NONEQUILIBRIUM PHASE-TRANSITION AND CURRENT FILAMENTS IN EXTRINSIC SEMICONDUCTORS

A phenomenological model for charge transport in extrinsic semiconduct ors is presented. For shallow impurities, large saturated impact-ioniz ation coefficient and sufficiently low lattice temperature, the curren t-field characteristics is S-shaped (SNDC) due to the interplay betwee n impact ionization and scattering of carriers from acoustic phonons, ionized and neutral impurities. The present model provides and explana tion of the experimentally observed disappearance of SNDC above a crit ical lattice temperature. In the region of negative differential condu ctivity where a current-controlled uniform state is unstable, current filaments are calculated numerically.