Question

Consider the following statements A and B and identify the correct answer:
A) Germanium is preferred over silicon in the construction of Zener diodes.
B) Germanium has higher thermal stability than silicon in the construction of zener diodes.
A) Both A and B are true.
B) Both A and B are false.
C) A is true but B is false.
D) A is false but B is true.

Answer 1

Hint: Zener diode is a reverse-biased P-N junction diode operated in the breakdown region. Zener diodes operating in low Zener voltage will have a sharp, highly doped p-n junction. In this case, reverse conduction occurs due to quantum tunneling in the space between the p and n junction. This is called the Zener effect. Zener diodes operating at higher voltage involve avalanche breakdown.

Complete step by step solution:
Zener diode is a reverse-biased diode. It operates in the breakdown region. Due to the heavy doping, the width of the depletion layer is very narrow. When a reverse bias is applied, a very strong electric field is developed across the thin depletion layer.

The breakdown of a germanium diode is not sharp enough in reverse bias. Germanium does not give a constant voltage after $5V$. Silicon has a better breakdown in reverse bias. This shows that silicon is preferred over germanium for the construction of Zener diodes. Hence we can say that the first statement is false.
It is known that silicon has better thermal stability than germanium. This proves that the second statement is also false.

The answer is option (B), Both A and B are false.

Note: On applying reverse bias, a very strong electric field is formed across the narrow depletion layer. This will break the covalent bonds and an extremely large number of electrons and holes are produced. This is called the Zener breakdown. This reverse breakdown voltage is called the Zener breakdown voltage. This will also give rise to a reverse saturation current called the Zener current. Zener current is independent of the applied voltage. Zener diodes can be used as voltage regulators.