Bias induced strain in AlGaN∕GaN heterojunction field effect transistors and its implications

Abstract

We report gate bias dependence of the charge due to piezoelectric polarization obtained by using a fully coupled formulation based upon the piezoelectric constitutive equations for stress and electric displacement. This formulation is significant because it fully accounts for electromechanical coupling under the constraint of global charge control. The coupled formulation results in lower charge due to piezoelectric polarization as compared to the uncoupled formulation for a given Al mole fraction. With increasing two dimensional electron gas concentration, that is, for gate biases greater than threshold, the compressive strain along the c axis in the barrier AlGaN layer increases with a concomitant increase of in-plane stress. Current collapse is correlated to the increase in source and drain resistances through their dependence upon surface charge. An alternate explanation of current collapse using local charge neutrality is also presented.