Two-dimensional transient simulations of AlGaAs/GaAs HFETs are performed in which substrate traps and surface states are considered. When the drain voltage is raised abruptly, the drain current overshoots the steady-state value, and when it is lowered abruptly, the drain current remains at a low value, showing drain-lag behavior. Turn-on characteristics are also calculated when both the gate voltage and the drain voltage are changed abruptly, and quasi-pulsed I-V curves are derived from them. It is shown that the drain lag due to substrate traps could become a cause of so-called current compression of the HFETs. It is also shown that gate lag due to surface states could become a major cause of the current compression.
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