Transistor NPN and PNP Working - UNSOLVED PRACTICE SET
Chapter: Semiconductor Electronics | Topic: Transistor NPN and PNP Working
TRANSISTOR NPN AND PNP WORKING - UNSOLVED PRACTICE SET
Topic: Transistor NPN and PNP Working
Multiple Choice Questions
Q1. A transistor has:
- Two p-n junctions and two semiconductor layers
- Two p-n junctions and three semiconductor layers
- One p-n junction and two semiconductor layers
- Three p-n junctions and four semiconductor layers
Q2. In an NPN transistor, the emitter is:
- n-type and heavily doped
- p-type and lightly doped
- n-type and lightly doped
- p-type and heavily doped
Q3. In a properly biased transistor:
- The emitter-base junction is reverse biased and the collector-base junction is forward biased
- The emitter-base junction is forward biased and the collector-base junction is reverse biased
- Both junctions are forward biased
- Both junctions are reverse biased
Q4. The base of a transistor is:
- Heavily doped and thick
- Lightly doped and thin
- Moderately doped and thick
- Heavily doped and thin
Q5. In a transistor, the emitter current I_E, base current I_B, and collector current I_C are related by:
- I_E = I_B + I_C
- I_E = I_B โ I_C
- I_E = I_B ร I_C
- I_E = I_C / I_B
Q6. The current gain ฮฑ of a transistor is defined as:
- I_C / I_B
- I_C / I_E
- I_E / I_C
- I_B / I_E
Short Answer Questions
Q7. Draw the structure of an NPN transistor and label the emitter, base, and collector.
Q8. Why is the emitter heavily doped, the base lightly doped and thin, and the collector moderately doped with a large area?
Q9. Explain the working of an NPN transistor as a current amplifier. Draw a diagram showing the flow of electrons.
Q10. What is the difference between ฮฑ (current gain in common base) and ฮฒ (current gain in common emitter)? Derive the relationship between them.
Q11. Why does a transistor not work if the emitter-base junction is reverse biased?
Q12. Draw the circuit symbol for both NPN and PNP transistors and indicate the direction of conventional current flow in each.
Long Answer Questions
Q13. Describe the construction and working of an NPN transistor. Explain the flow of charge carriers and derive the relationship I_E = I_B + I_C.
Q14. Explain why the base of a transistor must be thin and lightly doped for the transistor to function as an amplifier. What would happen if the base were thick or heavily doped?
Q15. Compare NPN and PNP transistors in terms of:
(i) Construction
(ii) Biasing requirements
(iii) Direction of current flow
(iv) Majority charge carriers
Numerical & Application-based Problems
Q16. In a transistor, the emitter current is 10 mA and the base current is 0.2 mA.
(a) Calculate the collector current.
(b) Calculate the current gain ฮฑ.
(c) Calculate the current gain ฮฒ.
(d) Verify the relationship between ฮฑ and ฮฒ.
Q17. A transistor has ฮฒ = 100. The collector current is 5 mA.
(a) Calculate the base current.
(b) Calculate the emitter current.
(c) Calculate ฮฑ.
(d) If the base current is increased by 20 ฮผA, calculate the change in collector current.
Q18. In your school electronics lab, a student is experimenting with transistors.
(a) She measures the currents in an NPN transistor circuit: I_E = 12 mA, I_C = 11.8 mA. Calculate I_B, ฮฑ, and ฮฒ. Is this a good transistor? Explain why.
(b) The student then builds a simple circuit to light an LED using a transistor as a switch. The LED requires 20 mA. If ฮฒ = 50, calculate the minimum base current needed to saturate the transistor.
(c) She connects a microphone (which produces a small AC signal of 10 mV) to the base of a transistor amplifier. Explain how the transistor amplifies this signal and why the output is taken from the collector.
(d) A classmate asks why transistors are called 'bipolar' junction transistors. Explain the meaning of 'bipolar' and contrast it with unipolar devices like field-effect transistors.
(e) In India, the first transistor was manufactured at BEL (Bharat Electronics) in the 1970s. Today, India imports most chips but is pushing for domestic semiconductor manufacturing. Explain why transistor fabrication requires ultra-clean facilities and precise doping control.