Anisotropic Transient and Stationary Electron Velocity in Bulk Wurtzite GaN

The theoretical calculation of transient electron velocity overshoot in wurtzite
c
-axis GaN indicates a higher transient overshoot peak for transport in the basal plane ( Gamma-
M
and Gamma-
K
) than along the growth direction (Gamma-
A
). Characteristic rise times for the transient overshoot peak are found to be shorter for transport along the
c
-axis. Stationary electron velocity is significantly larger for transport oriented in the basal plane than along the
c
-axis. No significant anisotropy is observed, however, in either the transient or steady-state electron velocity as a function of field orientation within the basal plane itself. The higher peak transient and stationary velocities in the basal plane are directly attributable to the anisotropy of the electronic dispersion, which exhibits lower effective mass along the Gamma-
M
and Gamma-
K
directions and greater nonparabolicity along the Gamma-
A
direction.