PN Junction Diode Interview Questions and Answers:

1. 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.

2. Why silicon is usually preferred over germanium for fabrication of semiconductor devices.

Ans. The silicon semiconductor devices have, in general, higher PIV and current ratings and wider temperature range than germanium semiconductor devices and, therefore, silicon is preferred over germanium in the manufacture of semiconductor devices.

3. What does the arrowhead represent in a schematic symbol of a P-N junction?

Ans. The arrowhead in the schematic symbol of a P-N junction indicates the direction of conventional current flow when the diode is forward biased.

4. How diode acts as a rectifier?

Ans. Rectifier is a device which converts the sinusoidal ac voltage into unidirectional pulsating voltage. Diode is an electronic device consisting of two elements known as cathode and anode. Since in a diode electrons can flow in only one direction i.e., from cathode to anode, the diode provides the unilateral conduction necessary for rectification. This is true for diodes of all types—vacuum, gas-filled, crystal or semiconductor diodes.

5. In a semiconductor diode P-side is grounded and N-side is applied a potential of –3 V. Will the diode conduct? Explain.

Ans. The p-n diode connected with p-side grounded and with n-side to –ve potential of 3 V, the diode will conduct, because n-side is at negative potential w.r.t. positive side.

6. What is a junction diode?

Ans. Diode is a two-terminal device which conducts exponentially in one direction and has a poor conduction in reverse direction. It will be an ideal case if its conduction in one direction is like a perfect conductor and in other direction as perfect insulator.

7. What is the main difference between the characteristic of a simple switch and those of an ideal diode?

Ans. The important difference between the characteristic of a simple switch and those of an ideal diode is that an ideal diode acts like an automatic switch—the switch is closed when the diode is forward biased and is opened when reverse biased.

8. Name the two types of reverse breakdowns which occur in a P-N junction diode?

Ans. Avalanche breakdown and Zener breakdown.

9. Name the breakdown mechanism in a lightly doped P-N junction under reverse biased condition.

Ans. Avalanche breakdown.

10. Name the breakdown mechanism in a heavily duped P-N junction under reverse biased condition.

Ans. Zener breakdown.

11. Define Zener breakdown.

Ans. Zener breakdown occurs in a P-N diode with heavy doping (P-type semiconductor moderately doped and N-type heavily doped) and it is due to very large electric field inside the depletion region.

12. What is Avalanche breakdown?

Ans. The minority carriers, under reverse-biased conditions, flowing through the junction acquire a kinetic energy which increases with the increase in reverse voltage. At a sufficiently high reverse voltage (say 5 V or more), the kinetic energy of minority carriers becomes so large that they knock out electrons from the covalent bonds of the semiconductor material. As a result of collision, the liberated electrons in turn liberate more electrons and the current becomes very large leading to the breakdown of the crystal structure itself. This phenomenon is called the avalanche breakdown. The breakdown region is the ‘knee’ of the characteristic curve. Now the current is not controlled by the junction voltage but rather by the external circuit.

13. Clearly distinguish between Zener breakdown and Avalanche Breakdown.

Ans. Zener breakdown occurs in a P-N diode with heavy doping (P-type semiconductor moderately doped and N-type heavily doped) and it is due to very large electric field inside the depletion region.

Avalanche Breakdown occurs in lightly doped P-N junctions. It is due to cumulative breakdown of the bonds due to fast moving carriers inside the depletion region. The breakdown region is the “knee” of the characteristic curve.

14. What is reverse saturation current?

Ans. Reverse current of a diode is due to minority carriers and is caused when the diode is reversed biased. The reverse current is constant, independent of the applied reverse bias, consequently it is called the reverse saturation current.

15. Is reverse saturation current of a junction diode independent of reverse bias voltage?

Ans. Yes.

16. Germanium is more temperature dependent than silicon. Why?

Ans. Because the reverse saturation current in case of a germanium diode is approximately 1,000 times larger.

17. What is the effect of temperature on the reverse saturation current of a diode?

Ans. Reverse saturation current, theoretically, increases by 8 per cent per °C for silicon and 11 per cent per °C for germanium. But from experimental data it is found that the reverse saturation current increases 7 per cent per °C for both silicon and germanium. This is because a surface leakage current component of reverse saturation current is independent of temperature. Since (1.07)¹⁰ ≈ 2.0, the reverse saturation current approximately doubles for every 10°C. rise in temperature.

18. Explain the effect of temperature on the V-I characteristics of the diode.

Ans. Reverse saturation current I₀ and volt equivalent of temperature V_T increase with the increase in temperature. I₀ doubles for every 10°C rise in temperature. This causes lowering of threshold or cutin voltage. With the increase in temperature, forward characteristic becomes more ideal. In reverse bias, the breakdown voltage increases with the increase in temperature.

19. What is static resistance of a diode?

Ans. The static or dc resistance of a diode is the resistance offered by it to the direct current. It is defined as the ratio of the diode voltage and current at the point of interest and is not sensitive to the shape of V-I characteristic curve. It decreases with the increase in diode current or voltage.

20. Define dynamic resistance of a P-N junction diode in forward biased condition.

Ans. The resistance offered by a P-N junction diode to the changing forward current is defined as the dynamic resistance.

It may be given as AC or dynamic resistance,

r = Small change in forward voltage/Small change is forward current

Thus ac resistance of a diode is sensitive to the shape of v-i characteristic curve in the region of interest. It decreases for higher level of diode current or voltage.

21. Explain what do you understand by the expression “linearization of a nonlinear device”.

Ans. A nonlinear device, such as a diode, has a nonlinear voltage-current characteristic. In a diode V and I are related by equation I = I₀ (e^{V/η VT} – 1)

Diode shows nonlinearity until the threshold or cutin voltage V_T is reached. Voltage-current characteristics of the diode follow a linear relationship after it has attained the cutin voltage, just as for a resistance of very low magnitude. It means that diode exhibits nonlinearity for lower voltages (V < V_T) but linearity for voltages exceeding V_T.

22. What is step graded junction?

Ans. A junction is said to be step graded if there is an abrupt change from acceptor ion concentration on the P-side to donor ion concentration on the N-side such as alloyed or fused junctions.

23. What is linear graded junction?

Ans. A junction is said to be linearly graded if the charge concentration varies gradually with the distance in its transition region such as a growth junction.

24. Explain why a P-N junction possesses capacitance?

Ans. When a P-N junction is reverse biased, the depletion region acts like an insulator or dielectric material while the P- and N-type regions on either side have a low resistance and act as the plates. Thus P-N junction may be considered a parallel plate capacitor. The junction capacitance is termed as space-charge capacitance or transition capacitance or depletion region capacitance and is denoted by C_T.

When a P-N junction is forward biased, a capacitance, which is much larger than the transition capacitance, comes into play. This type of capacitance is called the diffusion capacitance, C_D. We know that for a forward-biased P-N junction, the potential barrier is reduced. Now the electrons from N-side enter the P-side and holes from P-side enter into the N-side. These charge carriers diffuse away from the junction and progressively recombine. The density of charge carriers is high near the junction and decays exponentially with distance. Thus a charge is stored on both sides of the junction when forward biased. It is observed that the amount of stored charge varies with the applied potential as for a true capacitor. It is convenient to introduce an incremental capacitance, called diffusion or storage capacitance C_D given by the equation C_D = dQ/dV where dQ represents the change in number of minority carriers stored outside the depletion region when a change in voltage across diode, dV, is applied.

25. What are the two types of capacitances across P-N junction? Which of these is more important in case of forward bias?

Ans. In a P-N junction, there are two capacitive effects which are to be considered. Thus, there are two types of capacitances namely transition or depletion region capacitance and diffusion or storage capacitance. In the case of a forward biased diffusion capacitance is much larger than transition capacitance (more than a million times).

26. Differentiate between transition capacitance (C_T) and diffusion capacitance (C_D) of a P-N junction diode.

Ans. When a P-N junction is reverse biased, the depletion region acts like an insulator or dielectric material while the P- and N-type regions on either side have a low resistance and act as the plates. Thus P-N junction may be considered a parallel plate capacitor. The junction capacitance is termed as space-charge capacitance or transition capacitance or depletion-region capacitance and is denoted by C_T.

When a P-N junction is forward biased, a capacitance, which is much larger than the transition capacitance, comes into play. This type of capacitance is called the diffusion capacitance C_D.

27. How does diffusion capacitance C_D vary with de diode current?

Ans. In a P-N junction diode, the diffusion capacitance is directly proportional to the diode forward current.

28. How does transition capacitance vary with junction voltage in a P-N junction diode?

Ans. Transition capacitance is inversely proportional to the square root of the junction voltage.

29. What is the relation between the transition capacitance and reverse bias voltage in a PN diode?

Ans. Transition capacitance C_T = dQ/dV where dQ is the increase in charge due to increase in voltage dV.

Since depletion region increases with the increase in reverse-bias potential, the resulting transition capacitance C_T decreases. Transition capacitance C_T is inversely proportional to the square root Of the junction voltage.

30. What is reverse recovery time?

Ans. If the external voltage is suddenly reversed in a diode circuit which has been carrying current in the forward direction the diode current will not fall to its steady-state reverse value. The diode will continue to conduct for a time, known as diode reverse recovery time t_rr, until excess minority-carrier density has dropped down to zero.

Reverse recovery time is the maximum time for the device to switch from on to off.

31. Is ‘reverse recovery time’ due to majority carriers or the minority carriers?

Ans. Reverse recovery time is due to large number of minority carriers in both of the P- and N-regions.

32. How can the switching speed of a diode be improved?

Ans. Switching speed of a diode can be improved by adding efficient recombination centres to the bulk material. For silicon diodes Au doping can serve this purpose.

A diode switching time can also be improved by making the lightly doped neutral region shorter than a minority carrier diffusion length.

33. Write down four applications of a diode.

Ans. An ideal P-N junction diode is a two terminal polarity sensitive device that has zero resistance (i.e., diode conducts) when it is forward biased and infinite resistance (i.e., diode does not conduct) when reverse biased. Because of this property the diode finds use in many applications as enumerated below :

• As rectifiers in dc power supplies.
• In demodulation or detector circuits.
• In clamping networks employed as dc restorers in TV receivers and voltage multipliers.
• In clipping circuits used as wave shaping circuits in computers, radars, radio and TV receivers.
• As switches in digital logic circuits.