The transport of the charges in a semiconductor crystal under the influence of electric field i.e., by the drift current and a result of non-uniform concentration gradient i.e., by diffusion current
Drift Process:
Let us consider a semiconductor bar AB. The following sequence of events takes place when we apply electric potential to it.
(a)Conduction electrons are attracted towards end A and travel through the connecting wire into the positive terminal of the battery. At the same time, additional electrons leave the negative terminal of the battery and enter the semiconductor at B. Thus only electrons flow through the connecting wire in the anticlockwise direction.
( b ) Positive holes in the bar are attracted towards the end B. On reaching there, they are neutralized by the electrons entering the bar from the wire end near B. It is called a recombination process. At the same time, some valence electrons break their covalent bonds and enter into the connecting wire for going to the positive terminal of the battery. This is the charge generation process in which both electrons and holes are produced. Hence, we find that hoy appears at A move from A to B and then disappears at B. The net result is that there is a continuous flow of holes in a clockwise direction from A to B inside the bar.
( c ) There is the continuous generation and recombination of electrons and holes. However, there are no holes in the external circuit. They are produced inside the bar at point A and are neutralised at the other end B. Inside the semiconductor, the current has . two components i.e., Electron current le and Hole current In Total current I = I₂ + Ih
Diffusion Process:
In addition to conduction current, the transport of charges in a semiconductor may take place by means of diffusion. It is possible to have a nonuniform concentration of particles in a semiconductor. With reference to Fig . 1.8 the concentration of n electrons varies with distance x in the semiconductor. In an imaginary surface is drawn in a semiconductor the density of electrons immediately on one side of the surface is larger than the other side . The electrons in a random. motion as a result of their thermal energy. We may expect that in a given time interval more electrons will cross the surface from the FIG Diffusion Process side of greater concentration to the side of smaller concentration, then in the reverse direction.
This net transport of holes across the surface constitutes the current which is called diffusion current. The same phenomenon is also applicable to holes.
