Subthreshold current in undoped AlGaAs/GaAs MODFET structures

The subthreshold current in undoped AlGaAs/GaAs MODFET structures can have a negative gradient with respect to the gate voltage, whereas in doped structures it has normally either a positive gradient or is constant. A two-dimensional finite-difference analysis is used to clarify this difference. It is shown that the behavior of the threshold current in an undoped structure is due to the effect of gate bias on the leakage between the drain and the gate. This leakage occurs near the top surface of the AlGaAs layer, and is strongly affected by the lateral straggle of the n/sup +/ implants and by the surface states in AlGaAs. The charge-control model, which is conventionally applied to the doped case, does not describe the behavior of the subthreshold current in the undoped case because it neglects leakage currents.<
>