Drift and Diffusion current
The flow of current through a semiconductor material is normally referred to as one of the two types.
Drift current
• If an electron is subjected to an electric field in free space it will accelerate in a straight line form the –ve terminal to the + ve terminal of the applied voltage.
• However in the case of conductor or semiconductor at room temperature, a free electrons under the influence of electric field will move towards the +ve terminal of the applied voltage but will continuously collide with atoms all the ways as shown in figure
• Each time, when the electron strikes an atom, it rebounds in a random direction but the presence of electric field doesnot stop the collisions and random motion. As a result the electrons drift in a direction of the applied electric field.
• The current produced in this way is called as Drift current and it is the usual kind of current flow that occurs in a conductor.
Diffusion current
• The directional movement of charge carriers due to their concentration gradient produces a component of current known as Diffusion current.
• The mechanism of transport of charges in a semiconductor when no electric field is applied called diffusion. It is encountered only in semiconductors and is normally absent in conductors.
• With no applied voltage if the number of charge carriers (either holes or electrons) in one region of a semiconductor is less compared to the rest of the region then there exist a concentration gradient.
• Since the charge carriers are either all electrons or all holes they sine polarity of charge and thus there is a force of repulsion between them.
• As a result, the carriers tend to move gradually or diffuse from the region of higher concentration to the region of lower concentration. This process is called diffusion and electric current produced due to this process is called diffusion current.
• This process continues until all the carriers are evenly distributed through the material. Hence when there is no applied voltage, the net diffusion current will be zero.
