Overview

Additional friction due to Pauli constraint, channel self-heating, alloy scattering, and hot phonons is reconsidered.

Keywords: Two-dimensional channels, GaN, AlGaN, drift velocity; hot electrons; hot phonons.

1. Introduction

An undoped GaN high-electron-mobility transistor (HEMT) with a two-dimensional electron gas (2DEG) channel is a promising high-power active device 1. Transistor unity-gain cut-off frequency exceeds 150 GHz for short-gate devices2 ^,3. A linear dependence of the inverse cut-off frequency on the gate length yields an effective electron drift velocity. The extracted velocity ranges from 1 10 ⁷ cm/s to 1.75 10 ⁷ cm/s 1 ^,2 ^,3 ^,4 ^,5. The drift velocity decreases as the electron density increases5. Several times higher values follow from time-of-flight experiments on GaN p-i-n diodes6. Thus, drifting electrons suffer additional friction in biased HEMT channels. The following effects can be of importance at a high electron density: (i) 2DEG degeneracy, (ii) channel self-heating at a high density of current and a high bias, (iii) alloy scattering caused by an overlap of electron envelope function with that of random alloy potential, (iv) accumulation of non-equilibrium optical phonons (termed hot phonons). Our goal is to discuss recent advances in understanding of electron drift velocity limitation in nominally undoped 2DEG channels for high-power GaN HEMTs.

2. Pauli Constraint

Scattering events into occupied states are forbidden in a degenerate 2DEG channel. As a result, the electron drift velocity tends to increase.