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SPECIAL
DIODES
C H A P T E R
Learning Objectives
Ç A major application for zener diodes is voltage
regulation in dc power supplies. Zener diode maintains a nearly constant dc voltage under the proper operating conditions.
➣➣ ➣➣➣ Zener Diode ➣➣ ➣➣➣ Voltage Regulation ➣➣ ➣➣➣ Zener Diode as Peak Clipper ➣➣ ➣➣➣ Meter Protection ➣➣ ➣➣➣ Zener Diode as a Reference Element ➣➣ ➣➣➣ Tunneling Effect ➣➣ ➣➣➣ Tunnel Diode ➣➣ ➣➣➣ Tunnel Diode Oscillator ➣➣ ➣➣➣ Varactor Diode ➣➣ ➣➣➣ PIN Diode ➣➣ ➣➣➣ Schottky Diode ➣➣ ➣➣➣ Step Recovery Diode ➣➣ ➣➣➣ Gunn Diode ➣➣ ➣➣➣ IMPATT Diode
2112 Electrical Technology
54.1. Zener Diode
It is a reverse-biased heavily-doped silicon (or germanium) P-N junction diode which is oper- ated in the breakdown region where current is limited by both external resistance and power dissipa- tion of the diode. Silicon is perferred to Ge because of its higher temperature and current capability. As seen from Art. 52.3, when a diode breaks down, both Zener and avalanche effects are present although usually one or the other predominates depending on the value of reverse voltage. At reverse voltages less than 6 V, Zener effect predominates whereas above 6 V, avalanche effect is predomi- nant. Strictly speaking, the first one should be called Zener diode and the second one as avalanche diode but the general practice is to call both types as Zener diodes.
Zener breakdown occurs due to breaking of covalent bonds by the strong electric field set up in the depletion region by the reverse voltage. It produces an extremely large number of electrons and holes which constitute the reverse saturation current (now called Zener current, I (^) z ) whose value is limited only by the external resistance in the circuit. It is independent of the applied voltage****. Ava- lanche breakdown occurs at higher reverse voltages when thermally-generated electrons acquire suf- ficient energy to produce more carriers by collision.
( a ) V/I Characteristic A typical characteristic is shown by Fig. 54.1 in the negative quadrant. The forward characteristic is simply that of an ordinary forward-biased junction diode. The impor- tant points on the reverse characteristic are :
Vz = Zener breakdown voltage I (^) z min = minimum current to sustain breakdown I (^) z max = maximum Zener current limited by maximum power dissipation. Since its reverse characteristic is not exactly vertical, the diode possesses some resistance called Zener dynamic impedance *****. However, we will neglect it assuming that the characteristic is truly vertical. In other words, we will assume an ideal Zener diode for which voltage does not change once it goes into breakdown. It means that V (^) z remains constant even when Iz increases considerably.
The schematic symbol of a Zener diode and its equivalent circuit are shown in Fig. 54.2 ( a ). The complete equivalent circuit is shown in Fig. 54.2 ( b ) and the approximate one in Fig. 54.2 ( c ) where it looks like a battery of V (^) z volts.
The schematic symbol of Fig. 54.2 ( a ) is similar to that of a normal diode except that the line representing the cathode is bent at both ends. With a little mental effort, the cathode symbol can be imagined to look like the letter Z for Zener.
( b ) Zener Voltages Zener diodes are available having Zener voltages of 2.4 V to 200 V. This voltage is temperature dependent. Their power dissipation is given by the product V (^) z I (^) z ... maximum ratings vary from 150 mW to 50 W. ( c ) Zener Biasing For proper working of a Zener diode in any circuit, it is essential that it must
1. be reverse-biased; 2. have voltage across it greater than V (^) z ;
***** Its value is given by Z (^) z = ∆ V (^) z /∆ I (^) z. It is negiligible as compared to large external resistance connected in the circuit.
Fig. 54.
IF
I (^) Z IZ
VZ VF I (^) Z (^) min.
max.
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Example. 54.4. Calculate the value of E 0 in the given circuit of Fig. 54.6. Given Ein = 6 V and 20 V. (Electrical Engg. II, Indore Univ.) Solution. When Ein is 6 V, the diode acts like an open circuit. It is so because 6 V is not enough to cause Zener break-down which will take place only when Ein exceeds 10 V. Hence, in this case, E 0 = 0. When Ein = 20 V, breakdown occurs but voltage across diode remains constant at 10 V. The balance (20 − 10) = 10 V appears across 100 Ω resistor. Hence, E 0 = drop across R = 10 V.
54.2. Voltage Regulation
It is a measure of a circuit's ability to maintain a constant output voltage even when either input voltage or load current varies. A Zener diode, when working in the breakdown region, can serve as a voltage regulator. In Fig. 54.7, V (^) in is the input dc voltage whose variations are to be regulated. The Zener diode is reverse-connected across V (^) in. When p.d. across the diode is greater than V (^) z , it conducts and draws relatively large current through the series resistance R. The load resistance R (^) L across which a constant voltage V (^) out is required, is connected in parallel with the diode. The total current I passing through R equals the sum of diode current and load current i.e. I = I (^) z + I (^) L. It will be seen that under all conditions V (^) out = V (^) z. Hence, V (^) in = IR + V (^) out = IR + V (^) z. Case 1. Suppose R (^) L is kept fixed but supply voltage V (^) in is increased slightly. It will increase I. This increases in I will be absorbed by the Zener diode without affecting I (^) L. The increase in V (^) in will be dropped across R thereby keeping V (^) out constant. Conversely if supply voltage V (^) in falls, the diode takes a smaller current and voltage drop across R is reduced, thus againt keeping V (^) out constant. Hence, when V (^) in changes, I and IR drop change in such a way as to keep V (^) out ( = V (^) z ) constant. Case 2. In this case, V (^) in is fixed but I (^) z is changed. When I (^) L increases, diode current I (^) z decreases thereby keeping I and hence IR drop constant. In this way, V (^) out remains unaffected. Should I (^) L decrease, I (^) z would increase in order to keep I and hence IR drop constant. Again, V (^) out would remain unchanged because V (^) out = V (^) in − IR = V (^) in − ( I (^) z + I (^) L ) R Incidentally, it may be noted that R = ( V (^) in − V (^) out ) / ( Iz + I (^) L ) It may also be noted that when diode current reaches its maximum value, I (^) L becomes zero. In that case
R = in − out Zmax
V V I In Fig. 54.7, only one reference voltage level is available. Fig. 54.8 shows the circuits for estab- lishing two reference levels. Here, two diodes having different Zener voltages have been connected in series. Example 54.5. Calculate the battery current I, I (^) z and IL in the circuit of Fig. 54.9. How will these values be affected if source voltage increases to 70 V? Neglect Zener resistance. (Industrial Electronics, Pune Univ.) Solution. When V (^) in = 40 V Now, VAB = V (^) z = 10 V ∴ I = 30/3 K = 10 mA ∴ drop across 3K series (or line) resistor is = 40 − 10 = 30 V IL = V (^) z /RL = V (^) A B / R (^) L = 10/2 K = 5 mA
Fig. 54.
Fig. 54.
Special Diodes 2115
Fig. 54.8 Fig. 54. ∴ Iz = I − I (^) L = 10 − 5 = 5 mA Now, Pmax = V (^) z .Iz ( max ) or 0.5 = 10 × I (^) z ( max ) or I (^) z ( max ) = 0.5/10 = 0.05 A = 50 mA Obviously, diode current of 5 mA is very much within the current range of the diode. ( b ) When, V (^) in = 70 V Drop across R = 70 − 10 = 60 V ∴ I = 60/3 K = 20 mA IL = 5 mA (as before); I (^) z = I − I (^) L = 20 − 5 = 15 mA Example 54.6. Using the ideal Zener approximations, find current through the diode of Fig. 54.10 when load resistance RL is (i) 30 K (ii) 5 K (iii) 3 K. (Electronics, Madurai Kamraj Univ. ) Solution. ( i ) RL = 30 K VAB = V (^) z = 30 V ; drop across R = 60 − 30 = 30 V ∴ I = 30/3 K = 10 mA ; I (^) L = V (^) A B /R (^) L = 30/30 K = 1 mA ∴ Iz = I − I (^) L = 10 − 1 = 9 mA ( ii ) When RL = 5 K I = 10 mA — as before ; I (^) L = 30/5 K = 6 mA I (^) z = 10 − 6 = 4 mA ( iii ) When RL = 3K I = 10 mA — as before ; I (^) L = 30/3 K = 10 mA I (^) z = I − I (^) L = 10 − 10 = 0 In this case, it is obvious that the diode is just on the verge of coming out of breakdown region. If R (^) L is reduced further, the diode will come out of breakdown region and would no longer act as a voltage regulator.
Fig. 54.10 Fig. 54. Example 54.7. A 24-V, 600-mW Zener diode is to be used for providing a 24-V stabilized supply to a variable load (Fig. 54.11). If input voltage is 32 V, calculate the (i) series resistance R required (ii) diode current when R (^) L = 1200 Ω. (Applied Electronics, Punjab Univ. 1991) Solution. ( i ) V (^) z Iz ( max ) = 600 mW ; I (^) z ( max ) = 600/24 = 25 mA
∴ R = (^3) (max)
32 24 25 10
in out z
V V I −
− (^) −
×
= 320 ΩΩΩΩΩ
( ii ) When RL = 1200 Ω, I (^) L = V (^) z /RL = 24/1200 = 20 mA; I (^) z = 25 − 20 = 5 mA
VZ =20 V
VZ =10 V
R
50 V _
_
Pmax=
VZ =10V
I (^) Z
IL
RL
3 K
40-70 V
_
A +
B
2 K
I
0.5W
VZ =30 V
IZ R
R = 3 K IL
60 V
B
I A
3-30 K
L
R
RL
Pmax (^) = 0.6W
V (^) Z= 24 V
Special Diodes 2117
54.6. Tunneling Effect
In a normally-doped P-N junction, the depletion layer is relatively wide and a potential barrier exists across the junction. The charge carriers on either side of the junction cannot cross over unless they possess sufficient energy to overcome this barrier (0.3 V for Ge and 0.7 V for Si). As is well-known, width of the depletion region depends directly on the doping den- sity of the semiconductor. If a P-N junction is doped very heavily (1000 times or more)*, its depletion layer
becomes extremely thin (about 0.00001 mm). It is found that under such conditions, many carriers can ‘punch through’ the junction with the speed of light even when they do not possess enough energy to over- come the potential barrier. Consequently, large for- ward current is produced even when the applied bias is much less than 0.3 V. This conduction mechanism in which charge carriers (possessing very little energy) bore through a barrier directly instead of climbing over it is called tunneling.
Explanation
Energy band diagrams (EBD) of N -type and P - type semiconductor materials can be used to explain this tunneling phenomenon. Fig. 54.16 shows the en- ergy band diagram of the two types of silicon sepa- rately. As explained earlier (Art. 51.21), in the N -type semiconductor, there is increased concentration of electrons in the conduction band. It would be further increased under heavy doping. Similarly, in a P - type material, there is increased concentration of holes in the valence band for similar reasons. ( a ) No Forward Bias When the N -type and P -type materials are joined, the EBD under no-bias conditiion becomes as shown in Fig. 54.17 ( a ). The junction barrier produces only a rough alignment of the two materials and their respective valence and conduction bands. As seen, the depletion region between the two is extremely narrow due to very heavy doping on both sides of the junction. The potential hill is also increased as shown.
Fig. 54.
Fig. 54.
Fig. 54.
- Much more than even for a Zener diode.
2118 Electrical Technology
Fig. 54.
Fig. 54.
I (^) P
VP VV V (^) T
i (^) T
IV
C
B
A
0
Negative Resistance
( b ) Small Forward Bias When a very small forward voltage (≅ 0.1 V) is applied, the EBDs become as shown in Fig. 54.17 ( b ). Due to the downward movement of the N -region, the P -region valence band becomes exactly aligned with the N -region conduction band. At this stage, electrons tunnel through the thin depletion layer with the velocity of light thereby giving rise to a large current called peak current I (^) p. ( c ) Large Forward Bias When the forward bias is increased further, the two bands get out of alignment as shown in Fig. 54.17 ( c ). Hence, tunneling of electrons stops thereby decreasing the current. Since current decreases with increase in applied voltage ( i.e. dV/dI is negative), the junction is said to possess negative resis- tance at this stage. This resistance increases throughout the negative region. However, it is found that when applied forward voltage is increased still further, the current starts increasing once again as in a normal junction diode.
54.7. Tunnel Diode
This diode was first introduced by Dr. Leo Easki in 1958. ( a ) Construction It is a high-conductivity two-terminal P-N junction diode having doping density about 1000 times higher as compared to an ordinary junction diode. This heavy doping produces follow- ing three unusual effects :
1. Firstly, it reduces the width of the depletion layer to an extremely small value (about 0.00001 mm). 2. Secondly, it reduces the reverse breakdown voltage to a very small value (approaching zero) with the result that the diode appears to be broken down for any reverse voltage. 3. Thirdly, it produces a negative resistance section on the V/I characteristic of the diode. It is called a tunnel diode because due to its extremely thin depletion layer, electrons are able to tunnel through the potential barrier at relatively low forward bias voltage (less than 0.05 V). Such diodes are usually fabricated from germanium, gallium-arsenide (GaAs) and gallium antimonide (GaSb). The commonly-used schematic symbols for the diode are shown in Fig. 54.18. It should be handled with caution because being a low-power device, it can be easily damaged by heat and static elec- tricity.
( b ) V/I Characteristic
It is shown in Fig. 54.19. As seen, forward bias produces immediate conduction i.e. as soon as forward bias is applied, significant current is produced. The current quickly rises to its peak value I (^) p when the applied forward voltage reaches a value V (^) p (point A ). When forward voltage is increased fur- ther, diode current starts decreasing till it achieves its mini- mum value called valley current I (^) v corresponding to valley voltage V (^) v (point B ). For voltages greater than V (^) v , current starts increasing again as in any ordinary junction diode.
Discrete commercialSi tunnel diode
2120 Electrical Technology
Fig. 54.
LS
RS
_R C (^) N
iT iT
(^0) VT 0 VT
Valley
Q
Q
Bias Bias V ( a ) ( b )
iT
VT
R (^) VR
0 Vp Vv VT
Q
B
Bias Voltage
V
D
S
A
C
( a ) ( b )
( f ) Biasing the Diode The tunnel diode has to be biased from some dc source for fixing its Q -point on its characteristic when used as an amplifier or as anoscillator and modulator.
Fig. 54.20 Fig. 54. The diode is usually biased in the negative region [Fig. 54.21 ( a )]. In mixer and relaxation oscillator applications, it is biased in the positive-resistance region nearest zero [Fig. 54.21 ( b )]. ( g ) Applications Tunnel diode is commonly used for the following purposes :
1. as an ultrahigh-speed switch-due to tunneling mechanism which essentially takes place at the speed of light. It has a switching time of the order of nanoseconds or even picoseconds; 2. as logic memory storage device − due to triple-valued feature of its curve for current. 3. as microwave oscillator at a frequency of about 10 GHz − due to its extremely small capaci- tance and inductance and negative resistance. 4. in relaxation oscillator circuits − due to its negative resistance. In this respect, it is very similar to the unijunction transistor. ( h ) Advantages and Disadvantages The advantages of a tunnel diode are : 1. low noise, 2. ease of operation, 3. high speed, 4. low power, 5. Insensitivity to nuclear radiations The disadvantages are : 1. the voltage range over which it can be operated properly is 1 V or less; 2. being a two-terminal device, it provides no isolation between the input and output circuits.
54.8. Tunnel Diode Oscillator
The basic job of an oscillator is to convert dc power into ac power. Ordinarily, we do not expect an ac signal from a circuit which has no input ac source. But the circuit shown in Fig. 54.22 does exactly that as explained below. The value of R is so selected as to bias the diode D in the negative-resistance region A − B. The working or quiescent point Q is almost at the centre of the curve A − B. When S is closed, the current immediately rises to a value determined by R and the diode resistance which are in series. The applied voltage V divides across D and R according to the ratio of their resistances. However, as V (^) T exceeds V (^) P , diode is driven into the negative area and its resistance starts to increase (Art 4.7). Hence, V (^) T increases further till it becomes equal to valley voltage V (^) V (point B ). At this point, further increase in V (^) T drives the diode into the positive- resistance region BC [Fig. 54.
Special Diodes 2121
( b )]. The resulting increase in current now increases V (^) R but correspondingly decreases V (^) T , thereby bringing the diode back into the negative-resistance region. This decrease in V (^) T increases the circuit current till point A is reached when V (^) T equals V (^) P. It describes one cycle of operation. In this way, the circuit will continue to oscillate back and forth through the negative-resistance region i.e. between points A and B on its characteristic. Its output across R is like a sine wave. Fig. 54.23 shows a practical circuit drawn in two slightly different ways. Here, R 2 sets the proper bias level for the diode whereas R 1 (in parallel with the LC tank circuit) sets proper current level for it. The capacitor CC is the coupling capacitor. As the switch S is closed, the diode is set into oscillations whose frequency equals the resonant frequency of the tank.
54.9. Varactor Diode
The varactor diode is a semiconductor, voltage-dependent variable capacitor alternatively known as varicap or voltacap or voltage-variable capacitor ( VVC ) diode. Basically, it is just a reverse-biased junction diode whose mode of operation depends on its transition capacitance ( CT ). As explained earlier in Art. 52.4, reverse-biased junctions behave like capacitors whose capacitance is ∝ 1/ V (^) Rn where n varies from 1/3 to 1/2. As reverse voltage V (^) R is increased, depletion layer widens thereby decreasing the junction capacitance. Hence, we can change diode capacitance by simply changing V (^) R. Silicon diodes which are optimised for this variable capacitance effect are called varactors. The picture, schematic symbol and a simple equivalent circuit for a varactor are shown in Fig. 54.24. Varactors may be of two types as shown in Fig. 54.25. The doping profile of the abrupt-junction diode is shown in Fig. 54.25 ( a ) and that of the hyperabrupt-junction diode in Fig. 54.25 ( b ). The abrupt-junction diode has uniform doping and a capacitive tuning ratio ( T R ) of 4 : 1. For example, if its maxi- mum transition capacitance is 100 pF and minimum 25 pF, then its T R is 4 : 1 which is not enough to tune a broadcast receiver over its entire frequency range of 550 to 1050 kHz. The hyperabrupt-junction diode has highest impurity concentration near the junction. It results in narrower depletion layer and larger capacitance. Also, changes in V (^) R produce larger capacitance changes. Such a diode has a tuning range of 10 : 1 enough to tune a broadcast receiver through its frequency range of nearly 3 : 1.
Fig. 54.24 Fig. 54.
Fig. 54.
Varactor diode
CT
RS
P
P
N
N
Doping Density DopingDensity
( a ) ( b )
P
N
P N
Special Diodes 2123
across the 2 Ω resistance. Hence, there is hardly any signal output. In practice instead of mechanically switching the diode-biasing supply from − 25 V to + 25 V, a transistor is used to do this switching operation. In this way, we can turn a very high frequency signal (MHz range) OFF and ON with the speed of a transistor switching circuit. ( ii ) Use as AM Modulator. The way in which the 500-MHz signal is modulated at 1 kHz rate is illustrated in Fig. 54.29. A 1 kHz signal is fed into a PNP transistor where it varies its dc output current at the same rate. This varying dc current is applied as biasing current to the PIN diode as shown in Fig. 54.29. It varies the diode ac resistance as seen by the 500 MHz signal. Hence, the signal is modulated at 1 kHz rate as shown. ( d ) Applications ( i ) as a switching diode for signal frequencies upto GHz range; ( ii ) as an AM modulator of very high frequency signals.
54.11. Schottky Diode
It is also called Schottky barrier diode or hot-carrier diode. It is mainly used as a rectifier at signal frequencies exceeding 300 MHz. It has more uniform junction region and is more rugged than PIN diode − its main rival. ( a ) Construction It is a metal-semiconductor junction di- ode with no depletion layer****. It uses a metal (like gold, silver, platinum, tungsten etc.) on the side of the junction and usually an N - type doped silicon semiconductor on the other side. The diode and its schematic sym- bol are shown in Fig. 54.30. ( b ) Operation When the diode is unbiased, electrons on the N -side have lower energy levels than electrons in the metal. Hence, they cannot surmount the junction barrier (called Schottky barrier) for going over to the metal. When the diode is forward-biased, conduction electrons on N -side gain enough energy to cross the junction and enter the metal. Since these electrons plunge into the metal with very large energy, they are commonly called ‘ hot-carriers ’. That is why this diode is often referred to as hot-carrier diode. ( c ) Applications This diode possesses two unique features as compared to an ordinary P-N junction diode :
1. it is a unipolar device because it has electrons as majority carriers on both sides of the junction. An ordianry P-N junction diode is a bipolar device because it has both electrons and holes as majority carriers; 2. since no holes are available in metal, there is no depletion layer or stored charges to worry about. Hence, Schottky diode can switch OFF faster than a bipolar diode.
Fig. 54.
R 2
R 1 Modulated Signal
PNP
+V
Pin 500 MHz
1 kHz
R 3
Schottky barrier diode family expands to meet needs of power related application
2124 Electrical Technology
Doping Density P 20 MHz N
Sr Diode
R
One Cycle
t
( a ) ( b )
Vo
Vin Vo
Because of these qualities, Schottky diode can easily rectify signals of frequencies exceeding 300 MHz. As shown in Fig. 54.31, it can produce an almost perfect half-wave rectified output. The present maximum current rating of the device is about 100 A. It is commonly used in switch- ing power supplies that operate at frequencies of 20 GHz. Another big advantage of this diode is its low noise figure which is extremely important in communication receivers and radar units etc. It is also used in clipping and clamping circuits, computer gating, mixing and detecting networks used is communication systems.
Fig. 54.30 Fig. 54.
54.12. Step Recovery Diode
It is another type of VVC diode having a graded doping profile where doping density decreases near the junction as shown in Fig. 54.32. This results in the production of strong electric fields on both sides of the junction. ( a ) Theory At low frequencies, an ordinary diode acts as a rectifier. It conducts in the forward direction but not in the reverse direction i.e. it recovers immediately from ON state to the OFF state. However, it is found that when driven forward-to-reverse by a high-frequency signal (above a few MHz), the diode does not recover immediately. Even during the negative half-cycle of the input signal when the diode is reverse-biased, it keeps conducting for a while after which the reverse current ceases abruptly in one step. This reverse conduction is due to the fact that charges stored in the depletion layer during the period of forward bias take time to drain away from the junction.
Fig. 54.32 Fig. 54. Fig. 54.33 ( a ) shows a step-recovery diode being driven by a 20-MHz signal source. As seen from Fig. 54.33 ( b ), it conducts in the forward direction like any diode. During the reverse half-cycle, we get reverse current due to the draining of the stored charge after which current suddenly drops to zero. It looks as though diode has suddenly snapped open during the early part of the reverse cycle. That is why it is sometimes called a snap diode. The step or sudden recovery from reverse current ON to reverse current OFF gives the diode its name.
Metal
500 MHz N (^) Vin
Vo
R
t ( a ) ( b )
(^03) p 5 p
Vo
2126 Electrical Technology
Fig. 54.36 Fig. 54.37 Fig. 54.
5. A 9 V stabilized voltage supply is required to run a car stereo system from car’s 12 V battery. A Zener diode with V (^) z = 9 V and Pmax = 0.25 W is used as a voltage regulator as shown in Fig. 54.40.
Fig. 54.39 Fig. 54. Find the value of the series resistor R. [120 ΩΩΩΩΩ ]
6. A load of 1kW is connected across a 10 V Zener regula- tor as shown in Fig. 54. 41. The zener current can vary between 5mA to 55mA while maintaining the voltage constant. Find the minimum and maximum voltage level at input. (Electronic Devices and Circuits, Nagpur Univ. Summer, 2004) 7. A 24V, 600 mW zener diode is used for providing a 24V stabilized supply to a variable load. If the input voltage is 32V, calculate ( i ) the value of series resistance required. ( ii ) diode current when the load is 1200 Ω (Electronics Engg., Bangalore Univ. 2003)
OBJECTIVE TESTS – 54
1. Silicon is preferred for manufacturing Zener diodes because it ( a ) is relatively cheap ( b ) needs lower doping level ( c ) has higher temperature and current capacity ( d ) has lower break-down voltage. 2. When used in a circuit, a Zener diode is always ( a ) forward-biased ( b ) connected in series
( c ) troubled by overheating ( d ) reverse-biased.
3. The main reason why electrons can tunnel through a P-N junction is that ( a ) they have high energy ( b ) barrier potential is very low ( c ) depletion layer is extremely thin ( d ) impurity level is low. 4. The I (^) P/I (^) V ratio of a tunnel diode is of primary importance in
Vin 10 V 1 K
500 W
Fig. 54.
Special Diodes 2127
( a ) determining tunneling speed of electrons ( b ) the design of an oscillator ( c ) amplifier designing ( d ) computer applications.
5. Mark the INCORRECT statement. A varactor diode ( a ) has variable capacitance ( b ) utilizes transition capacitance of a junction ( c ) has always a uniform doping profile ( d ) is often used as an automatic frequency control device. 6. The microwave device used as an oscillator within the frequency range 10-1000 GHz is ............................. diode. ( a ) Schottky ( b ) IMPATT ( c ) Gunn ( d ) Step Recovery. 7. A PIN diode is frequently used as a ( a ) peak clipper ( b ) voltage regulator ( c ) harmonic generator ( d ) switching diode for frequencies upto GHz range. 8. Mark the WRONG statement. A Schottky diode ( a ) has no depletion layer ( b ) has metal-semiconductor junction ( c ) has fast recovery time ( d ) is a bipolar device ( e ) is also called hot-carrier diode ( f ) can easily rectify high-frequency signals. 9. A special purpose diode which uses metals like gold, silver or platinum on one side of the junction, n -type doped silicon on another side and has almost no charge storage in the junction, is a ( a ) Schotty diode ( b ) tunnel diode ( c ) varactor diode ( d ) zener diode 10. A step-recovery diode ( a ) has an extremely short recovery time ( b ) conducts equally well in both direc- tions
( c ) is mainly used a harmonic generator ( d ) is an ideal rectifier of high-frequency signals.
11. A semiconductor device that resembles a voltage variable capacitor is called diode. ( a ) tunnel ( b ) PIN ( c ) Schottky ( d ) varactor 12. A diode that has no depletion layers and operates with hot carriers is called ...... diode. ( a ) Schottky ( b ) Gunn ( c ) step recovery ( d ) PIN 13. In switching devices, gold doping is used to ( a ) improve bonding ( b ) reduce storage time ( c ) increase the mobility of the carrier ( d ) protect the terminals against corrosion 14. When the reverse bias voltage of a varactor diode increases, its ( a ) capacitance decreases ( b ) lealkage current decreases ( c ) negative resistance increases ( d ) depletion zone decreases. 15. Which of the following are negative- resistance microwave diodes oscillator applications? ( a ) Gunn ( b ) IMPATT ( c ) step recovery ( d ) both ( a ) and ( b ) ( e ) both ( b ) and ( c ). 16. A negative-resistance microwave diode hav- ing a thin slice of a semiconductor material sandwiched between two metal conductors is called ............... diode. ( a ) Schottky ( b ) PIN ( c ) Gunn ( d ) varactor. 17. Zener diodes are used primarily as ( a ) rectifiers ( b ) voltage regulators ( c ) oscillators ( d ) amplifiers.
