Influence of surface states on the two-dimensional electron gas in AlGaN/GaN heterojunction field-effect transistors

The electron transfer into the two-dimensional electron gas (2DEG) of AlGaN/GaN heterojunction field-effect transistors (HFETs) is examined theoretically using a charge-control model. The model is based on a self-consistent solution of the Poisson, Schrodinger, and charge balance equations, together with the k.p Hamiltonian for the valence band states. Realistic surface boundary conditions are imposed, and surface states are included using Fermi-Dirac statistics. Based on the assumption that surface donors are the underlying cause of the 2DEG, a wide range of published data on the 2DEG can be explained. For instance, the variation of the 2DEG density with the AlGaN layer thickness and mole fraction can be accounted for, along with other experimental results, such as the reduction of the 2DEG density when the HFET is capped with a GaN layer, the saturation of the 2DEG density for thick GaN caps, and the increase in the 2DEG density when the surface is passivated. (C) 2003 American Institute of Physics. [DOI: 10.1063/1.1530729].