Types of Extrinsic Semiconductor

Depending on the type of dopants we use, there are two different types of extrinsic semiconductors formed:

• N-type extrinsic semiconductor
• P-type extrinsic semiconductor

N-type semiconductor

It is formed when pentavalent impurities were added to the semiconductor. It is also known as donor impurity.

When any dopant atom is added to a semiconductor crystal, like silicon, it replaces the silicon atom and makes a bond with another silicon atom, when we add a pentavalent impurity, five out of four electrons are only able to make the bond with their neighboring silicon atom, but that one extra electron is not able to make any bond, and it freely moves in the entire semiconductor crystal. As a result of which, the conductivity of semiconductors increases. As we see by adding a pentavalent impurity to the semiconductor, it donates an electron in the crystal, and that is why it is also known as a donor impurity atom.

Examples of pentavalent dopant atoms are Phosphorus, Arsenic Antimony, etc.

In N type semiconductor, the majority charge carriers are electrons and minority charge carriers are holes.

Condition in N type semiconductor : N_e >>> N_h

Therefore the current in N type semiconductor is due to electrons only, I_total ≅ I_e.

And the conductivity is also due to electrons only σ_total ≅ σe

N_e ≅ N_D where N_D is Donar atom density

The block diagram of an N-type semiconductor:

The energy level diagram of an N-type semiconductor:

b) P-type semiconductor

It is formed when trivalent impurities(Boron family) were added to the semiconductor. It is also known as acceptor impurity. When any dopant atom is added to a semiconductor crystal, like silicon, it replaces the silicon atom and makes a bond with another silicon atom, and when we add a trivalent impurity, all three electrons make bonds with the neighboring silicon atom and a vacancy is created in the semiconductor crystal known as a hole. All the neighboring electrons try to fill this vacancy, resulting in a creation of a new vacancy at their site. This is how the movement of the hole takes place. As a result of which, the conductivity of semiconductors increases. As we see by adding a trivalent impurity to the semiconductor, it accepts an electron from the crystal, and that is why it is also known as an acceptor impurity atom.

Examples of trivalent dopant atoms are Boron, Aluminium, Indium, etc.

In P type semiconductor, the majority charge carriers are holes and minority charge carriers are electrons.

Condition in P type semiconductor : N_h >>> N_e

Therefore the current in P type semiconductor is due to holes only, I_total ≅ I_h.

And the conductivity is also due to holes only σ_total ≅ σ_h

N_h ≅ N_A where N_A is Acceptor atom density

The block diagram of a P-type semiconductor:

The energy level diagram of a P-type semiconductor:

As we all know, based on conductivity, we divide the material into three categories: Conductor, Semiconductor, and Insulator. In which the conductivity of the conductor is highest, insulators have the lowest conductivity, and semiconductors have intermediate conductivity. The reason behind this is the number of free electrons present in the crystal of each material. If we talk about the conductivity of semiconductors, two different types of semiconductors are classified based on the number of electrons and holes present in the crystal: Intrinsic and Extrinsic semiconductors. In this article, we will discuss these semiconductors, especially extrinsic semiconductors, their properties, formation, needs, and various applications.