MODELING OF SHORT-PULSE THRESHOLD VOLTAGE SHIFTS DUE TO DX CENTERS IN ALXGA1-XAS/GAAS AND ALXGA1-XAS/INYGA1-YAS MODFETS

The DX center related short-pulse threshold voltage shift (SPTVS) in Al(x)Ga(1-x)As based MODFET's is modeled using CBAND, a simulator that solves Poisson equation self consistently with Schrodinger equation and donor statistics. Using values given in the literature for the DX energy level in Al(x)Ga(1-x)As this technique gives good agreement between measured and simulated SPTVS for Al0.3Ga0.7As/GaAs and Al0.3Ga0.7As/In0.2Ga0.8As MODFET's. Both simulation and experiment show that the use of Al0.2Ga0.8As in the donor layer reduces the SPTVS relative to the structures using Al0.3Ga0.7As. However, the measured shifts at this composition are considerably lower than the simulated values, indicating a DX energy level that may be higher than the value extrapolated from the literature, possibly due to the existence of multiple trap levels. Despite this discrepancy, these results support the use of strained-channel layers and lower Al(x)Ga(1-x)As compositions in MODFET's for digital and other large-signal applications requiring good threshold stability.