10.5 : 半導体の基礎

Carrier transport in semiconductors generates current, through drift and diffusion mechanisms.

Drift current arises when an external electric field causes charged particles to accelerate between collisions.

Considering the electrons, the additional velocity added to the electrons' thermal motion is called the drift velocity. It can be obtained by equating the momentum lost during collisions and the momentum gained from the applied field.

The proportionality factor-mobility- characterizes electron and hole transport due to drift.

The drift current densities for both carriers can be expressed as the product of charge density, mobility, and electric field intensity. The sum of these two gives the total drift current density. Here, the term in parentheses gives conductivity.

Diffusion current results from the random thermal motion of carriers from high to low carrier density regions.

The diffusion current density equals the product of carrier flux and proportionality constant, which includes elementary charge and diffusion coefficient. Its ratio with mobility gives the Einstein relation.

If both an electric field and concentration gradient are present, both the drift and diffusion current contribute to the total current.