Hot-phonon-induced velocity saturation in GaN

In highly polar semiconductors with electron densities typically found in heterostructure field-effect transistors (HFETs), transport cannot be described without taking hot phonons into account. Here we describe a simple analytical model applied to the case of bulk GaN, taking the nonparabolicity of the conduction band into account, and show that the production of longitudinal optical (LO) phonons reduces the mobility and causes the drift velocity to saturate at a value around 107 cm/s, depending on the density of electrons. Transfer of electrons to higher valleys is expected to be delayed to much higher fields than commonly predicted. The effect of possible hot products of the LO decay is also considered. We relate the results for bulk material to the situation in HFETs, in which real-space transfer is inhibited, by considering the effect of spatial spreading of the channel electrons.