Drift Current and Diffusion Current in Semiconductor:
The two basic processes which cause electrons and holes to move in a semiconductor are :
- drift, which is the movement caused by electric fields, and
- diffusion, which is the flow caused by variations in the concentration, i.e., concentration gradients.
Such gradients can be caused by a non-homogeneous doping distribution, or by injection of a quantity of electrons or holes into a region.
Drift Current in Semiconductor:
When a steady electric field E volts/metre is applied to a metal, the electrons move to the positive terminal of the applied voltage. In their way, they continually collide with the atoms and rebound in a random fashion, as illustrated in Fig. 6.8. Each collision being inelastic i.e., the electrons lose some kinetic energy.
After the collision, the electrons are accelerated and gain certain component of velocity in the direction opposite to that of applied electric field (- E) and lose their energy at the next collision. Thus, the applied electric field does not stop collisions and random motion but makes the electrons to drift towards the positive terminal. Consequently, the electrons gain average drift velocity v in the direction opposite to that of the applied electric field. It is observed that the drift velocity v is proportional to the applied electric field E, i.e.,
where
- μe is called the electron mobility and is expressed in m2/V-s.
This steady-state drift velocity is superimposed on the random motion of free electrons caused by thermal agitation. This steady flow of electrons in one direction caused by the applied electric field constitutes an electric current, called the drift current.
Diffusion Current in Semiconductor:
A second type of current, called the diffusion current, also exists in a semiconductor. The diffusion of charge carriers is as a result of a gradient of carrier concentration (i.e., the difference of carrier concentration from one region to another). In this case concentrations of charge carriers (either electrons or holes) tend to distribute themselves uniformly throughout the semiconductor crystal. This movement continues until all the carriers are evenly distributed throughout the material. This type of movement of charge carriers is called the diffusion current.
Diffusion Current in Semiconductor occurs when charge carriers diffuse from a point of concentration, to spread uniformly throughout the volume of a piece of material, as illustrated in Fig. 6.9.
Both Drift Current and Diffusion Current occur simultaneously in a semiconductor device.