Gate capacitance—Voltage characteristic of MODFET's: Its effect on transconductance

Gate capacitance C
g
in modulation-doped field-effect transistors (MODFET's) is one of the most important parameters for small- and large-signal applications as well as for understanding device operation at dc and high frequencies. We have modeled the gate capacitance-voltage characteristics of AlGaAs/GaAs MODFET's and obtained good agreement with experiments at 300 and 77 K. Rigorous numerical simulations relating the Fermi level at the GaAs/AlGaAs heterointerface to the two-dimensional electron gas (2DEG) concentrations were fitted with analytical functions to simplify the model. The fit is quite reasonable with an accuracy of 1 percent in a wide range of sheet carrier concentrations. By incorporating AlGaAs charge response to gate voltage near full turn-on, discrepancies between the experiments and earlier models (predicting a nearly constant capacitance-voltage characteristic) were minimized. The model also shows that the
C_{g}(V)
characteristic below threshold is governed by the doping level in the buffer layer, in the intermediate region by the 2DEG and near complete turn-on by the 2DEG and the AlGaAs layer. Considering the contribution of the AlGaAs layer, a detailed RC network was developed. The model also explains the rise in capacitance and fall in the transconductance for large forward gate voltages (low frequency) due basically to the contribution of AlGaAs to the gate capacitance but almost no contribution to the current conduction. In contrast, the transconductance and capacitance both increase in MESFET's.