RGCET / II CSE

1

UNIT V

Special Semiconductor Devices - Varactor diode, Tunnel diode, PIN diode, LED, LCD, Seven segment display,Opto – isolator. UJT – Characteristics and equivalent circuit - intrinsic standoff ratio – UJT relaxation Oscillator, SCR – Two transistor model, DIAC and TRIAC – operation, characteristics and their application.

VARACTOR DIODE

A junction diode which acts as a variable capacitor under changing reverse bias is known as a varactor diode.

When a pn junction is formed, depletion layer is created in the junction area. Since there are no charge carriers within the depletion zone, the zone acts as an insulator. The p-type material with holes(considered positive) as majority carriers and n-type material with electrons (−ve charge) as majority carriers act as charged plates. Thus the diode may be considered as a capacitor with n-region and p-region forming oppositely charged plates and with depletion zone between them acting as a dielectric. This is illustrated in Fig. (i). A varactor diode is specially constructed to have high capacitance under reverse bias. Fig. (ii) shows the symbol of varactor diode. The values of capacitance of varactor diodes are in the picofarad (10−12 F) range

OPERATION:

For normal operation, a varactor diode is always*reverse biased. The capacitance of varactor diode is found as :

CT= ε A/ Wd

where CT =Total capacitance of the junction ,ε=Permittivity of the semiconductor material

A=Cross-sectional area of the junction & Wd=Width of the depletion layer

When reverse voltage across a varactor diode is increased, the width Wd of the depletion layer increases. Therefore, the total junction capacitance CT of the junction decreases. On the other hand, if the reverse voltage across the diode is lowered, the width Wd of the depletion layer decreases. Consequently, the total junction capacitance CT increases.

RGCET / II CSE

2

Fig. shows the curve between reverse bias voltage VR across varactor diode and total junction capacitance CT . Note that CT can be changed simply by changing the voltage VR For this reason, a varactor diode is sometimes called voltage controlled capacitor.

Tunnel diode

A tunnel diode or Esaki diode is a type of semiconductor that is capable of very fast operation, well into the microwave frequency region, made possible by the use of the quantum mechanical effect called tunneling.

It was invented in August 1957 by Leo Esaki when he was with Tokyo Tsushin Kogyo, now known as Sony. In 1973 he received the Nobel Prize in Physics, jointly with Brian Josephson, for discovering the electron tunneling effect used in these diodes. Robert Noyce independently came up with the idea of a tunnel diode while working for William Shockley, but was discouraged from pursuing it.[1]

These diodes have a heavily doped p–n junction only some 10 nm (100 Å) wide. The heavy doping results in a broken bandgap, where conduction band electron states on the n-side are more or less aligned with valence band hole states on the p-side

Tunnel diodes were first manufactured by Sony in 1957[2] followed by General Electric and other companies from about 1960, and are still made in low volume today.[3] Tunnel diodes are usually made from germanium, but can also be made from gallium arsenide and silicon materials. They are used in frequency converters and detectors.[4] They have negative differential resistance in part of their operating range, and therefore are also used as oscillators, amplifiers, and in switching circuits using hysteresis.

There is another type of tunnel diode called a metal–insulator–metal (MIM) diode, but present application appears restricted to research environments due to inherent sensitivities.[7] There is also a metal–insulator–insulator–metal MIIM diode which has an additional insulator layer. The additional insulator layer allows "step tunneling" for precise diode control.

RGCET / II CSE

3

Forward bias operation

Under normal forward bias operation, as voltage begins to increase, electrons at first tunnel through the very narrow p–n junction barrier because filled electron states in the conduction band on the n-side become aligned with empty valence band hole states on the p-side of the p-n junction. As voltage increases further these states become more misaligned and the current drops. This is called negative resistance because current decreases with increasing voltage. As voltage increases yet further, the diode begins to operate as a normal diode, where electrons travel by conduction across the p–n junction, and no longer by tunneling through the p–n junction barrier. The most important operating region for a tunnel diode is the negative resistance region. Its graph is different from normal p-n junction diode.

Reverse bias operation

Main article: Backward diode

When used in the reverse direction, tunnel diodes are called back diodes (or backward diodes) and can act as fast rectifiers with zero offset voltage and extreme linearity for power signals (they have an accurate square law characteristic in the reverse direction). Under reverse bias, filled states on the p-side become increasingly aligned with empty states on the n-side and electrons now tunnel through the pn junction barrier in reverse direction.

In a conventional semiconductor diode, conduction takes place while the p–n junction is forward biased and blocks current flow when the junction is reverse biased. This occurs up to a point known as the “reverse breakdown voltage” when conduction begins (often accompanied

RGCET / II CSE

4

by destruction of the device). In the tunnel diode, the dopant concentrations in the p and n layers are increased to the point where the reverse breakdown voltage becomes zero and the diode conducts in the reverse direction. However, when forward-biased, an odd effect occurs called quantum mechanical tunnelling which gives rise to a region where an increase in forward voltage is accompanied by a decrease in forward current. This negative resistance region can be exploited in a solid state version of the dynatron oscillator which normally uses a tetrode thermionic valve (or tube).

The tunnel diode showed great promise as an oscillator and high-frequency threshold (trigger) device since it operated at frequencies far greater than the tetrode could, well into the microwave bands. Applications for tunnel diodes included local oscillators for UHF television tuners, trigger circuits in oscilloscopes, high-speed counter circuits, and very fast-rise time pulse generator circuits. The tunnel diode can also be used as low-noise microwave amplifier.[9] However, since its discovery, more conventional semiconductor devices have surpassed its performance using conventional oscillator techniques. For many purposes, a three-terminal device, such as a field-effect transistor, is more flexible than a device with only two terminals. Practical tunnel diodes operate at a few milliamperes and a few tenths of a volt, making them low-power devices.[10] The Gunn diode has similar high frequency capability and can handle more power.

Tunnel diodes are also more resistant to nuclear radiation than other diodes. This makes them well suited to higher radiation environments such as those found in space.

The tunnel diode is a form of very fast semiconductor diode that can operate well into the microwave radio frequency region.

It differs from other forms of semiconductor diode in that it uses a quantum mechanical effect called tunnelling. This provides the tunnel diode with a negative resistance region in its IV characteristic curve that enables it to be used as an oscillator and as an amplifier.

Although they are not as widely used as some devices today, these devices do have their place within RF technology. They were used in television receiver front end oscillators and oscilloscope trigger circuits, etc. They have been shown to have a very long life and can offer a very high level of performance when used as an RF pre-amplifier. However today, their applications are often limited because more traditional three terminal devices can offer a better level of performance in many areas.

Tunnel diode development

The tunnel diode was discovered by a Ph.D. research student named Esaki in 1958 while he was investigating the properties of heavily doped germanium junctions for use in high speed bipolar transistors.

In the course of his research Esaki produced some heavily doped junctions for high speed bipolar transistors and as a result he found that they produced an oscillation at microwave frequencies as a result of the tunnelling effect.

Then in 1973, Esaki received the Nobel prize for Physics for his work on the tunnel diode.

RGCET / II CSE

5

After the work by Esaki, other researchers demonstrated that other materials also showed the tunnelling effect. Holonyak and Lesk demonstrated a Gallium Arsenide device in 1960, and others demonstrated Indium tin, and then in 1962 the effect was demonstrated in materials including Indium Arsenide, Indium Phosphide and also Silicon.

Tunnel diode circuit symbol

Despite the operation of the tunnel diode. its circuit symbol is based on that for the standard diode, but has 'tails' added to the bar element of the symbol to differentiate it from other forms of PN junction diode.

Tunnel diode circuit symbol

Advantages and disadvantages

The tunnel diode is not as widely used these days as it was oat one time. With the improvement in performance of other forms of semiconductor technology, they have often become the preferred option. Nevertheless it is still worth looking at a tunnel diode, considering its advantages and disadvantages to discover whether it is a viable option.

Advantages

Very high speed: The high speed of operation means that the tunnel diode can be used for microwave RF applications.

Longevity: Studies have been undertaken of the tunnel diode and its performance has been shown to remain stable over long periods of time, where other semiconductor devices may have degraded.

Disadvantages

Reproducibility: It has not been possible to make the tunnel diode with as reproducible performance to the levels often needed.

Low peak to valley current ratio: The negative resistance region and the peak to valley current is not as high as is often be required to produce the levels of performance that can be attained with other devices.

One of the main reasons for the early success of the tunnel diode was its high speed of operation and the high frequencies it could handle. This resulted from the fact that while many other devices are slowed down by the presence of minority carriers, the tunnel diode only uses majority carriers, i.e. holes in an n-type material and electrons in a p-type material. The minority

RGCET / II CSE

6

carriers slow down the operation of a device and as a result their speed is slower. Also the tunnelling effect is inherently very fast.

The tunnel diode is rarely used these days and this results from its disadvantages. Firstly they only have a low tunnelling current and this means that they are low power devices. While this may be acceptable for low noise amplifiers, it is a significant drawback when they are sued in oscillators as further amplification is needed and this can only be undertaken by devices that have a higher power capability, i.e. not tunnel diodes. The third disadvantage is that they are problems with the reproducibility of the devices resulting in low yields and therefore higher production costs.

Applications

Although the tunnel diode appeared promising some years ago, it was soon replaced by other semiconductor devices like IMPATT diodes for oscillator applications and FETs when used as an amplifier. Nevertheless the tunnel diode is a useful device for certain applications.

Applications for the tunnel diode included uses as an oscillator, although it was also used as an amplifier and a mixer.

One of the major advantages of the tunnel diode which is currently beginning to be experienced is its longevity. Once manufactured its performance remains stable over long periods of time despite its use. Other devices might degrade slightly over time.While the tunnel diode is a semiconductor device using the same materials as other forms of diode and active devices, the very high levels of dopant used, cause the devices to operate in a very different manner.

The device theory shows that it does not act as a diode, but instead exhibits a negative resistance region in the forward direction.

The IV characteristic curve, combined with the very high speed of the diode means that the it can be used in a variety of microwave RF applications as an active device.

Tunnel diode theory basics

The characteristic curve for a tunnel diode shows an area of negative resistance. When forward biased the current in the diode rises at first, but later it can be seen to fall with increasing voltage, before finally rising again.

It is also interesting to note that current also flows in the reverse direction - the reverse breakdown voltage is actually zero and the diode conducts in the reverse direction. The characteristics near the original are virtually symmetrical.

RGCET / II CSE

7

Tunnel diode IV characteristic

The reason for this is that there are a number of different components to forming the overall curve.

Normal diode current: This is the 'normal' current that would flow through a PN junction diode.

Tunnelling current: This is the current that arises as a result of the tunnelling effect. Excess current: This is a third element of current that contributes to the overall current

within the diode. It results from what may be termed excess current that results from tunnelling though bulk states in the energy gap, and means that the valley current does not fall to zero.

Tunnel diode current components

These three main components sum together to provide the overall level of current passed by the tunnel diode.

RGCET / II CSE

8

Tunnelling mechanism & theory

Tunnelling is an effect that is caused by quantum mechanical effects when electrons pass through a potential barrier. It can be visualised in very basic terms by them "tunnelling" through the energy barrier.

The tunnelling only occurs under certain conditions. It occurs within tunnel diodes because of the very high doping levels employed.

At reverse bias, the electrons tunnel from the valence band in the p-type material to the conduction band in the n-type material, and the level of the current increase monotonically.

For the forward bias situation there are a number of different areas. For voltages up to Vpe, electrons from the conduction band find increasing availability of empty states in the valence band and the level of current increases up to a point where the current equals Ipe.

Once this point is reach, it is found that number of empty states available for electrons with the level of energy they are given by the increased voltage level starts to fall. This means that the current level falls in line with this. The overall current level falls away relatively swiftly, dropping to near zero.

As the current from the tunnelling effect falls, so the diffusion current, which is the same action as occurs in a normal PN junction diode starts to increase and steadily becomes the dominant mechanism.

Tunnel diode characteristics

The diagram towards the top of the page shows the tunnel diode IV characteristic. This has a form of 'N' shaped curve. With an area of negative resistance between the peak voltage, Vpe and the valley voltage Vv.

The values for these voltages depend upon the diode material and also upon its individual characteristics.

Tunnel Diode Properties for Different Materials

Parameter Germanium Gallium Arsenide Silicon

Vpe (mV) 40 - 70 90 -120 80 - 100

Vv (mV) 250 - 350 450 -600 400 - 500

Ipe/Iv 10 -15 10 - 20 3 - 5

RGCET / II CSE

9

One of the useful figures of merit for a tunnel diode characteristic is the peak to valley current ratio, Ipe / Iv. From the values in the table it can be seen that silicon has a very low value and as a result, this means that it is not normally one of the best options for a tunnel diode.

Tunnel diode structure basics

The tunnel diode is similar to a standard p-n junction in many respects except that the doping levels are very high. Densities of the order of 5x10^19 cm^-3 are common.

A further difference is that the depletion region, the area between the p-type and n-type areas, where there are no carriers is very narrow. Typically it is in the region of between five to ten nano-metres, which equates to a width of only a few atoms.

As the depletion region is so narrow this means that if it is to be used for high frequency operation the diode itself must be made very small to reduce the high level of capacitance resulting from the very narrow depletion region.

In terms of the material used for these diodes, the favoured semiconductor is germanium. Although other materials can be used and have been used, germanium has the advantage that it has a small energy gap that allows for more efficient tunnelling.

Tunnel diode fabrication structures

Tunnel diode structures generally fall into one of three basic structures:

Ball alloy: This type of tunnel diode format is fabricated as a mesa structure. To achieve this form of structure, the fabrication technique involves bringing an alloy containing the required dopants into contact with a heavily doped substrate. The temperature used is around 500°C at which point the dopants quickly melt and diffuse into the substrate. The overall structure geometry is then defined by etching the diode to the required proportions.

Tunnel diode ball alloy structure

Pulsed bond: This is a relatively straightforward structure to create, although careful process control is required during the fabrication process. The diode is created by using

RGCET / II CSE

10

a wire coated with an alloy containing the required dopants. This is pressed hard onto the heavily doped substrate, and then a voltage pulse is applied. The effect of this is that the junction forms by a process of local alloying.

Tunnel diode pulsed bond structure

Despite this, there are drawbacks to this process because it can only produce a small

junction, and the exact properties, including the area of the junction cannot be controlled

tightly.

Planar structure: Planar technology can be used to create the diode. Using this approach for the fabrication process, the heavily doped n+ substrate is masked off by an insulating layer to leave a small area exposed. This exposed area is then open to become the active area of the diode.

Tunnel diode planar structure

The doping for the area can be introduced by one of a number of means. It can be

introduced by diffusion, alloying or epitaxial growth. Alternatively it is possible to grow an

epitaxial layer over the whole surface and then etch away those areas that are not

required to leave a mesa structure.

All three structures enable high performance diodes to be obtained.

Although these are three popular structures for tunnel diodes, new developments are occurring using different materials and also involving new structures that offer a greater variety of characteristics, or they may be tailored to the needs of a particular material that may be used.

RGCET / II CSE

11

PIN diode

A PiN diode is a diode with a wide, lightly doped 'near' intrinsic semiconductor region between a p-type semiconductor and an n-type semiconductor regions. The p-type and n-type regions are typically heavily doped because they are used for ohmic contacts. The wide intrinsic region is in contrast to an ordinary PN diode. The wide intrinsic region makes the PIN diode an inferior rectifier (the normal function of a diode), but it makes the PIN diode suitable for attenuators, fast switches, photodetectors, and high voltage power electronics applications.

Layers of a PIN diode

It is composed of three regions. In addition to the usual N and P regions, an intrinsic layer is sandwitched between them, to form the PIN structure. Being intrinsic, the intermediate layer offers relatively high resistance which gives it two advantages compared to an ordinary PN diode. The are decrease in capacitance between P and N regions as it is inversely proportional to the separation between these regions. It allows a faster response time for the diode. Hence, PIN diodes are used at high frequencies. Possibility of greater electric field between the P and N junctions, so that the charge carries drift towards their majority carrier side. This enhances faster response of the diode.

Operation:

A PiN diode operates under what is known as high-level injection. In other words, the intrinsic "i" region is flooded with charge carriers from the "p" and "n" regions. Its function can be likened to filling up a water bucket with a hole on the side. Once the water reaches the hole's level it will begin to pour out. Similarly, the diode will conduct current once the flooded electrons and holes reach an equilibrium point, where the number of electrons is equal to the number of holes in the intrinsic region. When the diode is forward biased, the injected carrier concentration is typically several orders of magnitudes higher than the intrinsic level carrier concentration. Due to this high level injection, which in turn is due to the depletion process, the electric field extends deeply (almost the entire length) into the region. This electric field helps in speeding up of the transport of charge carriers from p to n region, which results in faster operation of the diode, making it a suitable device for high frequency operations.

LED

Introduction:

LED is Light Emitting Diode and there are many types of LED. It is made of semiconductors which is related with Gallium and Arsenic. Some can emit light, and some can detect light, and some can even detect radiation. LED is used as light indicators which can be seen in almost every electronic product (such as TV remote controller, sign of computer screen, Num Lock light in keyboard, score board, etc). It works exactly the same as diode, but when electrons jump from n-type to p-type, they lose some energy which is converted to light. Because all the energy

RGCET / II CSE

12

is converted to light, LEDs do not consume much energy and can operate at low voltages. They should not be operated at more than 40 mA (miliampere) or 0.04 A (ampere) or 2.2 V (volts).

Led Symbols:

Characteristics and parameters:

LED Characteristics are similar to those of other semiconductor diodes, except that the typical forward voltage drop is 1.6V. Note also that the reverse breakdown voltage can be as low as 3V. In some circuits it reverse breakdown voltage can be as low as 3 V. In some circuits it is necessary to include a diode with a high reverse breakdown voltage in series with a LED. The forward current used with a LED is usually in the 10mA to 20mA range, but (depending on the particular device) the peak current can be as high as 90mA.LED luminous intensity depends on the forward current level; it is usually specified at 20mA.

Typical applications include:

Indicator lights: These can be two-state (i.e., on/off), bar-graph, or alphabetic-numeric readouts.

LCD panel backlighting: Specialized white LEDs are used in flat-panel computer displays.

Fiber optic data transmission: Ease of modulation allows wide communications bandwidth with minimal noise, resulting in high speed and accuracy.

Remote control: Most home-entertainment "remotes" use IREDs to transmit data to the main unit.

optoisolator: Stages in an electronic system can be connected together without unwanted interaction.

LCD

LCDs - Liquid Crystal Displays:

A liquid crystal display (LCD) is a thin, flat panel used for electronically displaying information such as text, images, and moving pictures.

Its uses include monitors for computers, televisions, instrument panels, and other devices ranging from aircraft cockpit displays, to every-day consumer devices such as video players, gaming devices, clocks, watches, calculators, and telephones.

Among its major features are its lightweight construction, its portability, and its ability to be produced in much larger screen sizes than are practical for the construction of cathode ray tube (CRT) display technology.

RGCET / II CSE

13

Its low electrical power consumption enables it to be used in battery-powered electronic equipment. It is an electronically-modulated optical device made up of any number of pixels filled with liquid crystals and arrayed in front of a light source (backlight) or reflector to produce images in color or monochrome.

The earliest discovery leading to the development of LCD technology, the discovery of liquid crystals, dates from 1888. By 2008, worldwide sales of televisions with LCD screens had surpassed the sale of CRT units.

THE PHOTODIODE.

Photo-diode

The construction of the Photodiode light sensor is similar to that of a conventional PN-junction diode except that the diodes outer casing is either transparent or has a clear lens to focus the light onto the PN junction for increased sensitivity. The junction will respond to light particularly longer wavelengths such as red and infra-red rather than visible light.

This characteristic can be a problem for diodes with transparent or glass bead bodies such as the 1N4148 signal diode. LED‟s can also be used as photodiodes as they can both emit and detect light from their junction. All PN-junctions are light sensitive and can be used in a photo-conductive unbiased voltage mode with the PN-junction of the photodiode always “Reverse Biased” so that only the diodes leakage or dark current can flow.

The current-voltage characteristic (I/V Curves) of a photodiode with no light on its junction (dark mode) is very similar to a normal signal or rectifying diode. When the photodiode is forward biased, there is an exponential increase in the current, the same as for a normal diode. When a reverse bias is applied, a small reverse saturation current appears which causes an increase of the depletion region, which is the sensitive part of the junction. Photodiodes can also be connected in a current mode using a fixed bias voltage across the junction. The current mode is very linear over a wide range.

RGCET / II CSE

14

Photo-diode Construction and Characteristics

When used as a light sensor, a photodiodes dark current (0 lux) is about 10uA for geranium and 1uA for silicon type diodes. When light falls upon the junction more hole/electron pairs are formed and the leakage current increases. This leakage current increases as the illumination of the junction increases.

Thus, the photodiodes current is directly proportional to light intensity falling onto the PN-junction. One main advantage of photodiodes when used as light sensors is their fast response to changes in the light levels, but one disadvantage of this type of photodevice is the relatively small current flow even when fully lit.

The following circuit shows a photo-current-to-voltage converter circuit using an operational amplifier as the amplifying device. The output voltage (Vout) is given as Vout = Ip × Rf and which is proportional to the light intensity characteristics of the photodiode.

This type of circuit also utilizes the characteristics of an operational amplifier with two input terminals at about zero voltage to operate the photodiode without bias. This zero-bias op-amp configuration gives a high impedance loading to the photodiode resulting in less influence by dark current and a wider linear range of the photocurrent relative to the radiant light intensity. Capacitor Cf is used to prevent oscillation or gain peaking and to set the output bandwidth (1/2πRC).

RGCET / II CSE

15

Photo Diode

A photodiode is a type of photodetector capable of converting light into either current or voltage, depending upon the mode of operation. Photodiodes are similar to regular semiconductor diodes except that they may be either exposed (to detect vacuum UV or X-rays) or packaged with a window or optical fiber connection to allow light to reach the sensitive part of the device. Many diodes designed for use specifically as a photodiode will also use a PIN junction rather than the typical PN junction.

Photodiodes are basically diodes which are used in reverse bias and they are turned ON when a light intensity above the threshold level is incident on it. It has only two possible levels of outputs, either ON or OFF because of which it can only differentiate between two different intensities of light. It is suitable for applications where detecting a single light threshold is necessary. For example, if you are using this to make a Shadow Counter type of circuit, then this sensor will be most suitable.

Properties:

Quicker response time

Low cost Temperature sensitive Digital in nature Unidirectional

Principle of operation

A photodiode is a p–n junction or PIN structure. When a photon of sufficient energy

strikes the diode, it creates an electron-hole pair. This mechanism is also known as the inner

photoelectric effect. If the absorption occurs in the junction's depletion region, or one diffusion

length away from it, these carriers are swept from the junction by the built-in electric field of the

depletion region. Thus holes move toward the anode, and electrons toward the cathode, and a

photocurrent is produced. The total current through the photodiode is the sum of the dark

current (current that is generated in the absence of light) and the photocurrent, so the dark

current must be minimized to maximize the sensitivity of the device

Photovoltaic mode

When used in zero bias or photovoltaic mode, the flow of photocurrent out of the device is restricted and a voltage builds up. This mode exploits the photovoltaic effect, which is the basis for solar cells – a traditional solar cell is just a large area photodiode.

Photoconductive mode

In this mode the diode is often reverse biased (with the cathode driven positive with respect to the anode). This reduces the response time because the additional reverse bias increases the width of the depletion layer, which decreases the junction's capacitance. The reverse bias also

RGCET / II CSE

16

increases the dark current without much change in the photocurrent. For a given spectral distribution, the photocurrent is linearly proportional to the illuminance (and to the irradiance).[3]

Although this mode is faster, the photoconductive mode tends to exhibit more electronic noise. [4] The leakage current of a good PIN diode is so low (<1 nA) that the Johnson–Nyquist noise of the load resistance in a typical circuit often dominates.

Other modes of operation

Avalanche photodiodes have a similar structure to regular photodiodes, but they are operated with much higher reverse bias. This allows each photo-generated carrier to be multiplied by avalanche breakdown, resulting in internal gain within the photodiode, which increases the effective responsivity of the device.

SYMBOL

Applications

P–n photodiodes are used in similar applications to other photodetectors, such as photoconductors, charge-coupled devices, and photomultiplier tubes. They may be used to generate an output which is dependent upon the illumination (analog; for measurement and the like), or to change the state of circuitry (digital; either for control and switching, or digital signal processing).

Photodiodes are used in consumer electronics devices such as compact disc players, smoke detectors, and the receivers for infrared remote control devices used to control equipment from televisions to air conditioners. For many applications either photodiodes or photoconductors

RGCET / II CSE

17

may be used. Either type of photosensor may be used for light measurement, as in camera light meters, or to respond to light levels, as in switching on street lighting after dark.

Photosensors of all types may be used to respond to incident light, or to a source of light which is part of the same circuit or system. A photodiode is often combined into a single component with an emitter of light, usually a light-emitting diode (LED), either to detect the presence of a mechanical obstruction to the beam (slotted optical switch), or to couple two digital or analog circuits while maintaining extremely high electrical isolation between them, often for safety (optocoupler).

Photodiodes are often used for accurate measurement of light intensity in science and industry. They generally have a more linear response than photoconductors.

They are also widely used in various medical applications, such as detectors for computed tomography (coupled with scintillators), instruments to analyze samples (immunoassay), and pulse oximeters.

PIN diodes are much faster and more sensitive than p–n junction diodes, and hence are often used for optical communications and in lighting regulation.

P–n photodiodes are not used to measure extremely low light intensities. Instead, if high sensitivity is needed, avalanche photodiodes, intensified charge-coupled devices or photomultiplier tubes are used for applications such as astronomy, spectroscopy, night vision equipment and laser rangefinding.

Pinned photodiode is not a PIN photodiode, it has p+/n/p regions in it. It has a shallow P+ implant in N type diffusion layer over a P-type epitaxial substrate layer. It is used in CMOS Active pixel sensor.

APD diodes

Avalanche photodiode:

An avalanche photodiode (APD) is a highly sensitive semiconductor electronic device that exploits the photoelectric effect to convert light to electricity. APDs can be thought of as photodetectors that provide a built-in first stage of gain through avalanche multiplication. From a functional standpoint, they can be regarded as the semiconductor analog to photomultipliers. By applying a high reverse bias voltage (typically 100-200 V in silicon), APDs show an internal current gain effect (around 100) due to impact ionization (avalanche effect). However, some silicon APDs employ alternative doping and beveling techniques compared to traditional APDs that allow greater voltage to be applied (> 1500 V) before breakdown is reached and hence a greater operating gain (> 1000). In general, the higher the reverse voltage the higher the gain. Among the various expressions for the APD multiplication factor (M), an instructive expression is given by the formula

RGCET / II CSE

18

where L is the space charge boundary for electrons and α is the multiplication coefficient for electrons (and holes). This coefficient has a strong dependence on the applied electric field strength, temperature, and doping profile. Since APD gain varies strongly with the applied reverse bias and temperature, it is necessary to control the reverse voltage to keep a stable gain. Avalanche photodiodes therefore are more sensitive compared to other semiconductor photodiodes.

If very high gain is needed (105 to 106), certain APDs can be operated with a reverse voltage above the APD's breakdown voltage. In this case, the APD needs to have its signal current limited and quickly diminished. Active and passive current quenching techniques have been used for this purpose. APDs that operate in this high-gain regime are in Geiger mode. This mode is particularly useful for single photon detection provided that the dark count event rate is sufficiently low.

A typical application for APDs is laser rangefinders and long range fiber optic telecommunication. New applications include positron emission tomography and particle physics.APD arrays are becoming commercially available.

APD applicability and usefulness depends on many parameters. Two of the larger factors are: quantum efficiency, which indicates how well incident optical photons are absorbed and then used to generate primary charge carriers; and total leakage current, which is the sum of the dark current and photocurrent and noise. Electronic dark noise components are series and parallel noise. Series noise, which is the effect of shot noise, is basically proportional to the APD capacitance while the parallel noise is associated with the fluctuations of the APD bulk and surface dark currents. Another noise source is the excess noise factor, F. It describes the statistical noise that is inherent with the stochastic APD multiplication process.

RGCET / II CSE

19

UNI-JUNCTION TRANSISTOR

The UJT as the name implies, is characterized by a single pn junction. It exhibits negative resistance characteristic that makes it useful in oscillator circuits.

The symbol for UJT is shown in fig.i. The UJT is having three terminals base1 (B1), base2 (B2) and emitter (E). The UJT is made up of an N-type silicon bar which acts as the base as shown in fig. ii. It is very lightly doped. A P-type impurity is introduced into the base, producing a single PN junction called emitter. The PN junction exhibits the properties of a conventional diode.

Fig. i

Fig .ii

A complementary UJT is formed by a P-type base and N-type emitter. Except for the polarity of voltage and current the characteristic is similar to those of a conventional UJT.

A simplified equivalent circuit for the UJT is shown in fig.iii . VBB is a source of biasing voltage connected between B2 and B1. When the emitter is open, the total resistance from B2 to B1 is simply the resistance of the silicon bar, this is known as the inter base resistance RBB. Since the N-channel is lightly doped, therefore RBB is relatively high, typically 5 to 10K ohm. RB2 is the resistance between B2 and point „a', while RB1 is the resistance from point „a' to B1, therefore the interbase resistance RBB is

RBB = RB1 + RB2

RGCET / II CSE

20

Fig.iii

The diode accounts for the rectifying properties of the PN junction. VD is the diode's threshold voltage. With the emitter open, IE = 0, and I1 = I 2 . The interbase current is given by

I1 = I2 = VBB / R BB .

Part of VBB is dropped across RB2 while the rest of voltage is dropped across RB1. The voltage across RB1 is

Va = VBB * (RB1 ) / (RB1 + RB2 )

The ratio RB1 / (RB1 + RB2 ) is called intrinsic standoff ratio

h = RB1 / (RB1 + RB2 ) i.e. Va = h VBB .

The ratio h is a property of UJT and it is always less than one and usually between 0.4 and 0.85. As long as IB = 0, the circuit of behaves as a voltage divider.

Assume now that vE is gradually increased from zero using an emitter supply VEE . The diode remains reverse biased till vE voltage is less than h VBB and no emitter current flows except leakage current. The emitter diode will be reversed biased.

When vE = VD + h VBB, then appreciable emitter current begins to flow where VD is the diode's threshold voltage. The value of vE that causes, the diode to start conducting is called the peak point voltage and the current is called peak point current IP.

VP = VD + h VBB.

RGCET / II CSE

21

The graph of fig. iv shows the relationship between the emitter voltage and current. vE is plotted on the vertical axis and IE is plotted on the horizontal axis. The region from vE = 0 to vE = VP is called cut off region because no emitter current flows (except for leakage). Once vE exceeds the peak point voltage, IE increases, but v E decreases. up to certain point called valley point (VV and IV). This is called negative resistance region. Beyond this, IE increases with vE this is the saturation region, which exhibits a positive resistance characteristic.

The physical process responsible for the negative resistance characteristic is called conductivity modulation. When the vE exceeds VP voltage, holes from P emitter are injected into N base. Since the P region is heavily doped compared with the N-region, holes are injected to the lower half of the UJT.

Fig. iv

The lightly doped N region gives these holes a long lifetime. These holes move towards B1 to complete their path by re-entering at the negative terminal of VEE. The large holes create a conducting path between the emitter and the lower base. These increased charge carriers represent a decrease in resistance RB1, therefore can be considered as variable resistance. It decreases up to 50 ohm.

Since h is a function of RB1 it follows that the reduction of RB1 causes a corresponding reduction in intrinsic standoff ratio. Thus as IE increases, RB1 decreases, h decreases, and Va decreases. The decrease in V a causes more emitter current to flow which causes further reduction in RB1, h, and Va. This process is regenerative and therefore Va as well as vE quickly drops while IE increases. Although RB decreases in value, but it is always positive resistance. It is only the dynamic resistance between VV and VP. At point B, the entire base1 region will saturate with carriers and resistance RB1 will not decrease any more. A further increase in Ie will be followed by a voltage rise.

The diode threshold voltage decreases with temperature and RBB resistance increases with temperature because Si has positive temperature coefficient.

SILICON-CONTROLLED RECTIFIER

A Silicon-Controlled Rectifier (SCR) is a four-layer (p-n-p-n) semiconductor device that doesn't allow current to flow until it is triggered and, once triggered, will only allow the flow of current in one direction. It has three terminals: 1) an input control terminal referred to as a 'gate'; 2) an output terminal known as the 'anode'; and 3) a terminal known as a 'cathode', which is common to both the gate and the anode.

CONSTRUCTION OF AN SCR

RGCET / II CSE

22

SCR - construction types

From fig a it is clear that SCR is essentially an ordinary rectifier (PN) and a junction transistor (N-P-N) combined in one unit to form PNPN device. Three terminals are taken: one from the outer P-type material, known as anode, second from the outer N-type material, known as cathode and the third from the base of transistor section known as the gate.

The basic material used for fabrication of an SCR is N-type silicon. It has a specific resistance of about 6 ohm-mm. Silicon is the natural choice as base material because of the following advantages

(i) ability to withstand high junction temperature of the order of 150° C

(ii) high thermal conductivity;

(iii) less variations in characteristics with temperature; and

(iv) less leakage current in P-N junction.

It consists, essentially, of a four layer pellet of P and N type silicon semiconductor materials. The junctions are diffused or alloyed. The material which may be used for P diffusion is aluminium and for N diffusion is phosphorous. The contact with anode can be made with an aluminium foil and through cathode and gate by metal sheet. Diffusion must be carried out at a proper temperature and for necessary duration to provide correct concentration because this decides the properties of the device. Low power SCRs employ the planar construction shown in fig a. Planar construction is useful for making a number of units from a silicon wafer. Here, all the junctions are diffused. The other technique is the mesa construction shown in fig.b. This technique is used for high power SCRs. In this technique, the inner junction J2 is obtained by diffusion, and then the outer two layers are alloyed to it. The PNPN pellet is properly braced with tungsten or molybdenum plates to provide greater mechanical strength and make it capable of handling large currents. One of these plates is hard soldered to a copper or an aluminium stud, which is threaded for attachment to a heat sink. This provides an efficient thermal path for conducting the internal losses to the surrounding medium. The uses of hard solder between the pellet and back-up plates minimises thermal fatigue, when the SCRs are subjected to temperature induced stresses. For medium and low power SCRs, the pellet is mounted directly on the copper stud or casing, using a soft solder which absorbs the thermal stresses set up by differential expansion and provides a good thermal path for heat transfer. For a larger cooling arrangement, which is required for high power SCRs, the press-pack or hockey-puck construction is employed, which provides for double-sided air for cooling.

RGCET / II CSE

23

Principle of Operation

The SCR is a four-layer, three-junction and a three-terminal device and is shown in fig.a. The end P-region is the anode, the end N-region is the cathode and the inner P-region is the gate. The anode to cathode is connected in series with the load circuit. Essentially the device is a switch. Ideally it remains off (voltage blocking state), or appears to have an infinite impedance until both the anode and gate terminals have suitable positive voltages with respect to the cathode terminal. The thyristor then switches on and current flows and continues to conduct without further gate signals. Ideally the thyristor has zero impedance in conduction state. For switching off or reverting to the blocking state, there must be no gate signal and the anode current must be reduced to zero. Current can flow only in one direction.

In absence of external bias voltages, the majority carrier in each layer diffuses until there is a built-in voltage that retards further diffusion. Some majority carriers have enough energy to cross the barrier caused by the retarding electric field at each junction. These carriers then become minority carriers and can recombine with majority carriers. Minority carriers in each layer can be accelerated across each junction by the fixed field, but because of absence of external circuit in this case the sum of majority and minority carrier currents must be zero.

A voltage bias, as shown in figure, and an external circuit to carry current allow internal currents which include the following terms:

The current Ix is due to

Majority carriers (holes) crossing junction J1 Minority carriers crossing junction J1 Holes injected at junction J2 diffusing through the N-region and crossing junction J1 and Minority carriers from junction J2 diffusing through the N-region and crossing junction J1.

Similarly I2 is due to six terms and I3 is due to four terms.

RGCET / II CSE

24

SCR Characteristics

As already mentioned, the SCR is a four-layer device with three terminals, namely, the anode, the cathode and the gate. When the anode is made positive with respect to the cathode, junctions J1 and J3 are forward biased and junction J2 is reverse-biased and only the leakage current will flow through the device. The SCR is then said to be in the forward blocking state or in the forward mode or off state. But when the cathode is made positive with respect to the anode, junctions J1 and J3 are reverse-biased, a small reverse leakage current will flow through the SCR and the SGR is said to be in the reverse blocking state or in reverse mode.

When the anode is positive with respect to cathode i.e. when the SCR is in forward mode, the SCR does not conduct unless the forward voltage exceeds certain value, called the forward breakover voltage, VFB0. In non-conducting state, the current through the SCR is the leakage current which is very small and is negligible. If a positive gate current is supplied, the SCR can become conducting at a voltage much lesser than forward break-over voltage. The larger the gate current, lower the break-over voltage. With sufficiently large gate current, the SCR behaves identical to PN rectifier. Once the SCR is switched on, the forward voltage drop across it is suddenly reduced to very small value, say about 1 volt. In the conducting or on-state, the current through the SCR is limited by the external impedance.

RGCET / II CSE

25

When the anode is negative with respect to cathode, that is when the SCR is in reverse mode or in blocking state no current flows through the SCR except very small leakage current of the order of few micro-amperes. But if the reverse voltage is increased beyond a certain value, called the reverse break-over voltage, VRB0 avalanche break down takes place. Forward break-over voltage VFB0 is usually higher than reverse breakover voltage,VRBO.

From the foregoing discussion, it can be seen that the SCR has two stable and reversible operating states. The change over from off-state to on-state, called turn-on, can be achieved by increasing the forward voltage beyond VFB0. A more convenient and useful method of turn-on the device employs the gate drive. If the forward voltage is less than the forward break-over voltage, VFB0, it can be turned-on by applying a positive voltage between the gate and the cathode. This method is called the gate control. Another very important feature of the gate is that once the SCR is triggered to on-state the gate loses its control.

The switching action of gate takes place only when

(i) SCR is forward biased i.e. anode is positive with respect to cathode, and

(ii) Suitable positive voltage is applied between the gate and the cathode.

Once the SCR has been switched on, it has no control on the amount of current flowing through it. The current through the SCR is entirely controlled by the external impedance connected in the circuit and the applied voltage. There is, however, a very small, about 1 V, potential drop across the SCR. The forward current through the SCR can be reduced by reducing the applied voltage or by increasing the circuit impedance. There is, however, a minimum forward current that must be maintained to keep the SCR in conducting state. This is called the holding current rating of SCR. If the current through the SCR is reduced below the level of holding current, the device returns to off-state or blocking state.

The SCR can be switched off by reducing the forward current below the level of holding current which may be done either by reducing the applied voltage or by increasing the circuit impedance.

Note : The gate can only trigger or switch-on the SCR, it cannot switch off.

Alternatively the SCR can be switched off by applying negative voltage to the anode (reverse mode), the SCR naturally will be switched off.

Here one point is worth mentioning, the SCR takes certain time to switch off. The time, called the turn-off time, must be allowed before forward voltage may be applied again otherwise the device will switch-on with forward voltage without any gate pulse. The turn-off time is about 15 micro-seconds, which is immaterial when dealing with power frequency, but this becomes important in the inverter circuits, which are to operate at high frequency.

Applications:

The six applications of SCR like power control, switching, zero-voltage switching, over-voltage protection, pulse circuits and battery charging regulator.

RGCET / II CSE

26

DIAC DIAC (Diode for Alternating Current)

Symbol

Construction:

The diac is basically a two terminal parellel-inverse combination of semiconductor layers that permits triggering in either direction. The basic arrangement of the semiconductor layers of the diac is shown in the figure, along with its graphical symbol. Nore that either terminal is referred as the cathode. Instead, there is an anode 1 and an anode 2. When the anode 1 is positive with respect to anode 2, the semiconductor

Operation:

Diac circuits use the fact that a diac only conducts current only after a certain breakdown voltage has been exceeded. The actual breakdown voltage will depend upon the specification for the particular component type. When the diac breakdown voltage occurs, the resistance of the component decreases abruptly and this leads to a sharp decrease in the voltage drop across the diac, and a corresponding increase in current. The diac will remain in its conducing state until the current flow through it drops below a particular value known as the holding current. When the current falls below the holding current, the diac switches back to its high resistance, or non-conducting state. Diacs are widely used in AC applications and it is found that the device is "reset" to its non-conducting state, each time the voltage on the cycle falls so that the current falls below the holding current. As the behaviour of the device is approximately equal in both directions, it can provide a method of providing equal switching for both halves of an AC cycle, e.g for triacs. Most diacs have a breakdown voltage of around 30 volts, although the exact specifications will depend upon the particular type of device.. Interestingly their behaviour is somewhat similar to that of a neon lamp, although they offer a far more precise switch on voltage and thereby provide a far better degree of switching equalization

RGCET / II CSE

27

TRIAC (Triode for Alternating Current)

The triac is a three terminal semiconductor device for controlling current. It is effectively a development of the SCR or thyristor, but unlike the thyristor which is only able to conduct in one direction, the triac is a biriectional device. As such the triac is an ideal device to use for AC switching applications because it can control the current flow over both halves of an alternating cycle. A thyristor is only able to control them over one half of a cycle. During the remaining half no conduction occurs and accordingly only half the waveform can be utilised.

There are three terminal on a triac. These are the Gate and two other terminals. These other triac terminals are often referred to as an "Anode" or "Main Terminal"

TRIAC circuit symbol:

RGCET / II CSE

28

On the triac, the gate that acts as the trigger to turn the device on. The current then flows betweent he two anodes or main terminals. These are usually designated Anode 1 and Anode 2 or Main Terminal 1 and Main Terminal 2 (MT1 and MT2).

It can be imagined from the circuit symbol that the triac consists of two thyristors back to back. The operation of the triac can be looked on in this fashion, although the actual operation at the semiconductor level is rather complicated. When the voltage on the MT1 is positive with regard to MT2 and a positive gate voltage is applied, one of the SCRs conducts. When the voltage is reversed and a negative voltage is applied to the gate, the other SCR conducts. This is provided that there is sufficient voltage across the device to enable a minimum holding current to flow.

TRIAC OPERATION

The structure of a triac may be considered as a p-n-p-n structure and the triac may be considered to consist of two conventional SCRs fabricated in an inverse parallel configuration.

In operation, when terminal A2 is positive with respect to A1, then a positive gate voltage will give rise to a current that will trigger the part of the triac consisting of p1 n1 p2 n2 and it will have an identical characteristic to an SCR. When terminal A2 is negative with respect to A1 a negative current will trigger the part of the triac consisting of p2 n1 p1 n3. In this way conduction on the triac occurs over both halves an alternating cycle.

RGCET / II CSE

29

TRIAC structure

Triacs do not fire symmetrically as a result of slight differences between the two halves of the device. This results in harmonics being generated, and the less symmetrical the triac fires, the greater the level of harmonics produced. It is generally undesirable to have high levels of harmonics in a power system and as a result triacs are not favoured for high power systems. Instead two thyristors may be used as it is easier to control their firing.

To help in overcoming this problem, a device known as a diac (diode AC switch) is often placed in series with the gate. This device helps make the switching more even for both halves of the cycle. This results from the fact that the diac switching characteristic is far more even than that of the triac. Since the diac prevents any gate current flowing until the trigger voltage has reached a certain voltage in either direction, this makes the firing point of the triac more even in both directions.

Characteristics of TRIAC

Typical V-I characteristics of a triac are shown in figure. The triac has on and off state characteristics similar to SCR but now the char acteristic is applicable to both positive and negative voltages. This is expected because triac consists of two SCRs connected in parallel but opposite in directions.

RGCET / II CSE

30

MT2 is positive with respect to MTX in the first quadrant and it is negative in the third quad rant. As already said in previous blog posts, the gate triggering may occur in any of the following four modes.

Quadrant I operation : VMT2, positive; VG1 positive Quadrant II operation : VMT21 positive; VGl negative Quadrant III operation : VMT21 negative; VGl negative Quadrant IV operation : VMT21 negative; VG1 positive

Where VMT21 and VGl are the voltages of terminal MT2 and gate with respect to terminal MT1.

The device, when starts conduction permits a very heavy amount of current to flow through it. This large inrush of current must be restricted by employing external resist ance, otherwise the device may get damaged.

The gate is the control terminal of the device. By applying proper signal to the gate, the firing angle of the device can be controlled.

The circuits used in the gate for triggering the device are called the gate-triggering circuits. The gate-triggering circuits for the triac are almost same like those used for SCRs.

These triggering circuits usually generate trigger pulses for firing the device. The trigger pulse should be of sufficient magnitude and duration so that firing of the device is assured. Usually, a duration of 35 us is sufficient for sustaining the firing of the device.

RGCET / II CSE

31

Application of TRIAC

Low power TRIACs are used in many applications such as light dimmers, speed controls for electric fans and other electric motors, and in the modern computerized control circuits of many household small and major appliances.

However, when used with inductive loads such as electric fans, care must be taken to assure that the TRIAC will turn off correctly at the end of each half-cycle of the AC power. Indeed, TRIACs can be very sensitive to high values of dv/dt between A1 and A2, so a phase shift between current and voltage (as in the case of an inductive load) leads to sudden voltage step which can make the device turn on in an unwanted manner.

Unwanted turn-ons can be avoided by using a snubber circuit (usually of the RC or RCL type) between A1 and A2. Snubber circuits are also used to prevent premature triggering, caused for example by voltage spikes in the mains supply.

Because turn-ons are caused by internal capacitive currents flowing into the gate as a consequence of a high voltage dv/dt, a gate resistor or capacitor (or both in parallel) may be connected between gate and A1 to provide a low-impedance path to A1 and further prevent false triggering. This, however, increases the required trigger current or adds latency due to capacitor charging. On the other hand, a resistor between the gate and A1 helps dragging leakage currents out of the device, thus improving the performance of the TRIAC at high temperature, where the maximum allowed dv/dt is lower. Values of resistors less than 1kΩ and capacitors of 100nF are generally suitable for this purpose, although the fine-tuning should be done on the particular device model.

For higher-powered, more-demanding loads, two SCRs in inverse parallel may be used instead of one TRIAC. Because each SCR will have an entire half-cycle of reverse polarity voltage applied to it, turn-off of the SCRs is assured, no matter what the character of the load. However, due to the separate gates, proper triggering of the SCRs is more complex than triggering a TRIAC.

In addition to commutation, a TRIAC may also not turn on reliably with non-resistive loads if the phase shift of the current prevents achieving holding current at trigger time. To overcome that, pulse trains may be used to repeatedly try to trigger the TRIAC until it finally turns on. The advantage is that the gate current does not need to be maintained throughout the entire conduction angle, which can be beneficial when there is only limited drive capability available.