Waves in semiconductors with nonconstant mobility

It is pointed out that the usual assumption of field-dependent mobility is incorrect when diffusion is included in the current equation. In general, the mobility depends on the carrier temperature, which should be included as a separate variable. However, at low frequencies and long wavelengths, the carrier temperature, and hence the mobility, may be taken as local and instantaneous functions of the average carrier velocity, but not of the electric field. This modification is shown to have important implications on the small-signal stability.