# Engineering Physics Questions and Answers – LED

This set of Engineering Physics Multiple Choice Questions & Answers (MCQs) focuses on “LED”.

1. A light emitting diode is _________
a) Heavily doped
b) Lightly doped
c) Intrinsic semiconductor
d) Zener diode

Explanation: A light emitting diode, LED, is heavily doped. It works under forward biased conditions. When the electrons recombine with holes, the energy released in the form of photons causes the production of light.

2. Which of the following materials can be used to produce infrared LED?
a) Si
b) GaAs
c) CdS
d) PbS

Explanation: GaAs has an energy band gap of 1.4 eV. It can be used to produce infrared LED. Various other combinations can be used to produce LED of different colors.

3. The reverse breakdown voltage of LED is very low.
a) True
b) False

Explanation: The reverse breakdown voltages of LEDs are very low, typically around 5 V. So, if access voltage is provided, they will get fused.

4. What should be the band gap of the semiconductors to be used as LED?
a) 0.5 eV
b) 1 eV
c) 1.5 eV
d) 1.8 eV

Explanation: Semiconductors with band gap close to 1.8 eV are ideal materials for LED. They are made with semiconductors like GaAs, GaAsP etc.

5. What should be the biasing of the LED?
a) Forward bias
b) Reverse bias
c) Forward bias than Reverse bias
d) No biasing required

Explanation: The LED works when the p-n junction is forward biased i.e., the p- side is connected to the positive terminal and n-side to the negative terminal.
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6. Which of the following would have highest wavelength?

a) A
b) B
c) C
d) D

Explanation: In the I-V characteristic of an LED, as the frequency increases, the voltage required to achieve the same current increases. Hence A would have the highest wavelength.

7. Increase in the forward current always increases the intensity of an LED.
a) True
b) False

Explanation: As the forward current is increased for an LED, the intensity of the light increases up to a certain maximum value. After that, the intensity starts decreasing.

8. Which process of the Electron-hole pair is responsible for emitting of light?
a) Generation
b) Movement
c) Recombination
d) Diffusion

Explanation: When the recombination of electrons with holes takes place, the energy is released in the form of photon. This photon is responsible for the emission of light.

9. What is the bandwidth of the emitted light in an LED?
a) 1 nm to 10 nm
b) 10 nm to 50 nm
c) 50 nm to 100 nm
d) 100 nm to 500 nm

Explanation: The bandwidth of the emitted light is 10 nm to 50 nm. Thus, the emitted light is nearly (but not exactly) monochromatic.

10. Which of the following is not a characteristic of LED?
a) Fast action
b) High Warm-up time
c) Low operational voltage
d) Long life

Explanation: The warm-up time required should be lower so that the lighting action can take place faster. This is one of the advantages LED have over incandescent lamps.

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