The effect of electromechanical coupling on the strain in AlGaN/GaN heterojunction field effect transistors

TL;DR

This paper investigates how electromechanical coupling affects strain calculations in AlGaN/GaN HFETs, showing that neglecting coupling can lead to significant errors, but the 2DEG screens this effect, validating the standard decoupled model.

Contribution

It provides an analytical and numerical analysis of electromechanical coupling effects on strain in AlGaN/GaN HFETs, demonstrating the screening effect of the 2DEG and validating simplified models.

Findings

Neglecting electromechanical coupling causes about 30% error in out-of-plane strain for Al fraction 0.3.

The 2DEG screens electromechanical effects, making the standard decoupled model reasonable.

Self-consistent Schrödinger-Poisson calculations support the analytical results.

Abstract

The strain in AlGaN/GaN heterojunction field-effect transistors (HFETs) is examined theoretically in the context of the fully-coupled equation of state for piezoelectric materials. Using a simple analytical model, it is shown that, in the absence of a two-dimensional electron gas (2DEG), the out-of-plane strain obtained without electromechanical coupling is in error by about 30% for an Al fraction of 0.3. This result has consequences for the calculation of quantities that depend directly on the strain tensor. These quantities include the eigenstates and electrostatic potential in AlGaN/GaN heterostructures. It is shown that for an HFET, the electromechanical coupling is screened by the 2DEG. Results for the electromechanical model, including the 2DEG, indicate that the standard (decoupled) strain model is a reasonable approximation for HFET calculataions. The analytical results are supported by a self-consistent Schr\"odinger-Poisson calculation that includes the fully-coupled equation of state together with the charge-balance equation.