Theoretical Evaluation of the Effects of Isolation-Feature Size and Geometry on the Built-In Strain and 2-D Electron Gas Density of AlGaN/GaN Heterostructures

Using a commercial self-consistent Poisson-Schrödinger solver with the built-in possibility of allowing elastic energy minimization, the strain and the sheet charge density induced at the pseudomorphically grown Ga-face Wurtzite AlGaN/GaN heterojunctions are evaluated in the context of 3-D simulation of heterostructure field-effect transistor (HFET) epilayers etched into a variety of isolation-feature sizes and geometries. Through these studies and in the presence of surface states, the extent of the relevance of strain minimization in the vicinity of the unconstrained boundaries of isolation features of different degrees of roundness and perimeter-to-area ratio to threshold-voltage engineering is assessed. Although it is demonstrated that threshold-voltage shift caused by this induced strain minimization is smaller than the amount of shift levied by the depleting effect of the negatively charged states on the sidewall facets, it is shown that the reduction of the isolation-feature size is capable of substantially reducing the average trace of the stress tensor across such heterointerfaces. Considering the importance of this factor to the long-term reliability of AlGaN/GaN HFETs, especially when undergoing self-heating at high-power levels, use of alternative isolation features such as small islands of a lateral area less than 1000 nm
²
is proposed as a solution.