Engineering Physics

8.7 Drift and diffusion currents

Often, we come across drift and diffusion currents in semiconductors. The drift current occurs due to an applied electric field on a semiconductor and the diffusion current occurs whenever carrier concentration gradient exists in the material. In a semiconductor, both currents are explained separately in the following way.

(a)  Drift current

Let us consider a semiconductor material of length ‘l’ at some (room) temperature T K. At this temperature, the free electron and hole concentrations are n and p, respectively. The free electrons and holes possess random motions so that they do not possess any resultant velocity in any direction inside the crystal. When a voltage ‘V’ is applied across the semiconductor, then ...