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

摘要

The strain in AlGaN/GaN heterojunction field-effect transistors (HFETs) isexamined theoretically in the context of the fully-coupled equation of statefor piezoelectric materials. Using a simple analytical model, it is shown that,in the absence of a two-dimensional electron gas (2DEG), the out-of-planestrain obtained without electromechanical coupling is in error by about 30% foran Al fraction of 0.3. This result has consequences for the calculation ofquantities that depend directly on the strain tensor. These quantities includethe eigenstates and electrostatic potential in AlGaN/GaN heterostructures. Itis shown that for an HFET, the electromechanical coupling is screened by the2DEG. Results for the electromechanical model, including the 2DEG, indicatethat the standard (decoupled) strain model is a reasonable approximation forHFET calculataions. The analytical results are supported by a self-consistentSchr\"odinger-Poisson calculation that includes the fully-coupled equation ofstate together with the charge-balance equation.