Influence of Fermi-Level Pinning at the GaAs Substrate on HEMT Threshold Voltage.

摘要

Two-dimensional quantum well, strong-inversion model of threshold in Aluminum Gallium Arsenide Gallium Arsenide high-electron-mobility transistors (HEMTs) has been extended to include the effects of Fermi-level pinning at the semi-insulating boundary results from an abundance of mid-gap traps in the substrate and couples the carrier channel at the Al GaAs/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 micrometer active-layer device having a typical acceptor doping of approx. 10 to the 14th Power/CC. 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, 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.