Perpendicular Transport in GaAs-GaAlAs High Electron Mobility Transistors

Abstract

The thickness of conventional GaAs-GaAlAs High Electron Mobility Transistors (HEMTs) is in the order of 1000 Å or smaller. Therefore, the perpendicular transport in such structures will strongly be influenced by tunneling effects. Investigating the tunneling current perpendicular to the GaAlAs barrier, which had a typical thickness of 500 Å on our samples, oscillatory behavior was observed in dI/dV on HEMT structures having a shallow alloyed gate contact. Using a Fowler-Nordheim tunneling theory, we were able to determine the conduction band discontinuity from the observed oscillations. The fit of the data gave a value of ΔE_c/ΔE_g=0.61±0.04 for aluminum concentrations of 30%, 36%, and 40%.

On samples having a semitransparent Au Schottky-gate contact the bandstructure was varied by illumination. Sharp peaks were observed in the derivative of the tunneling current after illumination at liquid helium temperature. Using a self consistent model, these peaks could be explained by resonant tunneling via subband states in the GaAlAs.

The subband energies in the two-dimensional electron gas (2DEG) were measured by tunneling spectroscopy on samples, where the tunneling process starts from an accumulation layer, and conventional structures, where the electrons tunnel from a metal electrode into the 2DEG. Self-consistent calculations were performed to determine the depletion charge from the measured subband energies. Furthermore the influence of a back-gate voltage was investigated both experimentally and theoretically.