Free electron distribution in AlGaN/GaN heterojunction field-effect transistors

A detailed calculation of the free electron concentration and conduction and valence band edges of AlGaN/GaN heterojunction field-effect transistors is presented. The model is based on a self-consistent solution of the Schrödinger, Poisson, and charge balance equations and includes the effect of exchange correlation on the Coulomb interaction. It also includes surface acceptor and donor states populated according to Fermi–Dirac statistics. The piezoelectric and spontaneous polarization discontinuities across the material interfaces are rigorously taken into account. The influence of the polarization discontinuity on the magnitude of the charge in the two-dimensional electron gas is investigated. From charge conservation, it is shown that the polarization discontinuity does not behave as dopants in the same manner as substitutional impurities. Any free electrons within the structure must originate from some other source, either from the surface through surface donors, or from the bulk through unintentional n-type dopants. The present model allows for δ doping and multiple AlGaN/GaN interfaces.