Basic Electronics: 2 Marks Important Questions with Answers

CHAPTER 1: BASIC ELECTRONICS

PART A

TWO MARKS QUESTIONS AND ANSWERS

1. What is forbidden gap?

Forbidden gap is the energy gap between valance and conduction bands. Insulators, the gap will be more and for conductors, it is nill. For silicon, it is 1.1 eV and for Germanium, it is 0.7 eV

2. How is hole formed in a semiconductor?

"HOLE" represents absence of an electron. The mass of hole is zero which reveals that it is an imaginary particle. As the electron moves from its position hole is formed that is representing the vacancy created by electron is represented by hole. The charge of hole is positive which is opposite to that value of charge of an electron.

3. Distinguish between intrinsic and extrinsic semiconductors.

Intrinsic semiconductors are pure form of semiconductors, where as extrinsic semiconductors are the impure form of semiconductors. Extrinsic semiconductor are formed by doping. There are two types of semiconductors namely, N‒type semiconductor and P‒type semiconductor.

4. Why silicon is preferred over germanium in the manufacture of semiconductor devices.

As the knee voltage of silicon is higher (0.7) than the knee voltage of germanium (0.3) silicon will be stable for temperature variation than Germanium.

5. Give the cut in voltage in germanium diode and silicon diode.

Germanium diode: cut in voltage = 0.3 V

Silicon diode: cut in voltage = 0.6 V

6. Mentions two application of zener diodes.

(i) used as a constant voltage source

(ii) used as a voltage regulator

7. Compare Avalanche and Zener breakdown.

Zener Breakdown

(i) Breakdown occurs due to the heavily doped junction and applied strong electric field.

(ii) Breakdown occurs at lower voltage compared to avalanche breakdown.

(iii) Doping level is high.

Avalanche Breakdown

(i) Breakdown occurs due to the avalanche multiplication between the thermally generated ions. As a chain of collisions breakdown in resulting.

(ii) Breakdown occurs at comparatively higher voltages.

(iii) Doping level is relatively low.

8. Define the terms knee voltage and breakdown voltage with respect to diodes.

Knee voltage: It is the forward voltage of a PN diode at which the current through the junction starts increasing rapidly.

Breakdown voltage: It is the reverse voltage of a PN junction diode at which the junction breaks down with sudden rise in the reverse current.

9. Define peak inverse voltage. (PIV)

Peak inverse voltage is the maximum reverse voltage that can be applied to the PN junction without damage to the junction. If the reverse voltage across the junction exceeds to its peak inverse voltage, the junction may be destroyed due to excessive heat.

10. Define and explain maximum forward current.

Maximum forward current is the highest instantaneous forward current that a PN junction can conduct without damage to the junction. If the forward current in a PN junction is more than this rating, the junction will be damaged due to overheating.

11. Explain the following with reference to a PN diode: Forward resistance, Reverse resistance and cut in voltage.

Forward resistance

The resistance offered by a diode to forward bias is known as forward resistance.

Reverse resistance

The resistance offered by the diode to the reverse bias is known as reverse resistance. The reverse resistance is very high compared to the forward resistance. In germanium diodes the ratio of reverse to forward resistance is 4 × 10⁴ : 1 whereas for the silicon diode the ratio is 1× 10⁶:1

Cut in voltage: This is the voltage at which the forward bias curve abruptly increase from the smaller value. The cut in voltage of germanium is 0.3 V and for silicon it is 0.6 V.

12. Explain diode forward and reverse recovery times.

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 immediately fall to its steady‒state reverse voltage value. Until such time as the injected or excess minority‒carrier has dropped nominally to zero, the diode will conduct easily. This time is known as diode reverse Recovery time. Similarly diode forward recovery time corresponds to sudden reversal of external voltage in the reverse direction.

13. A 5V battery is connected across the two diodes connected in series opposing. Find the voltage drop across each diode at room temperature.

At the two diodes are connected in opposite direction, one of the diode will be reverse biased. Therefore no current will flow in the circuit hence no voltage drop will be there.

14. What is avalanche multiplication in PN junction diode?

Consider a thermally generated carrier which has acquired kinetic energy from the applied voltage. Now the carrier is moving fastly with high kinetic energy. During its travel it is colliding with some other ions hence causing the rupture in covalent bonds. Therefore, one carrier may generated number of carrier by collisions. The newly generate carriers may also acquire enough energy from the applied field. They can collide with crystal ions and generate new electrons pairs. This process is continuous and cumulative. It is known as avalanche multiplication.

15. What is junction capacitance?

Any variation of the charge within a P‒n diode with an applied voltage variation yields a capacitance. This capacitance related to the depletion layer charge in a p‒n diode is called the junction capacitance.

16. What is ideal diode? Draw its V‒I characteristics.

An ideal diode is a diode which has,

(i) No forward resistance i.e., R_f=0.

(ii) Therefore no voltage drop across it i.e., v_f = 0. It acts as a conductor.

(iii) Under reverse bias, Resistance is infinite. It acts as a open circuit.

In simple terms, an ideal diode allows current to flow in the forward direction without any resistance and completely prevent the flow of current in reverse direction.

17. Define power dissipation of a zener diode.

It is the product of reverse recovery current & the reverse voltage.

Power dissipation = (reverse recover current) × (reverse voltage)

18. Mention the advantages and disadvantages of zener diode.

advantages

(i) It is a heavily doped diode

(ii) Depletion layer will be thin

(ii) Consequently breakdown occurs at lower reverse voltage

(iv) Zener diode is operated at reverse biased condition

(v) It can be operated as a voltage regulator

(vi) Size is small & less space is occupied

(vii) Operation is speed

Disadvantages

(i) Limited power rating

(ii) The operation of the diode is temperature sensitive

19. How does a PN junction diode behave under forward and reverse biased condition?

A PN junction diode will behave as, closed switch under forward bias, and, open switch under reverse bias conditions.

20. Sketch the VI characteristics of a PN junction diode and mark various operations.

21. What are majority and minority carriers in a semiconductor?

P‒type Semiconductor

Holes are the majority carriers and electrons are minority carriers.

n‒type Semiconductor

Electrons are majority carriers and holes are minority carriers.

For a particular semiconductors, the larger number of charge carriers is called to be majority carriers and the smaller number of charge carrier is called to be minority carriers.

22. Define drift current and diffusion current?

Drift current: It is defined as the flow of electric current due to the motion of the charge carriers under the influence of an external electric field.

Diffusion current: In a semiconductor, the charge carriers have the tendency to move from the region of higher concentration to the region of lower concentration, of the same type of charge carriers. Thus the movement of charge carriers takes place which results in a current called diffusion current.

23. Write the difference between the PN junction diode and Zener diode?

PN Junction diode

(i) The PN Junction diode is a semi conductor diode which is formed when n type and p type semiconductor crystals are joined together.

(iii) It is operated in forward bias region as well as reverse bias region.

(iv) Application (a) As a switch in logic circuits (b) As a signal diode in communication circuits.

Zener diode

(i) It is also a silicon special PN Junction diode which differs from a rectifier diode in the sense operated in the reverse break down.

(iii) It is operated only in reverse bias region.

(iv) Application (a) As a voltage regulator (b) As a fixed reference voltage in transistor biasing circuits.

24. Write any two salient points on a p‒n junction.

• It is lightly doped diode

• During reverse biased condition, Avalanche breakdown occurs.

• Used only in forward bias condition as switch for rectifier circuits.

25. What is meant by Zener effect?

Zener effect

The zener breakdown mechanism is fundamentally different from avalanche breakdown. Zener breakdown occurs when the electric field in the deflection layer increases to the point where it can break covalent bonds and generate electron‒hole pairs. The electrons generated in this way will be swept by the electric field into N‒side and holes into P‒sides. Thus, these electrons and holes constitute reverse current across the junction that helps to supports the external current. In terms of an energy band structure, in this breakdown process an electron makes a transition from the valance band to the conduction band without the interaction of any other particle. Once the Zener effect starts a large number of carriers can be generated with a negligible increase in the junction voltage. In fact the zener process is quantum tunnelling. However, later on the breakdown in junction which may result from avalanche multiplication or tunneling was in general called Zener breakdown.

The maximum reverse bias potential that can be applied before entering the zener region is called the peak inverse voltage or peak voltage.

26. Why is zener diode used as a voltage regulator?

From the VI characteristics we can see that, zener diode has a constant negative voltage regardless of the value of current flowing through the diode and remains nearly constant even with large change in current. This ability to control it's voltage can be used to regulate or stabilise a voltage source against supply or load variations.

27. What are the other names of Zener diode?

• Backward diode

• Transient voltage suppression diode

• Voltage regulator

28. Define breakdown voltage.

The breakdown voltage of a diode is the minimum reverse voltage to make the diode conduct in reverse.

29. What are the applications of diode?

(i) Switches

(ii) Rectifiers

30. What are the applications of zener diode?

(a) Used as a constant voltage source

(b) Used as a voltage regulator

31. Write any two salient points on a p‒n junction.

When N‒type and p‒type semiconductor crystals are joined together a PN junction diode is formed.

N‒type semiconductor material has high concentration of free electrons while p‒type material has high concentration of holes.

It is used as a switches and rectifiers.

32. Define the two breakdown conditions in zener diode.

Avalanche Breakdown

As the applied reverse bias voltage increases, the field across the junction increases corresponding, then the Avalanche breakdown occurs at comparatively higher breakdown voltage.

Zener Breakdown

When the reverse bias field across the junction is sufficiently high, then the zener breakdown occurs at comparatively lower breakdown voltage.

33. Draw the characteristics of zener diode.

34. Draw the circuit diagram of half wave rectifier.