SOLID-STATE DEVICE A SOLID-STATE DEVICE is an electronic device, which operates by virtue of the movement of electrons within a solid piece of semiconductor material. Eg – Diodes and Transistors.

DIODES Diodes are made up of Semiconductors.

The energy levels of a semiconductor can be modified so that a material (e.g. silicon or germanium) that is normally an insulator will conduct electricity. Energy level structure of a semiconductor is quit complicated, requires a quantum mechanical treatment.

Doping is a process where impurities are added to the semiconductor to lower its resistivity to make them conduct current. For example, Silicon has 4 electrons in its valence level. We add atoms which have a different number of valence shell electrons (3 or 5) to a piece of silicon. 5 valence electrons: Phosphorous, Arsenic, Antimony 3 valence electrons: Boron, Aluminium, Indium

• N type silicon: Adding atoms which have 5 valence electrons makes the silicon more negative. The majority carriers are the excess electrons.

• P type silicon Adding atoms which have 3 valence electrons makes the silicon more positive. The majority carriers are "holes". A hole is the lack of an electron in the valence shell.

Put a piece of N type silicon next to a piece of P type silicon to make a PN diode.



Unbiased diode

Forward biased diode (Junction diode)

Diode is forward biased when V

anode

.

Characteristics of a forward biased Diode: -

 Diode conducts current strongly.  Voltage drop across diode is (almost) independent of diode current.  Effective resistance (impedance) of diode is small.

Reversed biased diode

> V

cathode



< Diode is reverse biased when V

anode

V

cathode

.

Characteristics of a reverse biased diode:-

 Diode conducts current very weakly (typically < A).  Diode current is (almost) independent of voltage, until breakdown.  Effective resistance (impedance) of diode is very large.

Current-voltage relationship for a diode can be expressed as: I I

s

(e

eV/ kT

1) This is known as: "diode", "rectifier", or "Ebers-Moll" equation , where Is = reverse saturation current (typically < A) k = Boltzmann's constant, e = electron charge, T = temperature At room temperature, kT/e = 25.3 mV,

Types of diodes:-

Zener Diode A zener diode is a special type of diode that is designed to operate in the reverse breakdown region. An ordinary diode operated in this region will usually be destroyed due to excessive current.

This is not the case for the zener diode. A zener diode is heavily doped to reduce the reverse breakdown voltage. This causes a very thin depletion layer. As a result, a zener diode has a sharp reverse breakdown voltage VZ. This is clear from the reverse characteristic of zener diode shown in above figure. Note that the reverse characteristic drops in an almost vertical manner at reverse voltage VZ. As the curve reveals, two things happen when VZ is reached: (i) The diode current increases rapidly.



(ii) The reverse voltage VZ across the diode remains almost constant.

In other words, the zener diode operated in this region will have a relatively constant voltage across it, regardless of the value of current through the device. This permits the zener diode to be used as a voltage regulator.

Light-Emitting Diode (LED)

A light-emitting diode (LED) is a diode that gives off visible light when forward biased. Light-emitting diodes are not made from silicon or germanium but are made by using elements like gallium, phosphorus and arsenic. By varying the quantities of these elements, it is possible to produce light of different wavelengths with colours that include red, green, yellow and blue. For example, when a LED is manufactured using gallium arsenide, it will produce a red light. If the LED is made with gallium phosphide, it will produce a green light.

Theory:-

When light-emitting diode (LED) is forward biased as shown in figure below, the electrons from the n-type material cross the pn junction and recombine with holes in the p-type material. Recall that these free electrons are in the conduction band and at a higher energy level than the holes in the valence band. When recombination takes place, the recombining electrons release energy in the form of heat and light. In germanium and silicon diodes, almost the entire energy is given up in the form of heat and emitted light is insignificant. However, in materials like gallium arsenide, the number of photons of light energy is sufficient to produce quite intense visible light.

It is clear from the graph below that the intensity of radiated light is directly proportional to the forward current of LED.



Photo-diode

A photo-diode is a reverse-biased silicon or germanium pn junction in which reverse current increases when the junction is exposed to light. The reverse current in a photo-diode is directly proportional to the intensity of light falling on its pn junction. This means that greater the intensity of light falling on the pn junction of photo-diode, the greater will be the reverse current.

Principle:-

When a rectifier diode is reverse biased, it has a very small reverse leakage current. The same is true for a photo-diode. The reverse current is produced by thermally generated electron hole pairs which are swept across the junction by the electric field created by the reverse voltage. In a rectifier diode, the reverse current increases with temperature due to an increase in the number of electron-hole pairs. A photo-diode differs from a rectifier diode in that when its pn junction is exposed to light, the reverse current increases with the increase in light intensity and vice-versa. This is explained as follows. When light (photons) falls on the pn junction, the energy is imparted by the photons to the atoms in the junction. This will create more free electrons (and more holes). These additional free electrons will increase the reverse current. As the intensity of light incident on the pn junction increases, the reverse current also increases. In other words, as the incident light intensity increases, the resistance of the device (photo-diode) decreases.

Applications of Photo-diodes

There are a large number of applications of photodiodes. However, we shall give two applications of photodiodes for example.

1. Alarm circuit using photo-diode.

The use of photo-diode in an alarm system. Light from a light source is allowed to fall on a photo-diode fitted in the doorway. The reverse current IR will continue to flow so long as the light beam is not broken. If a person passes through the door, light beam is broken and the reverse current drops to the dark current level. As a result, an alarm is sounded.

2. Counter circuit using photo-diode.

A photodiode may be used to count items on a conveyor belt. Below figure shows a photo-diode circuit used in a system that counts objects as they pass by on a conveyor. In this circuit, a source of light sends a concentrated beam of light across a conveyor to a photo-diode. As the object passes, the light beam is broken, IR drops to the dark current level and the count is increased by one.

Tunnel Diode

A tunnel diode is a pn junction that exhibits negative resistance between two values of forward voltage (i.e., between peak-point voltage and valley-point voltage). A conventional diode exhibits *positive resistance when it is forward biased or reverse biased. However, if a semiconductor junction diode is heavily doped with impurities, it exhibits negative resistance (i.e. current



decreases as the voltage is increased) in certain regions in the forward direction. Such a diode is called tunnel diode.

Principle:-

The tunnel diode is basically a pn junction with heavy doping of p-type and n-type semiconductor materials. In fact, a tunnel diode is doped approximately 1000 times as heavily as a conventional diode. This heavy doping results in a large number of majority carriers. Because of the large number of carriers, most are not used during the initial recombination that produces the depletion layer. As a result, the depletion layer is very narrow. In comparison with conventional diode, the depletion layer of a tunnel diode is 100 times narrower. The operation of a tunnel diode depends upon the tunnelling effect and hence the name.

Tunnelling effect:-

The heavy doping provides a large number of majority carriers. Because of the large number of carriers, there is much drift activity in p and n sections. This causes many valence electrons to have their energy levels raised closer to the conduction region. Therefore, it takes only a very small applied forward voltage to cause conduction. The movement of valence electrons from the valence energy band to the conduction band with little or no applied forward voltage is called tunnelling. Valence electrons seem to tunnel through the forbidden energy band. As the forward voltage is first increased, the diode current rises rapidly due to tunnelling effect. Soon the tunnelling effect is reduced and current flow starts to decrease as the forward voltage across the diode is increased. The tunnel diode is said to have entered the negative resistance region. As the voltage is further increased, the tunnelling effect plays less and less part until a valley-point is reached. From now onwards, the tunnel diode behaves as ordinary diode i.e., diode current increases with the increase in forward voltage.

V-I Characteristic

Figure below shows the V-I characteristic of a typical tunnel diode.

(i) As the forward voltage across the tunnel diode is increased from zero, electrons from the N region “tunnel” through the potential barrier to the p-region. As the forward voltage increases, the diode current also increases until the peak-point P is reached. The diode current has now reached peak current IP (= 2.2 mA) at about peak-point voltage VP (= 0.07 V). Until now the diode has exhibited positive resistance.

(ii) As the voltage is increased beyond VP, the tunnelling action starts decreasing and the diode current decreases as the forward voltage is increased until valley-point V is reached at valley-point voltage VV (= 0.7V). In the region between peak-point and valley-point (i.e., between points P and V), the diode exhibits negative resistance i.e., as the forward bias is increased, the current decreases.This suggests that tunnel diode, when operated in the negative resistance region, can be used as an oscillator or a switch.

(iii) When forward bias is increased beyond valley-point voltage VV (= 0.7 V), the tunnel diode behaves as a normal diode. In other words, from point V onwards, the diode current increases with the increase in forward



voltage i.e., the diode exhibits positive resistance once again.It may be noted that a tunnel diode has a high reverse current but operation under this condition is not generally used.

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 below figure. A varactor diode is specially constructed to have high capacitance under reverse bias. Other figure shows the symbol of varactor diode. The values of capacitance of varactor diodes are in the Pico farad (10−12 F) range.

Shockley Diode

Named after its inventor, a Shockley diode is a PNPN device having two terminals as shown in figure. This device acts as a switch and consists of four alternate P-type and N-type layers in a single crystal. The various layers are labelled as P1, N1, P2 and N2 for identification. Since a P-region adjacent to an N-region may be considered a junction diode, the Shockley diode is equivalent to three junction diodes connected in series as shown in second Figure. The symbol of Shockley diode is shown in 3rd figure.

Working:-

(i) When Shockley diode is forward biased (i.e., anode is positive w.r.t. cathode), diodes D1 and D3 would be forward-biased while diode D2 would be reverse-biased. Since diode D2 offers very high resistance (being reverse biased) and the three diodes are in series, the Shockley diode presents a very high resistance. As the



forward voltage increases, the reverse bias across D2 is also increased. At some forward voltage (called break over voltage VBO), reverse breakdown of D2 occurs. Since this breakdown results in reduced resistance, the Shockley diode presents a very low resistance. From now onwards, the Shockley diode behaves as a conventional forward-biased diode; the forward current being determined by the applied voltage and external load resistance. This behaviour of Shockley diode is indicated on its V-I characteristic in below Figure.

ii) When Shockley diode is reverse biased (i.e., anode is negative w.r.t. cathode), diodes D1 and D3 would be reverse- biased while diode D2 would be forward-biased. If reverse voltage is increased sufficiently, the reverse voltage breakdown (point A in Fig) of Shockley diode is reached. At this point, diodes D1 and D3 would go into reverse-voltage breakdown, the reverse current flowing through them would rise rapidly and the heat produced by this current flow could ruin the entire device. For this reason, Shockley diode should never be operated with a reverse voltage sufficient to reach the reverse-voltage breakdown point.

Conclusion:- The above discussion reveals that Shockley diode behaves like a switch. So long as the forward voltage is less than break over voltage, Shockley diode offers very high resistance (i.e., switch is open) and practically conducts no current. At voltages above the break-over value, Shockley diode presents a very low resistance (i.e. switch is closed) and Shockley diode conducts heavily. It may be noted that Shockley diode is also known as PNPN diode or four layer diode or reverse blocking diode thyristor.

Note:- Once Shockley diode is turned ON (i.e., it starts conducting), the only way to turn it OFF is to reduce the applied voltage to such a value so that current flowing through Shockley diode drops below its holding current (IH) value. Diode D2 then comes out of its reverse-breakdown state and its high-resistance value is restored. This, in turn, causes the entire Shockley diode to revert to its high resistance (switch open) state.

RADAR

Full form of RADAR is Radio Detection and Ranging. Radar uses radio waves, which are a type of electromagnetic energy.

Principle:-

The basic principle of operation of primary radar is simple to understand. The implementation and operation of primary radar systems involve a wide range of disciplines such as building works, heavy mechanical and electrical engineering, high power microwave engineering, and advanced high speed signal and data processing techniques. Some laws of nature have a greater importance here. Radar measurement of range, or distance, is made possible because of the properties of radiated electromagnetic energy.

1. Reflection of electromagnetic waves :-

The electromagnetic waves are reflected if they meet an electrically leading surface. If these reflected waves are received again at the place of their origin, then that means an obstacle is in the propagation direction.



2. Electromagnetic energy travels through air at a constant speed, at approximately the speed of light, i.e.

300,000 kilometres per second. This constant speed allows the determination of the distance between the reflecting objects (airplanes, ships or cars) and the radar site by measuring the running time of the transmitted pulses.

This energy normally travels through space in astraight line,and will vary only slightly because of atmospheric and weather conditions. By using of special radar antennas this energy can be focused into a desired direction. Thus the direction (in azimuth and elevation of the reflecting objects can be measured) These principles can basically be implemented in a radar system, and allow the determination of the distance, the direction and the height of the reflecting object.

Radar has many advantages compared to an attempt of visual observation:

 Radar is able to operate day or night, in lightness or darkness over a long range;  Radar is able to operate in all weathers, in fog and rain, it can even penetrate walls or layers of snow;  Radar has very broad coverage; it is possible to observe the whole hemisphere;  Radar detects and tracks moving objects, a high resolution imaging is possible, that results in an object

recognition;  Radar can operate unmanned, 24 hours a day, 7 days a week.

SONAR

Sonar, an acronym for sound navigation and ranging, is a system that uses sound waves to detect and locate objects underwater.

Working Principle: The principle behind sonar is simple, a pulse of ultrasonic waves is sent into the water where it bounces off a target and comes back to the source (ultrasonic waves are pitched too high for humans to detect). The distance and location can be calculated by measuring the time it takes for the sound to return. By knowing the speed of sound in water, the distance is computed by multiplying the speed by one-half of the time travelled (for a one-way trip). This is active sonar ranging (echolocation).

Two types:

1. Active Sonar: Mode of echo location by sending a signal and detecting the returning echo. 2. Sensitive listening/Passive Sonar -only mode to detect the presence of objects making noise.

Most moving objects underwater make some kind of noise. Marine life, cavitation (small collapsing air pockets caused by propellers), hull popping of submarines changing depth, and engine vibration are all forms of underwater noise. In passive sonar ranging, no pulse signal is sent. Instead, the searcher listens for the characteristic sound of another boat or submarine. By doing so, the searcher can identify the target without revealing his own location. This method is most often used during wartime.

However, since a submarine is usually completely submerged, it must use active sonar at times, generally to navigate past obstacles. In doing so, the submarine risks alerting others of its presence. In such cases, the use of sonar has become a sophisticated military tactical exercise.

Sonar devices have become standard equipment for most commercial and many recreational ships. Fishing boats use active sonar to locate schools of fish. Other applications of sonar include searching for shipwrecks, probing harbours where visibility is poor, mapping the ocean floor, and helping submerged vessels navigate under the Arctic Ocean ice sheets.



LASER

Principles and Applications of Laser

Laser is the abbreviation of Light Amplification by the Stimulated Emission of Radiation. It is a device that creates a narrow and low-divergent beam [1] of coherent light, while most other light sources emit incoherent light, which has a phase that varies randomly with time and position. Most lasers emit nearly "monochromatic" light with a narrow wavelength spectrum.

Principle of Lasers

The principle of a laser is based on three separate features: a) stimulated emission within an amplifying medium, b) population inversion of electronics and c) an optical resonator.

Spontaneous Emission and Stimulated Emission According to the quantum mechanics, an electron within an atom can have only certain values of energy, or energy levels. There are many energy levels that an electron can occupy, but here we will only consider two. If an electron is in the excited state with the energy E2 it may spontaneously decay to the ground state, with energy E1, releasing the difference in energy between the two states as a photon. [2] (See Fig.2a) this process is called spontaneous emission, producing fluorescent light. The phase and direction of the photon in spontaneous emission are completely random due to Uncertainty Principle. Conversely, a photon with a particular frequency satisfying would be absorbed by an electron in the ground state. The electron remains in this excited state for a period of time typically less than 10-6 second. Then it returns to the lower state spontaneously by a photon or a phonon. These common processes of absorption and spontaneous emission cannot give rise to the amplification of light. The best that can be achieved is that for every photon absorbed, another is emitted.

Diagram of (a) spontaneous Emission; and (b) stimulated Emission

Alternatively, if the excited-state atom is perturbed by the electric field of a photon with frequency ω, it may release a second photon of the same frequency, in phase with the first photon. The atom will again decay into the ground state. This process is known as stimulated emission. The emitted photon is identical to the stimulating photon with the same frequency, polarization, and direction of propagation. And there is a fixed phase relationship between light radiated from different atoms. The photons, as a result, are totally coherent. This is the critical property that allows optical amplification to take place.

Population Inversion of the Gain Medium If the higher energy state has a greater population than the lower energy state, then the light in the system undergoes a net increase in intensity. And this is called population inversion.

Optical Resonator Although with a population inversion we have the ability to amplify a signal via stimulated emission, the overall single-pass gain is quite small, and most of the excited atoms in the population emit spontaneously