiency crystalline silicon cells is around 0.35%/°C.

The overall effect of temperature on cell efficiency can be computed using these factors in combination with the characteristic equation. However, since the change in voltage is much stronger than the change in current, the overall effect on efficiency tends to be similar to that on voltage. Most crystalline silicon solar cells decline in efficiency by 0.50%/°C and most amorphous cells decline by 0.15-0.25%/°C.

4.6 Effect of Series Resistance on the Current-Voltage Characteristics of a Solar Cell

As series resistance increases, the voltage drop between the junction voltage and the terminal voltage becomes greater for the same flow of current. The result is that the current-controlled portion of the I-V curve begins to sag toward the origin, producing a significant decrease in the terminal voltage V and a slight reduction in ISC. Very high values of RS will also produce a significant reduction in ISC; in these regimes, series resistance dominates and the behavior of the solar cell resembles that of a resistor.

4.7 Effect of Shunt Resistance on the Current-Voltage Characteristics of a Solar Cell

As shunt resistance decreases, the flow of current diverted through the shunt resistor increases for a given level of junction voltage. The result is that the voltage-controlled portion of the I-V curve begins to sag toward the origin, producing a significant decrease in the terminal current I and a slight reduction in VOC. Very low values of RSH will produce a significant reduction in VOC. Much as in the case of a high series resistance, a badly shunted solar cell will take on operating characteristics similar to those of a resistor.

Varactor Diodes

5. Varactor Diode

5.1 IntroductionThe name ‘varactor’ originates from the phrase variable reactor. It is a device, the

reactance of which can be changed in a controlled manner by a bias voltage. Since a p-n junction diode has a voltage dependent depletion region capacitance, it finds application as a varactor diode, also a called a varicap.

5.2 OperationThis is a junction diode with a small impurity at its junction which has the useful

property that its junction or transient capacitance is easily varied electronically. When any diode is reverse-biased a depletion layer is formed. Larger the reverse bias

applied across the diode the width of the depletion layer becomes wider. Conversely, by decreasing the reverse bias voltage the depletion layer width becomes narrower. The p and n regions are like the two plates of a capacitor and the depletion layer is like the dielectric material. When a diode is reverse-biased the width of the depletion layer increases with the reverse voltage.

Since the depletion layer gets wider with more reverse voltage, the capacitance becomes smaller. Therefore, capacitance is controlled by reverse voltage. As the capacitance is inversely proportional to the distance between the plates, the transient capacitance varies inversely with the reverse voltage. Consequently, an increase in reverse bias voltage will result in an increase in the depletion layer width and a subsequent decrease in transient capacitance.

5.3 Applications` a) Varactor is connected in parallel with an inductor to form a parallel resonant circuit. This circuit has only one frequency at which maximum impedance occurs. This frequency is called resonant frequency.

b) Varactor diodes can be used in the tuning stage of a radio receiver or in a TV tuner in place of bulky variable capacitors. The circuit thereby becomes compact and its performance improves.

c) Varactor diodes also find applications in harmonic generators, microwave frequency multipliers, self-balancing bridge circuits, active filters, and parametric amplifiers.

d) In very low noise microwave parametric amplifiers.

Tunnel Diode: (a) Schematic Symbol. (b) Current / Voltage Plot (c) Oscillator.

6. Tunnel Diodes

6.1 IntroductionTunnel diodes also known as Esaki diodes in honor of their Japanese inventor Leo

Esaki, exploit a strange quantum phenomenon called resonant tunneling to provide a negative-resistance forward-bias characteristics. When a small forward-bias voltage is applied across a tunnel diode, it begins to conduct current. As the voltage is increased, the current increases and reaches a peak value called the peak current (IP). If the voltage is increased a little more, the current actually begins to decrease until it reaches a low point called the valley current (IV). If the voltage is increased further yet, the current begins to increase again, this time without decreasing into another “valley.”

The forward voltages necessary to drive a tunnel diode to its peak and valley currents are known as peak voltage (VP) and valley voltage (VV), respectively. The region on the graph where current is decreasing while applied voltage is increasing (between VP and VV on the horizontal scale) is known as the region of negative resistance.

Tunnel diodes, are able to transition between peak and valley current levels very quickly, “switching” between high and low states of conduction much faster than even Schottky diodes. Tunnel diode characteristics are also relatively unaffected by changes in temperature.

6.2 ConstructionThese are usually fabricated form Germanium, Gallium Arsenide or Gallium

Antimonide but Silicon is not used because the ration of peak value of forward current to the value of forward current to the value of valley current is very low.

Tunnel diodes are heavily doped in both the P and N regions, 1000 times the level in a rectifier. The heavy doping produces an unusually thin depletion region. This produces an unusually low reverse breakdown voltage with high leakage. The thin depletion region causes high capacitance. To overcome this, the tunnel diode junction area must be tiny.

6.3 Reverse Breakdown Voltage Versus Doping Level. The forward diode characteristic consists of two regions: a normal forward diode

characteristic with current rising exponentially beyond VF, 0.3 V for Ge, 0.7 V for Si.

Between 0 V and VF is an additional “negative resistance” characteristic peak. This is due to quantum mechanical tunneling involving the dual particle-wave nature of electrons. The depletion region is thin enough compared with the equivalent wavelength of the electron that they can tunnel through. They do not have to overcome the normal forward diode voltage VF.

The energy level of the conduction band of the N-type material overlaps the level of the valence band in the P-type region. With increasing voltage, tunneling begins; the levels overlap; current increases, up to a point. As current increases further, the energy levels overlap less; current decreases with increasing voltage. This is the “negative resistance” portion of the curve.

Tunnel diodes are not good rectifiers, as they have relatively high “leakage” current when reverse-biased. Consequently, they find application only in special circuits where their unique tunnel effect has value. To exploit the tunnel effect, these diodes are maintained at a bias voltage somewhere between the peak and valley voltage levels, always in a forward-biased polarity.

Perhaps the most common application of a tunnel diode is in simple high-frequency oscillator circuits, where it allows a DC voltage source to contribute power to an LC “tank” circuit, the diode conducting when the voltage across it reaches the peak (tunnel) level and effectively insulating at all other voltages. The resistors bias the tunnel diode at a few tenths of a volt centered on the negative resistance portion of the characteristic curve. The L-C resonant circuit may be a section of waveguide for microwave operation. Oscillation to 5 GHz is possible.

Tunnel diodes are also resistant to radiation because of the heavy doping. Today various transistors operate at microwave frequencies. Even small signal tunnel diodes are expensive and difficult to find today. There is one remaining manufacturer of germanium tunnel diodes, and none for silicon devices. They are sometimes used in military equipment because they are insensitive to radiation and large temperature changes.

7. Schottky diodes7.1 Introduction

Schottky diodes are constructed of a metal-to-N junction rather than a P-N semiconductor junction. Also known as hot-carrier diodes, Schottky diodes are characterized by fast switching times (low reverse-recovery time), low forward voltage drop (typically 0.25 to 0.4 volts for a metal-silicon junction), and low junction capacitance.

7.2 ConstructionIf aluminum, which is an acceptor impurity in Silicon is attached to n-type Silicon as

lead the n-region near the surface where aluminum is deposited must be made highly n-type to achieve ohmic contact. If aluminum is directly deposited on n-type silicon, a rectifying contact is obtained.

Contact A is rectified and contact B is ohmic, producing a Schottky diode where the contact A is the anode. When the anode voltage is positive, electrons from n-type silicon move into the metal through the junction resulting in the flow of current.

7.3 Working PrincipleThe forward voltage drop (VF), reverse-recovery time (trr), and junction capacitance

(CJ) of Schottky diodes are closer to ideal than the average “rectifying” diode. This makes them well suited for high-frequency applications. Unfortunately, though, Schottky diodes typically have lower forward current (IF) and reverse voltage (VRRM and VDC) ratings than rectifying diodes and are thus unsuitable for applications involving substantial amounts of power.

Switching regulator power supplies operating at 100's of kHz cannot use conventional silicon diodes as rectifiers because of their slow switching speed . When the signal applied to a diode changes from forward to reverse bias, conduction continues for a short time, while carriers are being swept out of the depletion region. Conduction only ceases after this t r

reverse recovery time has expired. Schottky diodes have a shorter reverse recovery time. Regardless of switching speed, the 0.7 V forward voltage drop of silicon diodes causes poor efficiency in low voltage supplies. This is not a problem in, say, a 10 V supply. In a 1 V supply the 0.7 V drop is a substantial portion of the output. One solution is to use a schottky power diode which has a lower forward drop.

7.4 Applicationsa) Mostly, Schottky diodes find application as clamping diodes. Such type of diodes is

used in digital computers, low voltage bridge rectifiers.b) Schottky diode technology finds broad application in high-speed computer circuits,

where the fast switching time equates to high speed capability, and the low forward voltage drop equates to less power dissipation when conducting.

Laser Diodes

8. Laser Diodes 8.1 Introduction

The laser diode is a further development upon the regular light-emitting diode, or LED. The term “laser” itself is actually an acronym, despite the fact its often written in lower-case letters. “Laser” stands for Light Amplification by Stimulated Emission of Radiation, and refers to another strange quantum process whereby characteristic light emitted by electrons falling from high-level to low-level energy states in a material stimulate other electrons in a substance to make similar “jumps,” the result being a synchronized output of light from the material. This synchronization extends to the actual phase of the emitted light, so that all light waves emitted from a “lasing” material are not just the same frequency (color), but also the same phase as each other, so that they reinforce one another and are able to travel in a very tightly-confined, non-dispersing beam. This is why laser light stays so remarkably focused over long distances: each and every light wave coming from the laser is in step with each other.

8.2 Operation Incandescent lamps produce “white” (mixed-frequency, or mixed-color) light as in

Figure (a). Regular LEDs produce monochromatic light: same frequency (color), but different phases, resulting in similar beam dispersion in Figure above (b). Laser LEDs produce coherent light: light that is both monochromatic (single-color) and mono-phasic (single-phase), resulting in precise beam confinement as in Figure above (c).

Laser light finds wide application in the modern world: everything

from surveying, where a straight and non-dispersing light beam is very useful for precise sighting of measurement markers, to the reading and writing of optical disks, where only the narrowness of a focused laser beam is able to resolve the microscopic “pits” in the disk's surface comprising the binary 1's and 0's of digital information. Some laser diodes require special high-power “pulsing” circuits to deliver large quantities of voltage and current in short bursts. Other laser diodes may be operated continuously at lower power. In the continuous laser, laser action occurs only within a certain range of diode current, necessitating some form of current-regulator circuit. As laser diodes age, their power requirements may change (more current required for less output power), but it should be remembered that low-power laser diodes, like LEDs, are fairly long-lived devices, with typical service lives in the tens of thousands of hours.

9. PIN Diodes9.1 Introduction

A p–i–n photodiode, also called PIN photodiode, is a photodiode with an intrinsic (i) (i.e., undoped) region in between the n- and p-doped regions. Most of the photons are absorbed in the intrinsic region, and carriers generated therein can efficiently contribute to the photocurrent. In the given figure the cathode is a flat electrode, whereas the anode has the form of a ring (of which two opposite parts are seen in the shown cross section). The positive pole of the (reverse) bias voltage is connected to the cathode. On top of the p region, there is an anti-reflection coating.

9.2 Structure of PIN DiodesIt consists of a highly-doped transparent p-type contact layer on top of an undoped

absorbing layer and an n-type highly doped contact layer on the bottom. Discrete photodiodes are fabricated on a conductive substrate as shown in the figure, which facilitates the formation of the n-type contact and reduces the number of process steps. The top contact is typically a metal ring contact, which has a low contact resistance and still allows the light to be absorbed in the semiconductor. An alternative approach uses a transparent conductor such as Indium Tin Oxide (ITO). The active device area is formed by mesa etching or by proton implantation of the adjacent area, which makes it isolating. A dielectric layer is added around the active area to reduce leakage currents and to ensure a low parasitic capacitance of the contact pad.

Grading of the material composition between the transparent contact layer and the absorbing layer is commonly used to reduce the n-n+ or p-p+ barrier formed at the interface. The above structure evolved mainly from one basic requirement: light should be absorbed in the depletion region of the diode to ensure that the electrons and holes are separated in the electric field and contribute to the photocurrent, while the transit time must be minimal.This implies that a depletion region larger than the absorption length must exist in the detector. This is easily assured by making the absorbing layer undoped. Only a very small voltage is required to deplete the undoped region. If a minimum electric field is required throughout the absorbing layer, to ensure a short transit time, it is also the undoped structure, which satisfies this condition with a minimal voltage across the region, because the electric field is constant. An added advantage is that the recombination/generation time constant is longest for undoped material, which provides a minimal thermal generation current.It also implies that the top contact layer should be transparent to the incoming light. In silicon photodiodes one uses a thin highly doped contact layer to minimize the absorption. By using a contact layer with a wider band gap (also called the window layer) absorption in the contact layer can be eliminated (except for a small fraction due to free carrier absorption) which improves the responsivity.

Electron-hole pairs, which are absorbed in the quasi-neutral regions, can still contribute to the photocurrent provided they are generated within one diffusion length of the depletion region. However, the collection of carriers due to diffusion is relatively inefficient and leads to long tails in the transient response. It therefore should be avoided.Because of the large difference in refractive index between air and most semiconductors, there is a substantial reflection at the surface. The reflection at normal incidence between two materials with refractive index n1 and n2 is given by:

P

Depletion Region

Schematic Drawing of a p–i–n Photodiode.

9.3 Construction of PIN DiodesCompared with an ordinary p–n photodiode, a p–i–n photodiode has a thicker

depletion region, which allows a more efficient collection of the carriers and thus a larger quantum efficiency, and also leads to a lower capacitance and thus to higher detection bandwidth.

The most common p–i–n diodes are based on silicon. They are sensitive throughout the visible spectral region and in the near infrared up to 1 μm. At longer wavelengths, the absorption efficiency and thus the responsivity drops sharply, but the parameters of this cut-off depend on the thickness of the i region.

For longer wavelengths up to 1.7μm (or with extended spectral response up to 2.6μm), InGaAs p–i–n diodes are

available, although at significantly higher prices (particularly for large active areas). Germanium p–i–n diodes can be an alternative. The fastest p–i–n photodiodes have bandwidths of the order of tens of gigahertz. Their active areas typically have a diameter of only a few hundred microns. Some of them are available in fiber-coupled form and can be applied e.g. in receivers for optical fiber communications.

9.3 Working Mechanism

9.3.1 Generation of Electron-Hole PairsThe generation of electron-hole pairs in a semiconductor is directly related to the

absorption of light since every absorbed photon generates one electron-hole pair. The optical generation rate gop is given by:

where A is the illuminated area of the photodiode, Popt

is the incident optical power, is the absorption coefficient and h is the photon energy. Note that the optical power is position dependent and obtained by solving:

The resulting generation rate must be added to the continuity equation and solved throughout the photodiode, which results in the photocurrent.

9.3.2 Photocurrent Due to Absorption in the Depletion RegionAssuming that all the generated electron-hole pairs contribute to the photocurrent, the

photocurrent is simply the integral of the generation rate over the depletion region:

P N i N Intrinsic Region

where d is the thickness of the undoped region.

9.3.3 Photocurrent due to absorption in the quasi-neutral regionTo find the photocurrent due to absorption in the quasi-neutral region, we first have to

solve the diffusion equation in the presence of light. For holes in the n-type contact layer this means solving the continuity equation:

where the electron-hole pair generation gop depends on position. For an n-type contact layer with the same energy bandgap as the absorption layer, the optical generation rate equals:

The photocurrent due to holes originating in the n-type contact layer equals:

9.3.4 Absorption in the P-Contact RegionEven though the contact layer was designed so that no light absorbs in this layer, it

will become absorbing at shorter wavelengths. Consider a worst-case scenario where all the electron-hole pairs, which are generated in the p-type contact layer, recombine without contributing to the photocurrent. The optical power incident on the undoped region is reduced by exp(-*wp

’) where wp’ is the width of the quasi-neutral p region and * is the absorption

coefficient in that region.

IMPATT Diode: Oscillator Circuit and Heavily Doped P and N Layers.

10. IMPATT Diode

10.1 IntroductionAn IMPATT diode (IMPact Ionization Avalanche Transit-Time) is a form of high

power diode used in high-frequency electronics and microwave devices. They are typically made with silicon carbide owing to their high breakdown fields.

They operate at frequencies between about 3 and 100 GHz or more. A main advantage is their high power capability. These diodes are used in a variety of applications from low power radar systems to alarms. A major drawback of using IMPATT diodes is the high level of phase noise they generate. This results from the statistical nature of the avalanche process. Nevertheless these diodes make excellent microwave generators for many applications.

10.2 StructureThe IMPATT diode family includes many different junctions and metal

semiconductor devices. The first IMPATT oscillation was obtained from a simple silicon p-n junction diode biased into a reverse avalanche break down and mounted in a microwave cavity. Because of the strong dependence of the ionization coefficient on the electric field, most of the electron–hole pairs are generated in the high field region. The generated electron immediately moves into the region, while the generated holes drift across the p region. The time required for the hole to reach the contact constitutes the transit time delay.

The original proposal for a microwave device of the IMPATT type was made by Read and involved a structure. The Read diode consists of two regions namely i) The Avalanche region (p1 – region with relatively high doping and high field, in which avalanche multiplication occurs and (ii) the drift region (p2 – region with essentially intrinsic doping and constant field, in which the generated holes drift towards the - contact. Of course, a similar device can be built with the configuration, in which electrons generated from the avalanche multiplication drift through the intrinsic region.

A fabricated IMPATT diode generally is mounted in a micro wave package. The diode is mounted with its high – field region close to a copper heat sink so that the heat

generated at the diode junction can be readily conducted away by the copper heat sink. Similar microwave packages are used to house other microwave devices.

10.3 Principle of OperationIF a free electron with sufficient energy strikes on silicon atom , it can break covalent

bond of silicon and liberate an electron from the covalent bond if electron gains energy by electric field and liberate other electron from other covalent bond. This process can cascade (avalanche) very quickly into chain reaction producing number of electron and large current flow in diode. This phenomena is called impact avalanche. At breakdown, the n – region is punched through and forms the avalanche region of the diode. The high resistivity i – region is the drift zone through which the avalanche generated electrons move toward the anode.

Now if a dc bias VB is considered just short of that required to cause breakdown, applied to the diode, and an ac voltage of sufficiently large magnitude be superimposed on the dc bias, such that during the positive cycle of the ac voltage, the diode is driven deep into the avalanche breakdown. At t=0, the ac voltage is zero, and only a small pre-breakdown current flows through the diode. As t increases, the voltage goes above the breakdown voltage and secondary electron-hole pairs are produced by impact ionization.

As long as the field in the avalanche region is maintained above the breakdown field, the electron-hole concentration grows exponentially with t. Similarly this concentration decays exponentially with time when the field is reduced below at the negative swing of the ac voltage. The holes generated in the avalanche region disappear in the p+ region and are collected by the cathode. The electrons are injected into the i – zone where they drift toward the n+ region. Then, the field in the avalanche region reaches its maximum value and the population of the electron-hole pairs starts building up.

At this time, the ionization coefficients have their maximum values. The generated electron concentration does not follow the electric field instantaneously because it also depends on the number of electron-hole pairs already present in the avalanche region. Hence, the electron concentration at t will have a small value. Even after the field has passed its maximum value, the electron-hole concentration continues to grow because the secondary carrier generation rate still remains above its average value. For this reason, the electron concentration in the avalanche region attains its maximum value at t, when the field has dropped to its average value. Thus, it is clear that the avalanche region introduces a 90o phase shift between the ac signal and the electron concentration in this region.

With a further increase in t, the ac voltage becomes negative, and the field in the avalanche region drops below its critical value. The electrons in the avalanche region are then injected into the drift zone which induces a current in the external circuit which has a phase opposite to that of the ac voltage. The ac field, therefore, absorbs energy from the drifting electrons as they are decelerated by the decreasing field. It is clear that an ideal phase shift between the diode current and the ac signal is achieved if the thickness of the drift zone is such that the bunch of electron is collected at the n+ - anode at when the ac voltage goes to zero. This condition is achieved by making the length of the drift region equal to the wavelength of the signal. This situation produces an additional phase shift of 90o between the ac voltage and the diode current. The waveforms of the ac voltage, they injected electron charge, and the current induced in the external circuit.

Equivalent Circuit of a Gunn Diode

11. Gunn Diode

A gunn diode is solely composed of N-type semiconductor. As such, it is not a true diode. Figure below shows a lightly doped N- layer surrounded by heavily doped N+ layers. A voltage applied across the N-type gallium arsenide gunn diode creates a strong electric field across the lightly doped N- layer.

As voltage is increased, conduction increases due to electrons in a low energy conduction band. As voltage is increased beyond the threshold of approximately 1 V, electrons move from the lower conduction band to the higher energy conduction band where they no longer contribute to conduction. In other words, as voltage increases, current decreases a negative resistance condition. The oscillation frequency is determined by the transit time of the conduction electrons, which is inversely related to the thickness of the N -

layer. The frequency may be controlled to some extent by embedding the gunn diode into a

resonant circuit. The lumped circuit equivalent shown in Figure above is actually a coaxial transmission line or waveguide. Gallium arsenide gunn diodes are available for operation from 10 to 200 gHz at 5 to 65 mw power. Gunn diodes may also serve as amplifiers.

TABLE OF CONTENTS

1. Photodiodes......................................................................................................................1

1.1 Introduction...............................................................................................................1

1.2 Photodiode Types....................................................................................................1

1.3 Principle of Operation...........................................................................................1

1.4 Circuit and Characteristics of a Photodiode................................................2

1.5 Temperature Characteristics.............................................................................3

1.6 Applications...............................................................................................................3

1.7 Features of photodiodes.......................................................................................3

2. Light Emitting Diode....................................................................................................4

2.1 Introduction...............................................................................................................4

2.2 Construction..............................................................................................................4

2.3 Principle of Operation of LEDs..........................................................................4

2.4 Colours of LEDs.......................................................................................................5

2.5 Connecting LEDs in Series..................................................................................6

2.6 Connecting LEDs in Parallel...............................................................................6

2.7 Advantages of LEDs...............................................................................................6

2.8 Disadvantages...........................................................................................................6

3. Zener Diodes....................................................................................................................7

3.1 Introduction...............................................................................................................7

3.2 Construction..............................................................................................................7

3.3 Principle of Operation...........................................................................................7

3.4 V/I Characteristics..................................................................................................8

3.5 Uses of a Zener Diode...........................................................................................8

4. Solar Cells.........................................................................................................................9

4.1 Introduction...............................................................................................................9

4.2 Construction..............................................................................................................9

4.3 Connection to an External Load......................................................................10

4.4 Mechanism of Operation....................................................................................10

4.5 Effect of Temperature on the Current-Voltage Characteristics of a

Solar Cell..........................................................................................................................11

4.6 Effect of Series Resistance on the Current-Voltage Characteristics

of a Solar Cell.................................................................................................................11

4.7 Effect of Shunt Resistance on the Current-Voltage Characteristics

of a Solar Cell.................................................................................................................11

5. Varactor Diode..............................................................................................................12

5.1 Introduction.............................................................................................................12

5.2 Operation..................................................................................................................12

5.3 Applications.............................................................................................................12

6. Tunnel Diodes................................................................................................................13

6.1 Introduction.............................................................................................................13

6.2 Construction............................................................................................................13

6.3 Reverse Breakdown Voltage Versus Doping Level.................................13

7. Schottky diodes............................................................................................................15

7.1 Introduction.............................................................................................................15

7.2 Construction............................................................................................................15

7.3 Working Principle.................................................................................................15

7.4 Applications.............................................................................................................15

8. Laser Diodes..................................................................................................................16

8.1 Introduction.............................................................................................................16

8.2 Operation..................................................................................................................16

9. PIN Diodes......................................................................................................................17

9.1 Introduction.............................................................................................................17

9.2 Structure of PIN Diodes.....................................................................................17

9.3 Construction of PIN Diodes...............................................................................18

9.3 Working Mechanism............................................................................................18

9.3.1 Generation of Electron-Hole Pairs..........................................................................18

9.3.2 Photocurrent Due to Absorption in the Depletion Region......................................18

9.3.3 Photocurrent due to absorption in the quasi-neutral region....................................19

9.3.4 Absorption in the P-Contact Region.......................................................................19

10. IMPATT Diode...........................................................................................................20

10.1 Introduction..........................................................................................................20

10.2 Structure................................................................................................................20

10.3 Principle of Operation.......................................................................................21

11. Gunn Diode..................................................................................................................22