WAVE INSTABILITY IN SEMICONDUCTORS WITHOUT NEGATIVE DIFFERENTIAL CONDUCTIVITY

A traveling-wave instability at finite critical wave number is found i n extrinsic semiconductors at the onset of impact ionization. A condit ion for this instability is a strong increase of the carrier-generatio n rate as a function of the electric field, e.g., due to a sufficientl y low ionization energy of the impurities. The linear instability of t he uniform steady state is discussed for a simple model, and traveling waves are shown to exist by solving numerically the basic partial dif ferential equations.