The influence of Fermi-level pinning at the GaAs substrate on HEMT threshold voltage

Abstract

A two-dimensional quantum well, strong-inversion model of threshold in AlGaAs/GaAs high-electron-mobility transistors (HEMTs) has been extended to include the effects of Fermi-level pinning at the semi-insulating boundary of the thin active GaAs layer. The pinning of the Fermi level at the semi-insulating boundary results from an abundance of mid-gap traps in the substrate and couples the carrier channel at the AlGaAs/GaAs interface to the substrate. This communication between the carrier channel and the substrate causes the threshold voltage characteristics of the pinned devices to be substantially different from those of a conventional, semi-infinite HEMT structure. The discrepancy in the threshold voltage approaches 250 mV for a 0.1-μm active-layer device having a typical acceptor doping of ∼10 14 cm −3. The quantum-well HEMT threshold model has also been compared to a classical analysis of the threshold voltage. For low acceptor doping and thin GaAs layers (<0.5 μm), neglecting quantum effects can result in significant errors in the threshold voltage calculation. For state-of-the art AlGaAs/GaAs microcircuits, in which device dimensions are shrinking and unintentional acceptor densities are decreasing, analyses of the I- V characteristics of thin, fully depleted devices must be carried out in the electric quantum limit and include the effects of the semi-insulating substrate.