Diffusion Current

Diffusion Current

• It is seen that in conductors current flow is due to the free electrons. When conductor is subjected to an external voltage, free electrons move from negative to positive terminal with a steady velocity constituting a current. Such a current is called drift current which is due to drifting of free electrons under the externally applied voltage.

• In addition to the drift current, there may exist an additional current due to the transport of charges in a semiconductor. Such an additional current is due to the phenomenon called diffusion. This is the characteristic of semiconductors and cannot be observed in conductors. The current due to the diffusion is called diffusion current. Basically it is due to nonuniform concentration of charged particles in a semiconductor. 

Key Point : Diffusion is observed in nonuniformly doped semiconductors and not in the conductors.

• Consider a p-type semiconductor bar which is nonuniformly doped as shown in the Fig. 1.3.1. Due to nonuniform doping, the holes are large in number on one side while less in number on other side. Due to this there is high carrier concentration area on one side while low carrier concentration area on other side. This creates concentration gradient in a semiconductor.

• In p-type, there are large number of holes on one side which are similarly (positively) charged particles. Due to this there exists a force of repulsion between them and holes start moving from higher concentration to lower concentration area, to attain the uniform concentration.

• Such a movement of charge carriers, due to the concentration gradient in a semiconductor is called process of diffusion. When charge carriers move, the current is constituted in a bar. This current due to the diffusion is called diffusion current.

Key Point : Nonuniform doping creates concentration gradient, due to which diffusion of charge carriers exists.

• The concept of diffusion current is shown in the Fig. 1.3.2.

• There can exist such a diffusion in a n-type semiconductor if there is nonuniform doping and hence concentration gradient existing in it. In n-type, due to diffusion, free electrons move to constitute diffusion current.

Key Point: Such a diffusion plays an important role in understanding the theory of p-n junction.

• Both drift current as well as diffusion current occur in the semiconductor devices like p-n junction diode.

Review Question

1. What is diffusion current ?