Effect of hot phonon lifetime on electron velocity in InAlN/AlN/GaN heterostructure field effect transistors on bulk GaN substrates

We report on electron velocities deduced from current gain cutoff frequency measurements on GaN heterostructure field effect transistors (HFETs) with InAlN barriers on Fe-doped semi-insulating bulk GaN substrates. The intrinsic transit time is a strong function of the applied gate bias, and a minimum intrinsic transit time occurs for gate biases corresponding to two-dimensional electron gas densities near 9.3×1012 cm−2. This value correlates with the independently observed density giving the minimum longitudinal optical phonon lifetime. We expect the velocity, which is inversely proportional to the intrinsic transit time, to be limited by scattering with non equilibrium (hot) phonons at the high fields present in the HFET channel, and thus, we interpret the minimum intrinsic transit time in terms of the hot phonon decay. At the gate bias associated with the minimum transit time, we determined the average electron velocity for a 1.1 μm gate length device to be 1.75±0.1×107 cm/sec.