UNIT III-SPECIAL PURPOSE ELECTRONIC DEICES 1. Explain tunnel Diode operation with the help of energy band diagrams. TUNNEL DIODE: A tunnel diode or Esaki diode is a type of semiconductor diode which is capable of very fast operation, well into the microwave frequency region, by using quantum mechanical effects. Impurity concentration in normal diode is 1 part in 8 10 in Tunnel diode 1 part in 3 10.Normally a electron or hole must have energy greater than or equal to potential energy barrier, to move to other side of the barrier. For very thin barrier there is a large probability of electron penetrating through the barrier which is called as Tunneling:. 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. Thus the most important operating region for a tunnel diode is the negative resistance region. Reverse bias operation When used in the reverse direction they are called back diodes and can act as fast rectifiers with zero offset voltage and extreme linearity for power signals.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. ENERGY BAND STRUCTURE OF HIGHLY DOPED PN DIODE: GRIET-ECE G.Surekha Page 1
E E KT F C Similarly for P type E F = E +KT NC ln N for N type material If N C < N D, E F > E C D l n N N C A If N A > N C ; E F < E Fermi level lies in conduction band in N type as shown in Fig.5.3 (a). Fermi level lies in valence band in P type material. Fermi level is at same energy level on both sides. By reverse bailing barrier height increases as shown in fig.5.3 (b).fermi level on N side is lowered. Tunneling of electron from P to N side is the result. (From filled sates to empty states). If we increase the reverse bias, reverse current increases as shown in characteristics of (Fig.5.5) a. GRIET-ECE G.Surekha Page
Similarly for forward bias tunneling occurs from N to P type material as shown in Fig.5.4. Further increase in forward, the condition shown in 5.4(b)reached and maximum current follows (characteristics of Fig.5.5). Further increase will reduce the current as shown in fig.5.4(1) till a minimum current flows due to the condition shown in fig.5.4(d) section 3 of 5.5(a) Besides the above current due to tunneling normal diode current flows as shown in dotted lives in fig.5.5(a). Resultant is the graph shown in 5.5(b). The symbol and equivalent circuit is shown in the above fig5.6. Application: - 1. Used as oscillator at HF / UHF.. Ultra high speed switch. 3. Used as logic memory (storage) device. 4. Used as amplifier. Advantages Disadvantages 1) Low noise 1) Less voltage range of separation ) Ease of operation ) No isolation of input and output, 3) High speed as it is a two terminal device. 4) Low power.. Differences between Tunnel Diode and PN Diode. Impurity concentration in normal diode 1 part in 10 3 Impurity concentration in Tunnel diode 1 part in 10 Width of the junction barrio varies inversely as square root of impurity concentration GRIET-ECE G.Surekha Page 3 8
W B e A Where B is barrier potential, is the permitivity of material N A acceptor concentration. Width of junction for normal diode 5 microns 6 5 10 m Width of junction Tunnel Diode < 100 A 0 10-8 m Normally a electron or hole must have energy greater than or equal to potential energy barrier, to move to other side of the barrier. For very thin barrier there is a large probability of electron pert rating through the barrier which is called as Tunneling:. 3. Explain the principle and operation of varactor Diode. aractor or varicap diodes are used mainly in radio frequency (RF) circuits to be able to provide a capacitance that can be varied by changing a voltage in an electronics circuit. This can be used for tuning circuits including radio frequency oscillators and filters. Although both names: varactor and varicap diode are used, they are both the same form of diode. The name varactor meaning variable reactor, or reactance, and varicap meaning variable capacitance (vari-cap). Operation of a variable capacitor They key to understanding how a varactor or varicap diode works is to look at what a capacitor is and what can change the capacitance. As can be seen from the diagram below, a capacitor consists of two plates with an insulating dielectric between them.... the capacitance and the amount of charge that can be stored depends on the area of the plates and the distance between them... The capacitance of the capacitor is dependent upon the area of the plates - the larger the area the greater the capacitance, and also the distance between them - the greater the distance the smaller the level of capacitance. GRIET-ECE G.Surekha Page 4
A reverse biased diode has no current flowing between the P-type area and the N-type area. The N-type region and the P-type regions can conduct electricity, and can be considered to be the two plates, and the region between them - the depletion region is the insulating dielectric. This is exactly the same as the capacitor above. As with any diode, if the reverse bias is changed so does the size of the depletion region. If the reverse voltage on the varactor or varicap diode is increased, the depletion region of the diode increases and if the reverse voltage on varactor diode is decreased the depletion region narrows. Therefore by changing the reverse bias on the diode it is possible to change the capacitance. Change of varactor diode capacitance with reverse bias aractor or varicap circuit symbol The varactor diode or varicap diode is shown in circuit diagrams or schematics using a symbol that combines the diode and capacitor symbols. In this way it is obvious that it is being used as a varacor or varicap capacitor rather than a rectifying diode. Circuit symbol for a varactor diode / varicap diode When operated in a circuit, it is necessary to ensure the varactor diode remains reverse biased. This means that the cathode will be positive with respect to the anode, i.e. the cathode of the varactor will be more positive than the anode. aractor Diode or aractors are operated reverse biased so no current flows, but since the thickness of the depletion region varies with the applied bias voltage, the capacitance of the diode can be made to vary. Generally, the depletion region thickness is proportional to the square root of the applied voltage; and capacitance is inversely proportional to the depletion region thickness. GRIET-ECE G.Surekha Page 5
Applications: They are used in PLL, voltage controlled oscillators, harmonic generation, electronic tuning devices in tuners for television, mobiles, parametric amplification, AM radios, voltage-variable tuning, frequency multipliers, etc. 4. Describe the working principle of an SCR with -I characteristics and also explain two transistor analogy of an SCR. SILICON CONTROLLED RECTIFIER: Construction of SCR An SCR consists of four layers of alternating P and N type semiconductor materials. Silicon is used as the intrinsic semiconductor, to which the proper dopants are added. The junctions are either diffused or alloyed. The planar construction is used for low power SCRs (and all the junctions are diffused). The mesa type construction is used for high power SCRs. In this case, junction J is obtained by the diffusion method and then the outer two layers are alloyed to it, since the PNPN pellet is required to handle large currents. Construction Symbol GRIET-ECE G.Surekha Page 6
CHARACTERISTICS OF SCR. 1) SCR is a three terminal four layer semiconductor device. ) Leakage current is very small for SCR compared with germanium. 3) SCR acts as a switch when it is forward biased. 4) When gate is open i.e., I G = 0, and anode voltage is applied junctions P 1 N 1 and P N are forward biased where N 1 P is reverse biased. Only small reverse current flows. 5) If we increase anode voltage further, at one stage anode current increases suddenly and voltage across the SCR falls to holding voltage H. 6) Once SCR fires (conducts), it will remain in conduction till the current through the device is reduced less than IH, adding current by reducing applied voltage (to less than holding voltage) close to zero. 7) The firing angle can be varied by varying the Gate voltage. With very large positive (gate current break over may occur at very low voltage and SCR works as if it is a normal PN diode. TWO TRANSISTOR ERSION OF SCR. -T 1 is PNP and T is NPN. GRIET-ECE G.Surekha Page 7
I b1 = I A I e1 = I A - 1 I A = I A (1-1) - (1) I b1 = I c and I c = I k - () I b1 = I A (1-1) = I K - (3) We know I k = I A + Ig. ( I A = I C1 + I b1 ) - (4) Putting the value of I k from eqn. (4) in eqn. (3) I A (1-1) - (I A + Ig) I A (1-1) = (I A + Ig) I A (1-1 - ) = Ig. Or Ig I A -(5) 1 1 Equation 5 indicates that if( 1 + ) = 1, I A = - SCR is also called as Thyrister - Latching current (I L ) the min. current required to fire the device - Holding current (I H ) min. current to keep the SCR conductivity PI - oltage safety factor f RMS of operating voltage. alue of f is to.7. Application of SCRs 1. SCRs are mainly used in devices where the control of high power, possibly coupled with high voltage, is demanded.. Their operation makes them suitable for use in medium to high-voltage AC power control applications, such as regulators and motor control. 3. SCRs and similar devices are used for rectification of high power AC in high-voltage direct current power transmission. 5. Explain Rectifier circuits using SCR s. SCR Half wave Rectifier:- Below fig shows the circuit of an SCR half wave rectifier. GRIET-ECE G.Surekha Page 8
SCR does not conduct during negative half cycle (like normal PN diode) Firing angle depends on gate voltage Conduction angle is ( - ) During the positive half cycle of ac voltage appearing across secondary, the SCR will conduct provided proper gate current, the lesser the supply voltage at which the SCR is triggered ON. Reffering to above fig the gate current is adjusted to such a value that SCR is turned ON at a positive voltage 1 of ac secondary voltage which is less than the peak voltage m. Beyond this The SCR will be conducting till the applied voltage becomes zero. The angle at which the SCR starts conducting during the positive half cycle is called firing angle. There fore the conduction angle is (180 0 - ). Average DC output 1 av m sin wt. dwt 0 1 1 1 m m m m 1 cos cos wt cos 0 cos 1 cos GRIET-ECE G.Surekha Page 9
RMS OLTAGE: RMS is given by RMS m 1 sin 1/ SCR FULL WAE RECTIFIER The SCR Full wave Rectifier is shown in below fig. During the Positive half cycle of the input signal, anode of the SCR1 becomes positive and the at the same time the anode of SCR becomes negative. When the input voltage reaches 1 as shown in below fig (b), SCR1 starts conducting and therefore only the shaded portion of positive half cycle will pass through the load. During the negative half cycle of the input, the anode of SCR1 becomes negative and the anode of SCR becomes positive. Hence SCR1 does not conduct and SCR conducts when the input voltage becomes 1. DC m 1 cos 6. Explain the principle and working of Photo Diode. PHOTO DIODES: The diagrams shown below are construction, biasing and symbol of Photo diode. Construction Biasing Symbol A P N K P N GRIET-ECE G.Surekha Page 10
- When light falls on reverse biased PN photo junction, holes and electron pairs are liberated which leads to current flow through the external load. - Current will be zero only for a positive voltage T. Current luminous flux - LEDs are used for displays, including seven-segment display. - A photodiode is a type of photo detector capable of converting light into either current or voltage, depending upon the mode of operation. A photodiode is a p-n junction or PIN structure. It is designed to operate in reverse bias. When a photon of sufficient energy strikes the diode, it excites an electron, thereby creating a free electron (and a positively charged electron hole). 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 field of the depletion region. Thus holes move toward the anode, and electrons toward the cathode, and a photocurrent is produced. This photocurrent is the sum of both the dark current (without light) and the light current, so the dark current must be minimised to enhance the sensitivity of the device. Applications P-N photodiodes are used in similar applications to other photo detectors, such as photoconductors, charge-coupled devices, and photomultiplier tubes. They may be used to generate an output which is dependent upon the illumination. Photosensors of all types may be used to respond to incident light, or to a source of light which is part of the 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). 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. GRIET-ECE G.Surekha Page 11
7. Describe the operation of Schottky diode. Schottky Diode Junction of lightly doped n-type semiconductor with a metal electrode. The junction of a doped semiconductor (usually n-type) with a metal electrode can produce a very fast-switching diode which is mainly used in high frequency circuits or high speed digital circuits. Under forward bias, the electrons move from the n-type material to the metal and give up their energy quickly. There are no holes (minority carriers), so the conduction quickly stops upon change to reverse bias. Schottky diodes find application as rectifiers for high frequency signals. Construction A metal-semiconductor junction is formed between a metal and a semiconductor, creating a Schottky barrier (instead of a semiconductor semiconductor junction as in conventional diodes). Typical metals used are molybdenum, platinum, chromium or tungsten; and the semiconductor would typically be N-type silicon.the metal sides acts as the anode and N-type semiconductor acts as the cathode of the diode. This Schottky barrier results in both very fast switching and low forward voltage drop. Reverse recovery time The most important difference between p-n and Schottky diode is reverse recovery time, when the diode switches from non-conducting to conducting state and vice versa. Where in a p-n diode the reverse recovery time can be in the order of hundreds of nanoseconds and less than 100 ns for fast diodes, Schottky diodes do not have a recovery time, as there is nothing to recover from (i.e. no charge carrier depletion region at the junction). The switching time is ~100 ps for the small signal diodes, and up to tens of nanoseconds for special high-capacity power diodes. It is often said that the Schottky diode is a "majority carrier" semiconductor device. This means that if the semiconductor body is doped n-type, only the n-type carriers (mobile electrons) play a significant role in normal operation of the device. The majority carriers are quickly injected into the conduction band of the metal contact on the other side of the diode to become free moving electrons. Therefore no slow, random recombination of n- and p- type carriers is involved, so that this diode can cease conduction faster than an ordinary p-n rectifier diode. This property in turn allows a smaller device area, which also makes for a faster transition. This is another reason why Schottky diodes are useful in switch-mode power converters; the high speed of the diode means that the circuit can operate at frequencies in the range 00 khz to MHz, allowing the use of small inductors and capacitors with greater efficiency than would be possible with other diode types. Small-area Schottky diodes are the heart of RF detectors and mixers, which often operate up to 50 GHz. GRIET-ECE G.Surekha Page 1
Limitations The most evident limitations of Schottky diodes are the relatively low reverse voltage rating for silicon-metal Schottky diodes, 50 and below, and a relatively high reverse leakage current. Diode designs have been improving over time. oltage ratings now can reach 00. Reverse leakage current, because it increases with temperature, leads to a thermal instability. Problems Q)1. An SCR FWR is connected to 50. 50 Hz mains to supply ac voltage to resistive load of 10 for firing angle of 90. Find DC output voltage and load current. Solution: - Given RMS = 30, R L = 10, = 90 DC =? I L =? RMS DC max m 1 Cos Or max = RMS = 50 = 353.6 volts 353.6 1 cos90 11.6 volts I L DC R L 11.6 10 11.6Amps Q ) A sinusoidal voltage = 00 sin 314 t is applied to an SCR whose forward break down voltage is 150. Determine the time during which SCR remain off. Solution: - Given 1 = 150, m = 00 W = 314 =? t =? 1 150 3 1 m sin or sin 00 4 1 sin 3/ 4 48.6 m T = 1/f f =? w = f = 314 or f = 314/ = 50Hz. GRIET-ECE G.Surekha Page 13
T= 1/50 = 0.0sec = 0 m. sec. 48.6 t = T 0.7m sec 360 360 Q 3) A half wave rectifier employing SCR is adjusted to have a gate current of 1mA and its forward breakdown voltage is 150. If a sinusoidal voltage of 400 peak is applied, determine. i) Firing angle (ii) Average output voltage Given iii) Average current for a load resistance of 00 iv) Power output. 1 = 150, m = 400, =? DC =? I DC =? P DC =? R L = 00 Solution: - 1 = m sin, or Sin = 1 / m = 150/400 = 3/8 = 0.375. = Sin -1 0.375 =. DC m 400 400 1 cos (1 cos ) (1.97) 1.6volts I DC R DC L 1.6 0.613 Amps. 00 P. I 1.6 0.613 75.15 Watts. DC DC DC GRIET-ECE G.Surekha Page 14