Bias-Dependent Electron Velocity Extracted From N-Polar GaN Deep Recess HEMTs

Abstract

This article reports on the extraction of the electron velocity as a function of gate bias from N-polar GaN deep recess high-electron-mobility transistors (HEMTs) designed for mm-wave power amplification. Bias-dependent small-signal S-parameter measurements are used to obtain small-signal equivalent circuit parameters, which are applied to a transit delay model. The model accounts for fringing capacitance to arrive at an electron velocity associated with the transit of the physical gate length. A peak electron velocity of $1.4\times 10^{{7}}$ cm/s was obtained at a drain current of 700 mA/mm corresponding to a channel charge density of $0.3\times 10^{{13}}$ cm−2. At higher current, the velocity slowly decreased with the electron velocity crossing below $10^{{7}}$ cm/s at 1.8 A/mm. This behavior was found to be in good agreement with a previously proposed model based on optical phonon scattering at the source injection point. An analysis of the delay components is used to provide guidance for the factors influencing the device performance.