Bipolar Junction Transistors (BJT): Two Marks Important Questions and Answers

Electron Devices

Chapter 4: Bipolar Junction Transistors

Two Marks Questions with Answers

1. What is transistor?

Transistor is a three terminal device : Base, emitter and collector, can be operated in three configurations common base, common emitter and common collector. According to configuration it can be used for voltage as well as current amplification.

2. What are the terminals present in a transistor?

Three terminals: emitter, base, collector.

3. State the two types of transistors.

There are two types of transistors : Unipolar junction transistor and Bipolar junction transistor.

4. Why transistor is also called bipolar junction transistor ?

The current conduction in bipolar transistor is because of both the types of charge carriers, holes and electrons. Hence this is called Bipolar junction transistor.

5. Why transistor called a current controlled device ?

In BJT output current is controlled by input current and hence it is a current controlled device.

6. Mention the types of BJT.

The types of BJT are : 1. n‒p‒n type 2. p‒n‒p type

7. State the two junctions in the transistor.

A transistor has two p‒n junctions. One junction is between the emitter and the base and is called the emitter base junction or simply the emitter junction J_E. The other junction is between the base and the collector and and is called collector‒base junction or simply collector junction J_C.

8. What is doping?

The process by which impurities are added to a pure semiconductor is called doping.

9. Explain about the doping in transistor.

The base region is very thin and lightly doped. The emitter and collector are heavily doped. But the doping level in emitter is slightly greater than that of collector.

10. State the basic currents of the transistor.

The basic currents of the transistor are: base current, collector current and emitter current. They are related by equation I_E = I_B + I_C

11. Define α_dc and β_dc

 α_dc is the fraction of I_E that crosses over from emitter to the collector region.

 β_ac is defined as the ratio of the collector current to the base current.

12. Give the relation between α_dc and β_dc

 β = α / (1‒α)

 and α = β / (1+β)

13. Draw the neat circuit configuration of CB.

14. What is the characteristics of a transistor?

Input characteristics: It is the relationship plotted graphically between input voltage and input current while keeping output voltage as constant.

Output characteristics: It is the relationship plotted graphically between the output voltage and output current while keeping input current as constant.

15. State the three operating regions of transistor.

The three operating regions of transistor are Active, cut‒off and saturation.

16. State the relation between operating region and biasing of transistor junctions.

The table gives the relation between operating region and biasing biasing of transistor junctions.

17. When does a transistor act as a switch?

The transistor acts as a switch when it is operated at either cutoff region or saturation region.

18. What is early effect?

When reverse bias voltage V_CB increases, the width of depletion region also increases, which reduces the electrical base width. This effect is called as 'Early effect' or 'Base width modulation'.

19. What is a multi‒emitter transistor ?

A multi‒emitter transistor is a type of Bipolar Junction Transistor (BJT) that has more than one emitter terminal. Each emitter acts as a separate input, allowing the transistor to accept multiple logic signals simultaneously. It is mainly used in Transistor‒Transistor Logic (TTL) circuits.

20. What is the primary application of a multi‒emitter transistor?

The primary application of a multi‒emitter transistor is in the input stage of TTL logic gates, especially NAND gates. It replaces multiple diodes used in earlier logic families (like DTL), providing faster switching speed and compact circuit design.

21. Why is a multi‒emitter transistor preferred in TTL circuits ?

It is preferred because each emitter can serve as a separate input terminal, enabling multi‒input logic operations within a single transistor. This reduces the number of components, minimizes propagation delay and switching speed in digital ICs.