Bipolar Junction Transistors (BJTs) Interview Questions and Answers:

1. Explain why BJT is a polar device.

Ans. Since both of the charge carriers (electrons and well as holes) are involved in current flow through a BJT (may be either N-P-N or P-N-P), these devices are called the bipolar devices.

2. 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.

3. What is the significance of arrowhead in the transistor symbol?

Ans. Arrowhead is always marked on the emitter. The direction indicates the conventional direction of current flow (from emitter-to-base in case of P-N-P transistor and from base-to-­emitter in case of N-P-N transistor). Generally, no arrowhead is marked for collector since its reverse leakage current is always opposite to the direction of emitter current.

4. What is the need for biasing a transistor?

Ans. Necessity of Biasing. For most of the applications, transistors are required to operate as linear amplifiers (i.e., to amplify output voltage as a linear function of the input voltage). To achieve this, it is necessary to operate the transistor over region of its characteristic curves which are linear, parallel and equi-spaced for equal increments of the parameters. Such an operation can be ensured by proper selection of zero signal operating point and limiting the operation of the transistor over the linear portion of the characteristics. For proper selection of zero signal operating point, proper biasing i.e., application of dc voltages at emitter-to-base junction and collector-to-base junction is required.

If the transistor is not biased properly, it would work inefficiently and produce distortion in the output signal.

5. What is the biasing condition of base-emitter and collector-base junction in the active region of a CB BJT configuration?

Ans. In the active region of a common base BJT configuration, emitter-base junction is forward biased and collector base junction is reverse biased.

6. Which of the transistor currents is always the largest? Which is always the smallest? Which two currents are relatively close in magnitude?

Ans. The emitter current I_E is always the largest one. The base current I_B is always the smallest. The collector current l_C and emitter current I_E are relatively close in magnitude i.e., I_C ≈ I_E.

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

Ans. Because silicon transistor has smaller cutoff current I_CBO, small variations in I_CBO due to variations in temperature and high operating temperature as compared to those in case of germanium type.

8. Why is size of collector kept maximum in BJT?

Ans. The potential hill (bend in the Fenni level) between the base and collector region is very steep. Specifically the electrons must lose a considerable energy as they move from the base region to the collector region. Typically the electrons give up their energy in the form of heat, therefore, the collector region must be capable of dissipating this heat energy. It is for this reason that the size of the collector is kept maximum in BJT.

9. The thickness of base is typically smaller than emitter and collector. Why?

Ans. 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.

10. Why emitter is always forward biased?

Ans. Emitter is always forward biased w.r.t. base so as to supply majority charge carriers to the base.

11. Why collector is always reverse biased w.r.t. base?

Ans. Collector is always reverse biased w.r.t. base so as to remove the charge carriers away from the base-collector junction.

12. What are three regions of operation of a transistor?

Ans. The three regions of operation of a transistor are:

(i) Cutoff region, (ii) Active region and (iii) Saturation region.

13. Can a transistor be obtained by connecting two semiconductor diodes back to back?

Ans. No. Because in case of two discrete back to back connected diodes there are four doped regions instead of three and there is nothing that resembles a thin base region between an emitter and a collector.

14. In the active region, as temperature increases the current in BJT increases. Explain.

Ans. In BJT we know that I_C = βI_B + (1 + β)I_CO

where I_C is collector current, I_B is base current, β is current gain and I_O is leakage current.

I_CO increases with the increase in temperature in BJT, for 1°C rise in temperature I_CO increases by 7% and for 10°C rise in temperature I_CO doubles. β also increases with the increase in temperature. So collector current I_C increases in active region with the increase in temperature in BJT.

15. Indicate whether the β-value of a BJT increases or decreases with the increase in the values of the following parameters: (i) base width (ii) minority carrier lifetime in the base region. (iii) temperature (iv) collector current (v) collector voltage.

Ans. The β-value of a BJT

• decreases with the increase in base width.
• increases with the increase in minority carrier lifetime in the base region.
• increases with the increase in temperature.
• increases with the increase in collector current.
• increases with the increase in collector voltage.

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

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

17. Which transistor configuration CC, CB and CE is suitable for amplifier and why?

Ans. CE configuration is widely used as an amplifier because its current, voltage and power gains are quite high and the ratio of output impedance and input impedance is quite moderate.

18. Why is CE configuration preferred for cascaded amplifiers?

Ans. CE configuration, because of its moderate output to input impedance ratio, is preferred for cascaded amplifiers.

19. Why CC configuration is called a voltage buffer? What is its other name?

Ans. Because of its high input impedance and low output impedance, the common collector circuit finds wide application as a buffer amplifier between a high impedance source and low impedance load, that is why it is called a voltage buffer. Its other name is emitter follower.

20. Which configuration of BJT is used for impedance matching and why?

Ans. Common-collector configuration is used for impedance matching because it has relatively high input impedance and low output impedance.

21. What are the main purposes for which a CC amplifier may be used?

Ans. Because of its high input impedance and low output impedance, the common collector circuit finds wide application as a buffer amplifier between a high impedance source and low impedance load.

22. What is Early effect?

Ans. The modulation of the effective base width by the collector voltage is known as Early effect. An increase in collector voltage increases the space charge width at the output junction diode and thus the effective base width W_b is reduced.

23. Elucidate three consequences of the Early effect in bipolar junction transistor.

Ans. Three consequences of the Early effect in BJT are as follows:

1. There is less chance for recombination within the base Hence α and β increase with increasing collector-base voltage |V_CB|.
2. The concentration gradient of minority carriers is increased within the base, and consequently, the current of minority carriers injected across the emitter junction increases with increasing reverse collector voltage.
3. For extremely large collector-base voltage, effective base width W_b may be reduced to zero resulting in a phenomenon known as punch through.

24. Explain punch through effect?

Ans. When on increasing collector voltage effective base width is reduced to zero, the emitter barrier becomes smaller than normal value V₀ – |V_EB| because the collector voltage has reached through the base region. This lowering of emitter-junction voltage causes an excessively large emitter current, thus placing an upper limit on the magnitude of collector voltage. This phenomenon is known as punch through.

25. What do you understand by collector reverse saturation current? In which configuration does it have a greater value?

Ans. When input current (I_E in case of CB configuration and I_B in case of CE configuration) is zero, collector current l_C is not zero although it is very small. In fact this is the reverse leakage current or collector reverse saturation current (I_CBO or simply I_CO in CB configuration and I_CBO in CE configuration). In case of CE configuration it is much more than that in case of CB configuration.

26. What do you mean by operating point?

Ans. Quiescent point is a point on the dc load line which represents V_CE and I_C in the absence of ac signal and variations in V_CE and I_C take place around this point when ac signal is applied.

27. Why point contact transistors are not commercially used?

Ans. Because of poor reliability and reproducibility.

28. What is the major difference between the epitaxial mesa transistor and the diffused mesa transistor?

Ans. The major difference between the epitaxial mesa transistor and diffused mesa transistor is that an epitaxial layer is added on the original collector substrate in epitaxial mesa transistor.

29. What limits the maximum current a transistor is capable of handling?

Ans. The junction area of the collector region limits the maximum current a transistor can handle. Large power ratings are obtained from larger junction areas. More is the surface area, more is the effective heat dissipation and more is the magnitude of current, a transistor can handle.

30. Why heat sinks are used in power amplifiers?

Ans. Heat sink is a specially designed metal sheet over which the power transistor is mounted so that it dissipates heat more effectively and protects the power transistor from overheating. The idea behind the heat sink is to increase the area of contact with the atmosphere. It is to be noted that the ability of any heat sink to transfer heat to the surroundings depends upon its material, volume, area, shape, contact between case and sink and movement of air around the sink.