Drift Current and Diffusion Current

Drift Current and Diffusion Current

• In a semiconductor the flow of current is due to two actions namely drift and diffusion.

Drift Current

• When a voltage is applied to a semiconductor, the free electrons try to move in a straight line towards the positive terminal of the battery.

• The electrons, moving towards positive terminal collide with the atoms of semiconductor and connecting wires, along its way. Each time the electron strikes an atom, it rebounds in a random direction.

• But still the applied voltage make the electrons drift towards the positive terminal. This drift causes current to flow in a semiconductor, under the influence of the applied voltage.

• This current produced due to drifting of free electrons is called drift current and the velocity with which electrons drift is called drift velocity.

• Thus drift current means the flow of current due to bouncing of electrons from one atom to another, travelling from negative terminal to positive terminal of the applied voltage.

Key Point: The direction of conventional current is always opposite to the direction of drifting te to the electrons.

• This is shown in Fig. 1.11.1.

Diffusion Current

• This is the current which is due to the transport of charges occurring because of nonuniform concentration of charged particles in a semiconductor.

• Consider a piece of semiconductor which is nonuniformly doped. Due doped. Due to such nonuniform doping, one type of charge carriers occur at one end of a piece of semiconductor.

• The charge carriers are either electrons or holes, of one type depending upon the impurity used.

• They have the same polarity and hence experience a force of repulsion between them.

• The result is that there is a tendency of the charge carriers to move gradually i.e. to diffuse from the region of high carrier density to the low carrier density. This process is called diffusion.

• This movement of charge carriers under the process of diffusion constitutes a current called diffusion current. This is shown in Fig. 1.11.2.

• The diffusion current continues till all the carriers are evenly distributed throughout the material.

• A diffusion current is possible only in case of nonuniformly doped semiconductors while drift current is possible in semiconductors as well as conductors.

Key Point: The diffusion current exists without external voltage applied while drift current can not exist without an external voltage applied.

Review Question

1. Draw a diagram to illustrate drift current and diffusion current in a semiconductor material. Briefly explain.