Semiconductor Devices Viva Questions

Q.1. What is doping?

Ans. The process of adding impurity to a pure semiconductor is called doping. Doping is done at a rate such that only one atom of impurity is added per 10⁶ to 10¹⁰ semiconductor atoms.

Q.2. What are the types of extrinsic semiconductor?

Ans. Depending upon the types of impurity added, there are two types of extrinsic semiconductors:

(a) N-type semiconductor

(b) P-type semiconductor

Q.3. What is the energy gap?

Ans. The energy gap between the valence band and conduction band is known as the forbidden energy gap. A forbidden energy gap is a region in which no electron can stay as there is no allowed energy state. The greater the forbidden energy gap more tightly the valence electrons are bound to the nucleus.

Q.4. Define diode resistance.

Ans. An ideal diode offers zero resistance when forward bias and infinite resistance when it is reverse biased. But practical or real diode does not behave ideally. It means that the forward resistance of a real diode is quite small and reverse resistance is very high.

Q.5. What is forward resistance? Explain.

Ans. The resistance offered by the diode in the forward bias is called forward resistance. This resistance is different for the flow of direct current (d.c.) and for alternating current (a.c.). Hence, this resistance is of two types:

(a) d.c. forward resistance

(b) a.c. forward resistance.

Q.6. What is a P-N junction?

Ans. The contact surface between the layers of P-type and N-type semiconductor pieces placed together so as to form a P-N junction is called the P-N junction.

Q.7. What is the effect of reverse bias on the width of a P-N junction?

Ans. When the P-N junction is reverse biased, the width of the depletion region is increased.

Q.8. What types of carriers are present in space charge region?

Ans. No mobile carrier is present in the space charge region.

Q.9. What is cut-in voltage in semiconductors?

Ans. The forward voltage, at which the current through the P-N junction starts increasing rapidly is called the cut-in voltage.

Q.10. What do you mean by the breakdown mechanism? Explain.

Ans. If the reverse bias applied to a P-N-junction is increased, a point will reach when the junction breaks down, and the reverse current rises sharply to a value limited only by the external resistance connected in series. This specific value of the reverse bias voltage is called breakdown voltage (V_B). The breakdown voltage depends upon the width of the depletion layer. This width of depletion layer depends upon the doping level.

The following two processes cause junction breakdown due to the increase in reverse bias voltage: (i) Zener Breakdown (ii) Avalanche Breakdown.

Q.11. What is Zener's breakdown? Explain.

Ans. This form of breakdown occurs injunctions that are heavily doped. The depletion layer is narrow for heavily doped junctions. When the reverse bias voltage is increased, the electric field also increases across the narrow depletion layer. This strong electric field (about 10⁸ V/m) causes covalent bonds to break in the crystal structure. Due to this reason, a large number of carriers are generated. This causes a large current to flow.

Q.12. What is Zener voltage?

Ans. The voltage at which the Zener diode breaks down is called the Zener voltage.

Q.13. What happens to the series current, load current and Zener current when the de input voltage of a Zener regulator increases?

Ans. Zener current and series current increase while the load current remains unchanged.

Q.14. What is a Zener diode?

Ans. Zener diode is a P-N junction diode specially designed for operation in its breakdown region.

Q.15. What do you mean by the Varactor Diode?

Ans. A varactor diode is, basically, a reverse-biased PM junction, which utilizes the inherent capacitance of the depletion layer. It is also known as varicap, voltcap, or tunning diode. It is used as a voltage variable capacitor.

Q.16. Write few applications of BJT.

Ans.  

1.      Transistor is used in control systems.

2.      Transistor is used in satellites and mobile phones.

3.      In digital computer electronics, the transistor is used as a high-speed electronic switch.

4.      In communication systems, it is widely used as the primary component in the amplifier.

Q.17. What do you mean by operating point? Explain.

Ans. The zero signal values (no a.c. signal is applied) of collector current Ic and collector-to-emitter  voltage V_CE are known as the operating point.

Since the variations of I_C and V_CE take place about this point, therefore, this is known as the operating point.

This point is also called quiescent (stable) point or simply Q-point since it is a point on the output characteristics curve when the transistor is in a silent condition (i.e., the absence of a.c. input signal).

Q.18. Explain why an ordinary transistor is called bipolar?

Ans. Because the transistor operation is carried out by two types of charge carriers (majority and minor carriers), an ordinary transistor is called bipolar.

Q.19. What is done to the base region of a transistor to improve its operation?

Ans. The base is made of thin and very lightly doped in comparison to either emitter or collector so that  it may pass most of the injected charge carriers to the collector.

Q.20. Why transistor is called current controlled device?

Ans. The output voltage, current, or power is controlled by the input current in a transistor so it is called the current controlled device.

Q.21. Why silicon type transistors are more often used than germanium type?

Ans. Because. silicon has smaller cut-off current I_CBO, small variations in I_CBO due to variations in temperature as compared to those in case of germanium.

Q.22. Why the width of the base region of a transistor is kept very small compared to other regions?

Ans. The base region of a transistor is kept very small and very lightly doped so as to pass most of the injected charge carriers to the collector.

Q.23. Why is there a maximum limit of collector supply voltage for a transistor?

Ans. Although collector current is practically independent of the collector supply voltage over the transistor operating range, but if V_CB is increased beyond a certain value, collector current I_C eventually increased rapidly and possibly destroys the device.

Q.24. Why CE configuration is most popular in amplifier circuits?

Ans. CE configuration is mainly used because its current, voltage, and power gains are quite high and ratio of output impedance and input impedance are quite moderate.

Q.25. Why is CC configuration seldom used?

Ans. CC configuration is seldom used because its voltage gain is always less than unity.

Q.26. What is meant by the D.C. load line?

Ans. With the help of d.c. load line, we can easily analyze the performance of an amplifier circuit.

Q.27. What is meant by Operating point?

Ans. The zero signal values (no. a.c. signal is applied) of collector current Ic and collector-to-emitter voltage V_CE is known as the operating point.

Q.28. What do you mean by field-effect transistor amplifier?

Ans. The field-effect transistor (FET) has the capability to amplify a.c. signals like a bipolar junction transistor (BJT).  Depending upon the types of configuration, the  FET amplifiers may be  classified under the following three heads:

(i)                 Common source amplifier

(ii)               Common drain amplifier  

(iii)             Common gate amplifier

Q.29. Why are field-effect transistors called unipolar transistors?

Ans. In field-effect transistors, current conduction is by only one of type of majority carriers (ether by electrons or by holes) and, therefore are called unipolar transistors.

Q.30. How is drain current controlled in a JFET?

Ans. In a JFET, drain current is controlled by controlling the reverse bias given to its gate.

Q.31. What is meant by drain characteristics of FETs?

Ans. The curve is drawn between drain current and drain-source voltage with gate-to-source  voltage as the parameter is called the drain characteristic.

Q.32. What is meant by transfer characteristic of FETs?

Ans. The curve is drawn between drain current and gate-source voltage for a given value of drain-source voltage is called the transfer characteristic.

Q.33. What is pinch-off voltage in a JFET?

Ans. The value of the drain-source voltage at which channel is pinched-off  (i.e. all the free charges from the channel get removed) is called the pinch-off voltage.

Q.34. What is meant by saturation region?  

Ans. The region of drain characteristic of a FET in which drain current remains fairly constant is called the saturation or pinch-off region.

Q.35. Why is the input impedance of the FET very high?

Ans. FET has a very high input impedance because its input circuit (gate-to-source) is reverse biased and the input gate current is very small (of the order of a few nano-amperes).  

Feedback Viva Questions with answers