Current oscillations and negative differential conductivity in n-type germanium

The existence of several types of high-frequency majority carrier current instabilities in [100] and [110] oriented samples of n-type germanium has led to a renewed interest in the conductivity of germanium in a strong electric field. We review the experimental situation with regard to these instabilities and show how their properties can be understood in terms of bulk negative differential conductivity, which has been shown to occur in this material for temperatures below about 120°K. The present state of understanding of the origin of this BNDC effect in terms of the band structure and scattering mechanisms in germanium is summarized.