Electronic Devices And Circuits MCQ Questions & Answers | ECE

Power diodes are used in applications that require handling high currents and voltages.
Germanium diodes have a lower breakdown voltage compared to silicon diodes.
This means they are not suitable for high-power applications where high voltages are involved.
Silicon diodes, on the other hand, have a much higher breakdown voltage and can handle significantly larger currents.
Therefore, power diodes are generally made from silicon because of its ability to withstand higher power levels.
Hence, the correct answer is Option A: Silicon diodes.

Question: Why isn't silicon used to make light-emitting diodes (LEDs)?

Explanation:
LEDs work by emitting light when electrons fall from a higher energy level to a lower one. This happens most efficiently in materials with a direct band gap.

Silicon is an indirect band gap semiconductor. This means that when an electron drops to a lower energy level, it doesn't directly emit a photon (light particle). Instead, it often loses energy through other processes (like vibrations in the crystal lattice) which don't produce light.

Therefore, silicon isn't suitable for LEDs because its indirect band gap makes it inefficient at converting electrical energy into light. Materials like gallium arsenide (GaAs), which have a direct band gap, are much better choices for making LEDs.

The correct answer is A: An indirect band gap semiconductor

Light-dependent resistors (LDRs) are special resistors whose resistance changes depending on the amount of light shining on them.

To understand why, we need to know a bit about semiconductors.

Semiconductors are materials that are neither good conductors (like metals) nor good insulators (like rubber). Their conductivity can be changed.

Intrinsic semiconductors have a moderate number of charge carriers (electrons and holes) that allow them to conduct electricity, but this is relatively low.

Lightly doped semiconductors have a small amount of impurities added to increase their conductivity. This makes them slightly better at conducting.

Highly doped semiconductors have a large amount of impurities added, making them very good conductors.

LDRs work because light creates more charge carriers in the semiconductor material. More carriers mean better conductivity, and thus lower resistance. To make this effect strong, LDRs are usually made from lightly doped semiconductors. The small amount of doping provides the basis for a significant change in resistance when light strikes the material.

Therefore, the correct answer is C: Lightly doped semiconductor.