Anisotropy of the Conductivity ofn-Type Germanium at High Electric Fields

Current density vs electric field has been measured up to 10 000 V/cm in $n$-type germanium at 77° and 297°K for the field in the [100], [110], and [111] crystallographic directions. At 77°K for 100 V/cm the drift velocity in the [100] direction is 60% greater than in the [111] direction. By 10 000 V/cm, this difference (the longitudinal anisotropy) has decreased from its maximum value at 1000 V/cm to 15%. The [110] curve lies between the other two. This anisotropy can be explained in terms of the effective mass anisotropy and population changes of the conduction band valleys oriented differently with respect to the field. The drift velocity of a single valley and the change of intervalley scattering rate as a function of field are deduced. The latter increases by a factor of 30 over the range of applied field. At 297°K negligible anisotropy is found.