We have seen what is semiconductor? in our previous tutorial and today, we will have a look at types of Semiconductors. Semiconductors can be classified as:

• Intrinsic Semiconductor
• Extrinsic Semiconductor

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Intrinsic Semiconductor:

Definition:

• “A semiconductor in its extremely pure form is called intrinsic semiconductors.”

In this semiconductor, the conduction band is empty whereas the valence band is filled totally. When the temperature is increased, some heat energy can be supplied to it. So the electrons from the valence band moves towards the conduction band by leaving the valence band.

Examples:

Pure Ge and Si are Intrinsic Semiconductors. They belong to the fourth group of the periodic table. Their electrical behavior extremely depends upon its purity.

In the above figure the electronic configuration of both the elements that they have four electrons in their outermost shell or valence shell. When the temperature is increased, the electrons gain more thermal energy and thus break free from their shell. It creates a vacancy in the bond between the atoms. The hole is then occupied by a free electron, as a result of which the latter vacant position becomes a hole and the former becomes a neutral position. In an intrinsic semiconductor, the number of free electrons is equal to the number of holes.

Extrinsic Semiconductor:

Definition:

• “An impure form of semiconductor material is called the extrinsic semiconductor.”

Extrinsic semiconductors are semiconductors that are doped with specific impurities. The impurity modifies the electrical properties of the semiconductor and makes it more suitable for electronic devices such as diodes and transistors.

While adding impurities, a small amount of suitable impurity is added to pure material, increasing its conductivity by many times.

“The process of adding impurities deliberately is termed as doping and the atoms that are used as an impurity are termed as dopants.”
Extrinsic semiconductors are further divided into two types of Semiconductors:

• N-type semiconductors
• P-type semiconductors

N-type semiconductors:

Definition:

“The semiconductor which is formed by adding pentavalent impurity to a pure semiconductor is called N-type semiconductors.”

Example:

An atom belonging to the fifth group such as antimony (Sb), phosphorus (P), or arsenic (Ar) has five valence electrons. When it is added to Ge and Si having four electrons in their outermost orbit then four valence electron-like (P) form covalent bonds with Si while the fifth valence electron provides a free electron.

P-type Semiconductors:

Definition:

“The semiconductor which is formed by adding trivalent impurity to a pure semiconductor is called P-type semiconductors.”

Example:

An atom belonging to the third group (trivalent0 such as aluminum (Al), boron (B), gallium (Ga), or Indium (I) has three valence electrons. When it is added to Ge and Si having four electrons in their outermost orbit then three valence electrons like aluminum (Al) form three covalent bonds with Ge and Si atoms.

In n-type semiconductor, when a pure semiconductor like Ge and Si is doped by pentavalent impurity then, four electrons out of five valence electrons bonds with the four electrons of Ge or Si. The fifth electron of the dopant is free. Thus, the impurity atom donates a free electron for conduction in the lattice and is called “Donar“.

These types of Semiconductors are the main building blocks of PNP & NPN transistor.

So, the number of free electron increases by the addition of an impurity, the negative charge carrier’s increase.

In p-type semiconductor, when a pure semiconductor is doped with a trivalent impurity then, the three valence electrons of the impurity bonds with three of the four valence electrons of the semiconductor. This makes an absence of electron (hole) in the impurity. These impurity atoms which are ready to accept bonded electrons are called “Acceptors“.

So, with the increase in the number of impurities, holes (the positive charge carriers) are increased.