Semiconductors Devices PYQs

 

Semiconductor Devices - Important Questions & Answers

Q.1 (x) CE Amplifier

In common emitter amplifier, current gain is 80 and emitter current is 9 mA. Find base current.

Given: β = 80, IE = 9 mA

IE = IB + IC
β = IC / IB = 80
IC = 80IB

9 = IB + 80IB
9 = 81IB
IB = 9/81 = 1/9 mA

Answer: (d) 1/9 mA

Q.2 (vi) Phase Difference in CE Amplifier

In a Common Emitter (CE) amplifier, the output voltage is 180° out of phase with the input voltage.

Q.16 Light Emitting Diode (LED)

Definition:

A Light Emitting Diode (LED) is a p-n junction diode that emits light when forward biased.

Working:

1. LED is forward biased.
2. Electrons and holes move towards the junction.
3. Recombination occurs near the junction.
4. Energy is released as light.
5. The LED glows.

March 2022

Q.2 (iv) Write the Boolean expression for X-OR gate.

Y = A̅B + AB̅

Q.28 What is a Rectifier?

A rectifier is an electronic device that converts AC into DC.

Half Wave Rectifier Working:

1. During positive half cycle, diode is forward biased.
2. Current flows through load resistance.
3. During negative half cycle, diode is reverse biased.
4. No current flows.
5. Output is pulsating DC.

July 2022

Q.21 Define αDC and βDC. Obtain relation.

Common Base Current Gain:

αDC = IC / IE

Common Emitter Current Gain:

βDC = IC / IB

Relation:

α = β / (1 + β)

β = α / (1 - α)

February 2023

Q.1 (iii)

Y = (A + B)̅ represents NOR Gate.

Q.1 (vii)

LED emits visible light when holes and electrons recombine.

Q.20 Working of NPN Transistor in Common Base Configuration

1. Emitter-base junction is forward biased.
2. Collector-base junction is reverse biased.
3. Electrons move from emitter to base.
4. Very few electrons recombine in base.
5. Most electrons reach collector.
6. Collector current flows.

July 2023

Q.1 (viii)

Solar cell works on the principle of Photovoltaic Action.

Q.2 (viii)

Logic gate with one input and one output is NOT Gate.

Q.17 Half Wave Rectifier

Input Waveform: Complete AC Wave

Output Waveform: Positive half cycles only (Pulsating DC)

February 2024

Q.9 What is LED?

LED is a p-n junction diode that emits light when forward biased.

Symbol: Diode symbol with two arrows pointing outward.

Q.17 Define αDC and βDC.

Use the same definitions and relation given above.

July 2024

Q.1 (iv)

LED emits light when holes and electrons recombine.

Q.2 (vi)

A logic gate is an electronic circuit that performs a logical operation on binary inputs and produces a binary output.

Q.9 Transformer

Working:

1. Transformer works on mutual induction.
2. AC current in primary coil produces changing magnetic flux.
3. Flux links secondary coil.
4. EMF is induced in secondary coil.
5. Electrical energy is transferred without direct electrical connection.

Q.21 Zener Diode as Voltage Regulator

Voltage Regulation: Maintaining constant output voltage despite changes in input voltage or load current.

Working:

1. Zener diode is connected in reverse bias.
2. Operates in breakdown region.
3. Zener voltage remains constant.
4. Load voltage remains constant.

Specimen Paper - 1

Q.1 (i)

If switches are connected in series, operation is AND.

Q.27 Zener Diode

Zener diode maintains constant voltage by operating in reverse breakdown region.

Specimen Paper - 2

Q.1 (ii)

Output Frequency = 2 × Input Frequency
= 2 × 50
= 100 Hz

Answer: (c) 100 Hz

Q.2 (iii)

NAND Gate:
Y = (AB)̅

Q.22 Half Wave Rectifier

Same explanation as given above.

Specimen Paper - 3

Q.1 (iv)

Photodiode is used in Optocoupler.

Q.2 (ii)

αDC = IC / IE
= 6.20 / 6.28
= 0.987

Answer: αDC = 0.987

Q.15 Output Characteristics of CE Configuration

1. Shows variation of collector current (IC) with collector-emitter voltage (VCE).
2. Base current (IB) is kept constant.
3. Collector current increases slightly with VCE.
4. Higher IB gives higher IC.
5. Contains Cut-off, Active and Saturation regions.
6. Transistor works as amplifier in Active Region.