Under the condition of thermal equilibrium, the electrons and holes are uniformly distributed in the crystal and in the absence of an external stimulus their average velocity is zero and no current flows through the crystal. This is equally true for an intrinsic or an extrinsic semiconductor. An external agent can disturb equilibrium, causing charge carriers’ random motion to become directional, resulting in current flow. Electric fields and concentration gradients are examples of such disturbances.
Drift Current: When an electric field E is applied, charge carriers gain directional motion in addition to thermal motion, producing drift current.
Drift current occurs only when external electric field is present across the solid. Although electrons and holes move in opposite directions, the direction of conventional current flow due to both the carriers is in the same direction.
Diffusion Current: In semiconductors, current can flow even without an external electric field. When there is a spatial variation in carrier density (a concentration gradient), charge carriers move directionally, creating a diffusion current until uniform distribution is achieved (as shown in below Fig) .
A concentration gradient can be induced by applying heat or light locally. For example, in a p-type semiconductor, if light or heat is applied at one end, additional electron-hole pairs are generated, increasing carrier concentration at that end. The resulting difference causes carriers to diffuse from higher to lower concentration regions, restoring equilibrium. Since carriers are charged, their movement generates diffusion current. The diffusion current is proportional to the concentration gradient (rate of change of carrier concentration per unit length). If electrons move left to right, the resulting current flows right to left (negative x-direction).
