ELECTRONICS

Q.8: what are the majority carrier and minority carrier of p – type and n - type material?
Ans: Majority carrier and minority carrier of p – type and n - type material: In the intrinsic state, the number of free electrons in Ge or Si is due only to those few electrons in the valence bands that have acquired sufficient energy from thermal or light sources to break the covalent bond or to the few impurities that could not be removed. The vacancies left behind in the covalent bonding structure represent our very limited supply of holes. In an n – type material, the number of holes has not changed significantly from this intrinsic level. The net result therefore is that the number of electrons for outweighs the number of holes. For this reason –

In an n – type material the electron is called the majority carrier and the hole is the minority carrier.

Fig: N-type Fig: P – type

For p – type material the number of holes or outweighs the number of electron. Therefore –

In a p – type material the hole is the majority carrier and the electron is the minority carrier.

Q.9: What do you mean by bias?

Ans: Bias: The term bias refers to the application of an external voltage across the two terminals of the device to extract a response. In no – bias situation there is no external voltage applied that is applied voltage is zero and current is zero amperes.

Q.10: Describe semiconductor diode/P-N junction with diagram or the following condition –

1. No biasing


2. Forward biasing


3. Reverse biasing.

Ans: No biasing: Under no – bias condition any minority carriers (holes) in the n – type material find themselves within the depletion region and they can pass quickly into the p – type material for the greater attraction of the layer of negative ions and the less is the opposition offered by the positive ions in the depletion region of the n – type material. Depletion region

P-type N-type

Fig: No bias condition

The majority carriers of the n – type material overcome the attractive forces of the layer of positive ions in the n – type material and the shield of negative ions in the p – type material to migrate into the area beyond the depletion region of the p – type material. The number of majority carriers is so large in the n – type material but there are a small number of majority carriers with sufficient kinetic energy to pass through the depletion region into the p – type material. The same type of discussion can be applied to the majority carriers (holes) of the p – type materials. As a result, the current under no bias conditions is zero.

Reverse bias: If an external potential is applied across the p – n junction such that the positive terminal is connected to the n – type material and the negative terminal is connected to the p – type material makes the diode reverse bias. As a result, the numbered of uncovered positive ions in the depletion region of the n – type material will increase due to the large number of free electrons drawn to the positive potential of the applied voltage. Depletion region

P-type N-type

Fig: Reverse bias condition

For same region the number of uncovered negative ions will increase in the p – type material. As a result the width of the depletion region increase and established a great barrier for the majority carriers to overcome and effectively reducing flow to zero.

The number of minority carriers entering the depletion region will not change resulting in minority – carrier flow vectors of the same magnitude indicated no applied voltage.

Forward bias: A forward bias or “on” condition is established by applying the positive potential to the p – type material and the negative potential to the n – type material. Depletion region

Fig: Forward bias condition

The application of forward bias potential will “pressure” electrons in the n – type and holes in the p – type material to recombine with the ions near the boundary and reduce the width of the depletion region. The resulting minority carrier flow of electrons from the p – type material to the n – type material has not changed in magnitude but the reduction in the width of the depletion region has resulted in a heavy majority flow across the junction. An electron of the n – type material now “sees” a reduced barrier at the junction due to the reduced depletion region and a strong attraction for the positive potential applied to the p – type material. The depletion region will continue to decrease in width until a flood of electrons can pass through the junction, resulting in an exponential rise in current in the forward bias.