Charge Carriers in Intrinsic Semiconductors

 Charge Carriers in Intrinsic Semiconductors

Charge Carriers in Intrinsic Semiconductors :

In previous post (Intrinsic Semiconductor) we already knows that intrinsic semiconductor behaves as an insulator at absolute zero, temperature (in kelvin scale).

 

But in room temperature ( at 300K) valence electron of a semiconductor atom to move away from the influence of its nucleus. Thus a covalent bond is broken.

When this happen the electrons becomes free to move in the crystal.

This is shown in figure below

When an electron breaks a covalent bond and moves away, a vacancy is created in the broken covalent bond. This vacancy is called hole. Whenever a free electron is generated , a hole is created simultaneously. That is, free electrons and holes are always generated in pairs. Therefore , the concentration of free electrons and holes will always be equal in an intrinsic semiconductor. This type of generation of free electron-hole pairs is referred to as thermal generation.

Let us examine whether a hole has any charge associated with it.

The crystal is neutral. As soon as the electron-hole pair is generated, the electron leaves the covalent bond and moves away from it. Since, an electron is negatively charged, the site of a hole will be left with a net positive charge. Thus we say that is positive charge is associated with a hole or a hole is a positive charged.

 The hole too carries charge from one point to another. Although, strictly speaking, a hole is not a particle; for all practical purposes we can view it as a positive charged particle capable of conducing current. This concept of a hole as a positive charge particle merely helps in simplifying the explanation of current flow in semiconductors.

The following figure shows the generation of an electron-hole pair in crystal. The amount of energy required to break a covalent bond is 0.72 eV in case of germanium and 1.12 eV in case of silicon.

 Equivalently, we say that the energy needed for lifting an electron from the valence band to the conduction band is 0.72 eV for germanium and 1.12 eV for silicon. When the electron jumps the forbidden gap, it leaves a hole in the valence band as shown in above figure.

 

In silicon less number of electron-hole pair is generated as compared to germanium at room temperature because the the E_G is more in case of Silicon (1.12 eV).

The conductivity of silicon will be less than that of germanium at room temperature.