Threshold voltage dependence on design parameters and layer uniformity in high electron mobility transistors

Abstract

A threshold voltage sensitivity analysis of high electron mobility transistors (HEMTs) based on a self-consistent model for the electron distribution is presented. The calculated sensitivity values are related to experimentally obtained uniformity data on 2-in. wafers grown by molecular-beam epitaxy (MBE). The effect of doping concentration, compensation, and thickness of the doped and undoped (Al,Ga)As layers and the mole fraction are considered. Enhancement mode transistors are generally found to be less sensitive than transistors of depletion type. The analysis brings out that the most important design parameters to control are the thickness of the doped (Al,Ga)As layer and its doping concentration (compensation). Changes in the Al mole fraction are found to have a negligible influence. The effects of varying growth conditions are estimated and possible ways of improving the threshold uniformity are discussed.