Numerical studies of heterojunction transport and high electron mobility transistor (HEMT) devices

We present a comprehensive numerical study of III-nitride heterojunction transport properties and a numerical HEMT device simulator. Basically, the electron transport properties of III-nitride heterostructures presented in our research can be divided into two categories: zero-field and field-dependent mobility models. The calculations of the zero-field mobility in III-nitride heterojunction structures are made using a self-consistent solution of the Schroedinger, Poisson, charge and potential balance equations. It is found that the spontaneous and piezoelectric polarization fields act to significantly increase the two-dimensional (2D) sheet charge concentration while reducing the mobility. The mobility reduction results from the enhanced band bending and subsequent attraction of the electrons to the hetero-interface where they experience increased surface roughness scattering. Good agreement is obtained between the theoretical calculations and experimental measurements over the full temperature range examined. The field-dependent mobility model consists of a fully numerical self-consistent solution of the Schroedinger-Poisson equation with a Monte Carlo transport model. The 2D subband energies, wave functions and carrier scattering mechanisms are computed numerically and included within a Monte Carlo simulation. The electron energy, steady state and transient drift velocity and band occupancy are calculated as a function of electric field for different AlGaN/GaN heterostructure compositions. The effect of piezoelectrically induced strain fields on the transport dynamics is examined. A field dependent mobility model is also developed from the Monte Carlo results. In addition, we present a theoretical model of an AlGaN/GaN HEMT that includes a nonlinear model of the strain polarization fields produced at the heterointerface. Recent experimental work has indicated that the macroscopic polarization in III-nitride alloys is a nonlinear function of the material composition. It is well known that the behavior of a GaN/AlGaN HEMT depends greatly upon the properties of the strain induced polarization fields formed at the GaN/AlGaN heterointerface. The model is found to agree well with recent experimental measurements made for GaN/AlGaN HEMTs when the nonlinear polarization model is included. The cutoff frequency, transconductance and current-voltage characteristics are computed. The effect of the nonlinear polarization model on the sheet carrier density is also presented.