Abstract

AlGaN/GaN heterostructure field-effect transistors (HFETs) are strong candidates for high-power and high-frequency applications. Even in the absence of doping, thanks to high polarization fields, often a two-dimensional electron gas (2DEG) of unprecedented concentrations forms at these heterojunctions. Control over this carrier induction process is crucial in achieving normally-off field-effect transistors (i.e., transistors of zero standby power consumption). One way to achieve this is through polarization engineering. Mesa-isolation geometry seemingly offers interesting avenues to reduce the piezoelectric polarization at the heterointerface, and as a result means for polarization engineering.
Using a Poisson-Schrödinger self-consistent solver, the effect of strain on the sheet charge density is investigated in the context of one-, two- and three-dimensional simulations of AlGaN/GaN heterostructures. Properties of the two-dimensional electron gas are detailed and the influences of Aluminum mole fraction, AlGaN barrier thickness, GaN cap layer inclusion are investigated. The carrier confinement in the 2DEG is explored in the case of two-dimensional version of the simulations. Through these studies, the effect of shrinking the size of the mesa on lowering the 2DEG concentration is confirmed.
Through performing three-dimensional simulations, the effects of cross-sectional geometry on the average sheet charge density and the threshold voltage are presented. It is shown that as the perimeter-to-area ratio is increased, the carrier concentration decreases, and the threshold voltage becomes less negative. Via these studies, the degree of effectiveness of geometry as means for polarization engineering is, for the first time, theoretically quantified.