Important Two Marks Questions with Answers on Semiconductor Devices

TWO MARKS QUESTION AND ANSWERS

1. What is meant by valence electron?

The electrons in the outermost shell of an atom is called vallence electron.

 

2. Define forbidden energy gap.

The space between the valence band and conduction band is energy gap.

 

3. What are conductors?

Conductors are materials in which valence and conduction band overlap with each other.

Example: Copper, Silver

 

4. What do you mean by insulator?

Insulator is a material in which the valence and conduction band are far away from each other. The energy gap between the valence and conduction band is larger.

Example: Glass, Plastic

 

5. What is meant by semiconductor?

The material whose electrical property lies between that of conductors and insulator is called semiconductor.

Example: Silicon, Germanium

 

6. Define mass-action law.

Under thermal equilibrium the product of free electron concentration and hole concentration is constant regardless of the individual magnitude

 

7. Draw the Energy band diagram of conductors, semiconductors and Insulators.

 

8. Define intrinsic semiconductor.

The semiconductor in its purest form is called intrinsic semiconductor.

 

9. Define Extrinsic semiconductor.

If certain amount of impurity is added to intrinsic semiconductor then the resulting semiconductors is called Extrinsic semiconductor.

 

10. What are the types of Extrinsic semiconductor.

i. P-type semiconductor

ii. N-type semiconductor.

 

11. What is P-type semiconductor?

The semiconductor which is obtained by adding pentavalent impurity atom to an intrinsic semiconductor is called as P-type semiconductor.

Ex: Phosphorous, Antimony are pentavalent impurities

 

12. What is N-type semiconductor?

The semiconductor which is obtained by introducing trivalent impurity atom (Gallium, Indium) is called N-type semiconductor.

 

13. Define doping.

The process of adding impurity to an intrinsic semiconductor is called doping. The impurity is called dopant.

 

14. What is P-type semiconductor?

In P-type semiconductor, the majority charge carriers are holes and the minority charge carriers are electrons.

 

15. What is N-type semiconductor?

In N-type semiconductor, the majority charge carriers are electrons and the minority charge carriers are holes.

 

16. What is donor impurity?

Pentavalent impurity will donate the excess negative charge carriers (electrons) and hence, n-type or pentavalent impurity is called as donor impurity.

 

17. Why P-type or trivalent impurity is called acceptor impurity?

P-type impurity creates holes which can accept electron. So, these impurities are called acceptor impurities.

 

18. How PN diode is formed?

If a semiconductor is doped with P-type impurity on one half side and N-type impurity on other half side, then the semiconductor is called PN diode.

 

19. Define biasing.

Biasing is defined as the application of any external voltage to an electronic device.

 

20. What are the types of bias?

i. Forward bias

ii. Reverse bias

 

21. What is meant by forward bias?

The positive terminal of the battery is connected to P-type semiconductor and negative terminal is connected to n-type semiconductor.

 

22. What is meant by reverse bias.

The positive terminal of the battery is connected to n-type and negative terminal is connected to p-type semiconductor.

 

23. What is depletion region?

The region around the junction from which the mobile charge carriers (electrons and holes) are depleted is called depletion region.

It is also called space charge region because this region has electrically bd charged immobile ions.

 

24. Draw the V-I characteristics of PN diode.

 

25. What is barrier potential?

The oppositely charged ions in both sides of PN junction creates an electric potential across the junction. This is called barrier potential.

 

26. Define reverse saturation current.

The current due to the minority carriers in reverse bias is called reverse saturation current.

 

27. Define forward resistance.

The resistance offered by the p-n junction diode in forward biased condition is called forward resistance.

 

28. What are the types of forward resistance?

i. Static or dc resistance

ii. Dynamic or ac resistance

 

29. Define reverse resistance.

Reverse resistance is defined as the resistance offered by the PN junction diode under reverse bias condition.

 

30. Write the diode current equation.

 

31. What are the breakdown mechanisms in PN diode?

i. Avalanche breakdown

ii Zener breakdown

 

32. Define Fermilevel.

Fermilevel is defined as that energy level where the probability of occupation of charged carrier is 50%.

 

33. Define dynamic resistance.

The dynamic or ac resistance is the reciprocal of the slope V-I characteristics of diode.

 

34. Compare avalanche and Zener breakdown.

 

35. Define diffusion capacitances.

It is defined as the rate of change of injected charge with applied voltage.

 

36. What are the applications of PN diode?

i. Rectifier

ii. Switch

iii. Clippers

iv. Clampers

v. Detector

 

37. Define Zener effect.

Due to heavy doping, the electric field at the junction will be very high even at low reverse voltage. The electrons in covalent bonds break away from the bonds. This is called as Zener effect.

 

38. What is Zener diode?

A diode which exhibits Zener effect is called Zener diode.

 

39. What are the applications of Zener diode?

i. Voltage regulators

ii. Zener limiters

iii. Over-voltage protectors

 

40. Define rectifier.

A rectifier is an electrical device that converts alternating current (ac) to direct current (dc).

 

41. What are the types of rectifier?

i. Half-wave rectifier

ii. Full-wave rectifier

 

42. What is half-wave rectifier?

It converts an ac voltage into a dc voltage during one half of the ac cycle only.

 

43. What is full wave rectifier?

It converts an ac voltage into pulsating dc voltage during both half cycles of the applied voltage.

 

44. Define Ripple factor.

The ratio of rms value of ac component to the dc component in the output is known as ripple factor.

 

45. Define efficiency.

Efficiency is defined as the ratio of dc output power to ac input power.

 

46. Draw the input and output characteristics of a transistor in CE configuration and mark the cutoff, saturation and active regions. (AUC May 2009)

 

47. State the advantage of Optocoupler. (AUC May 2009)

i. An optocoupler (or an optoelectronic coupler) is basically an interface between two circuits which operate at different voltage levels. The key advantage of an optiocoupler is the electrical isolation between the input and output circuits.

ii. They can replace relays and reed relays, giving much faster switching (2005 YEM speeds, no contact bounce, better reliability, and usually better electrical isolation except for special configurations. However relays have high current capability, higher output voltage, lower on resistance and offset voltage and higher off resistance.

iii. They can replace pulse transformers in many floating applications. Opto- isolators can transmit DC signal components and low frequency AC, whereas pulse transformers couple only the high frequency components, (800 and a latch is required to restore the DC information. Pulse transformers have faster rise time than phototransistor optocouplers.

 

48. Name the operating modes of a transistor. (AUC Nov 2010)

i. Active region

ii. Cut-off region

iii. Saturation region

Active Region: It is defined in which transistor function is biased in reverse direction and emitter function in forward direction.

Cutoff Region: The region in which the collector and emitter functions are both reverse biased.

Saturation Region: The region in which both the collector and emitter functions are forward biased.

 

49. Which of the BJT configuration is suitable for impedance matching applications? Why? (AUC May 2007)

The common emitter configuration produces the highest current and power gain of all the three bipolar transistor configurations. This is mainl;y because the input impedance is LOW as it is connected to a forward biased PN-junction, while the output impedance is HIGH as it is taken from a reverse biased PN- junction.

 

50. What is meant by thermal run away? (AUC May 2007)

The continuous increase in collector current due to poor-biasing causes the (110 temperature at collector terminal to increase. If no stabilization i.e., done, the collector leakage current also increases. This further increases the temperature. This action becomes cumulative and ultimately the transistor turns out. The self- destruction of an unstabilized transistor is known as thermal runaway.

 

51. Why is necessary to stabilize the operating point of transistor? (AUC Nov 2005)

In order to keep the transistor operation in Active region, it is necessary to stabilize the transistor. If not, the transistor may go out of active region. (Saturation or cutoff region).

 

52. Derive the relationship between a and B. (AUC Dec 2008)

 

53. Draw and explain input and output characteristics of a transistor CB configuration. (AUC May 2005)

 

54. What is pinch off voltage? (AUC Dec 2011)

It is the voltage at which the channel is pinched off, (i.e.,) all the free charges ls from the channel get removed.

 

55. List the comparison between CB,CE,CC amplifiers. (AUC Dec 2007)

 

56. State the biasing conditions required for the three regions of operations of a BJT. (AUC May 2004)

Two external voltage sources are used to establish the required bias conditions for active-mode operation. The voltage V_BE causes the p-type base to be higher in potential than the n-type emitter, thus forward-biasing the emitter- base junction. The collector-base voltage V_CB causes the n-type collector to be higher in potential than the p type base, thus reverse-biasing the collector-base junction.

 

57. Give any two differences between E-MOSFET and D-MOSFET. (AUC Dec 2011)

In E-MOSFEThas no conducting channel between two terminal, source terminal and gateterminal. Whereas in the D-MOSFET has conducting channel between these two terminal, source and gate.

 

58. Compare JFET with BJT. (AUC May 2010)

 

59. Define amplification factor in JFET. (AUC May 2010)

It is the product of drain resistance and transconductance μ = R_d x g_m

R_d - Drain resistance, g_m – Transconductance

 

60. What are the special features of FET? (AUC Apr 2009)

i. Input impedance is very high. This allows high degree of isolation between the input and output circuit.

ii. Current carriers are not crossing the junctions hence noise is highly reduced.

iii. It has a negative temperature co-efficient of resistance. This avoids the thermal runaway.

 

61. Define: (a) Pinch off voltage and (b) Amplification factor in JFET. (AUC Apr 2009)

It is the voltage at which the channel is pinched off, (i.e.,) all the free charges from the channel get removed.

Amplification factor is the product of drain resistance and trans conductance.

Μ = R_d × g_m

R_d - Drain resistance, g_m – Transconductance

 

62. Why FET is called unipolar device? (AUC Dec 2004)

The operation of FET depends upon the flow of majority carriers only either

hour or electrons and hence FET is said to be unipolar device.

 

63. Define R_d g_m and μ of JFET. (May 2005)

The change in the drain source voltage due to change in drain current with constant V_GS can be determined using the drain resistance r_d

The change in the drain current due to change in gate to source voltage can be determined using the transconductance factor g_m

 

64. What do you understand by pinch off voltages and cutoff voltages? (AUC May 2006)

It is the voltage at which the channel is pinched off, (i.e.,) all the free charges from the channel get removed.

 

65. List the important features of FET. (AUC Dec 2004)

i. Input impedance is very high. This allows high degree of isolation between the input and output circuit.

ii. Current carriers are not crossing the junctions hence noise is highly reduced.

iii. It has a negative temperature co-efficient of resistance. This avoids the thermal runaway.

 

REVIEW QUESTIONS

1. What is meant by donor impurity and acceptor impurity?

2. Draw the energy band diagram for conductors, semiconductors and insulators.

3. Explain in detail about extrinsic and intrinsic semiconductors.

4. Explain the construction and operation of PN diode in forward and reverse bias conditions.

5. Draw the energy band diagram of a PN diode.

6. Sketch the V-I characteristics of the PN diode.

7. What is diffusion capacitance of a semiconductor diode? Explain.

8. Explain in detail about the current components in a PN diode.

9. Explain about transition and depletion capacitance in detail and derive its expression.

10. Explain the breakdown mechanisms of a diode.

11. What are the applications of a diode?

12. Explain the construction and working of Zener diode.

13. How does a Zener diode operate as voltage regulator? Explain.

14. What do you mean by rectifier? How it is classified.

15. Explain the static and dynamic resistance of a diode under forward and reverse bias.

16. Discuss briefly the temperature dependence characteristics of a PN diode.

17. How PN diode acts as rectifier? Explain.

18. Explain half wave rectifier in detail and derive its efficiency.

19. Discuss in detail the operation of full wave rectifier and derive its efficiency.

20. Compare Half wave, Full wave and Bridge rectifiers.

21. What is Zener effect? Explain the V-I characteristics of Zener diode.

22. Derive the PN diode current equation.

23. Draw the circuit for determining the transistor common base characteristics and explain how the characteristics are measured and draw the graphs. (AUC DEC'11)

24. Explain the switching characteristics of transistor with neat sketch. (AUC NOV'10)

25. Describe the static input and output characteristics of a CB transistor with neat circuit diagram. (AUC NOV'10)

26. Draw and explain the input and Output characteristics of a BJT in CE configuration. (AUC APRIL'09)

27. (i) What are the factors against which an amplifier needs to be stabilized? (ii) Explain any one method of biasing a single stage BJT amplifier. (AUC MAY'04)

28. Explain the construction and working of enhanced MOSFET and depletion MOSFET. Draw the characteristics. (AUC DEC'11)

29. Explain how the transconductance of a JFET varies with drain current and gate voltage characteristics and transfer characteristics. (AUC MAY'10)

30. A JFET has the following parameters I_DDS = 32 mA, V_GS(off) = -8 Volts, V_GS = -4.5 Volts. Find the values of drain current. (AUC MAY'10)

31. (i) Explain the working of n-channel enhancement type MOSFET. Sketch its typical characteristics. (AUC MAY'09)

(ii) Explain the application of FET as a voltage variable resistor. (AUC MAY'09)