Semiconductors: Overview, Questions, Preparation

We can classify Semiconductors into two

• Intrinsic Semiconductor
• Extrinsic Semiconductor

Intrinsic Semiconductors:

Intrinsic semiconductors are pure semiconducting materials like silicon and germanium. At absolute zero temperature, they have a very low number of charge carriers (electrons and holes) due to the absence of thermal energy. When heated, electrons are excited from the valence band to the conduction band, creating electron-hole pairs.

Extrinsic Semiconductors:

Extrinsic semiconductors are semiconductors which are doped with a small number of impurity atoms. semiconductors that are doped with specific impurities

Extrinsic Semiconductors can be divided into two. 

• n-type Semiconductor
• p-type Semiconductor

Consider a pure semiconductor doped with a trivalent impurity (B, Al, In, Ga). The three valence electrons of the impurity bond with three of the four valence electrons of the semiconductor. This causes an absence of electron in the impurity. These impurity atoms which are ready to accept bonded electrons are known as Acceptors. With an increase in the number of impurities, holes (the positive charge carriers) are increased. Hence, it is called a p-type semiconductor. Properties of p-type semiconductors are below.

• A p-type semiconductor is an intrinsic semiconductor doped with Boron or Indium.
• The hole density is much greater than the electron density in a p-type semiconductor
• The holes are the majority of carriers in p-type semiconductors.
• The minority carriers in a p-type semiconductor are Electrons.
• The acceptor energy level of the p-type is close to the valency bond and away from the conduction band.

Consider a pure semiconductor (Silicon or Germanium) is doped by pentavalent impurity (P, As, Sb, Bi), then four electrons out of five valence electrons bond with the four electrons of Ge or Si. The fifth electron of the dopant is free. Then the impurity atom donates a free electron for conduction in the lattice and is called Donar. Since the number of free electrons increases with the addition of an impurity, the negative charge carriers increase. So, it is called an n-type semiconductor. Properties of n-type semiconductors are below.

• n-type semiconductor is an intrinsic semiconductor doped with phosphorus or antimony as impurity.
• The electron density is much greater than the hole density in n type semiconductor.
• The electrons are the majority of charge carriers in n-type semiconductors.
• Minority carriers in a n type semiconductor are the holes.
• The donor energy level of n-type is close to the conduction band and away from the valency band.

Following are some properties of Semiconductors.

• Semiconductors show a negative temperature coefficient of resistance. 
• Semiconductors act as insulators at zero kelvin. When the temperature is increased it acts as a conductor.
• The resistivity of a semiconductor is more than a conductor but less than an insulator.
• When impurities are added, the conductivity of the semiconductors increases. The process of adding impurities to semiconductors is known as doping.

PN Junction

A PN junction is formed by bringing together a P-type and an N-type semiconductor. At the junction, electrons from the N-type material recombine with holes from the P-type material, creating a depletion region without charge carriers. This region acts as a barrier to the flow of current in one direction (reverse bias) and allows current in the other direction (forward bias).

Semiconductor Diode

A semiconductor diode is a two-terminal device formed by a PN junction. It allows the flow of current in one direction (forward bias) while blocking it in the other direction (reverse bias). It is used in rectification, signal clipping, and voltage regulation.

Transistor

Transistors are three-layer semiconductor devices (NPN or PNP) used as amplifiers, switches, and signal modulators. They consist of an emitter, base, and collector regions. The base current controls the current flow between the collector and emitter.

Integrated Circuits (ICs)

ICs are miniaturized electronic circuits consisting of thousands to millions of transistors and other components on a single chip. They revolutionized electronics by making devices smaller, more reliable, and efficient.

Semiconductor Materials

Silicon (Si) and Germanium (Ge) are the most commonly used semiconductor materials due to their electrical properties and availability.