The role of inefficient charge modulation in limiting the current-gain cutoff frequency of the MODFET

A model is presented that clarifies the role of inefficient charge modulation in limiting the current-gain cutoff frequency (fT) of conventional GaAs/AlGaAs and pseudomorphic InGaAs/AlGaAs MODFETs. Both modulation of parasitic charge in the electron-supplying layer and departure from the saturated velocity mode of operation are seen to reduce the efficiency with which the gate controls the drain current. Consequently, fT and the apparent electron velocity are reduced by a factor called the modulation efficiency. Using this model, the superior performance of the pseudomorphic MODFET is explained by its ability to achieve higher sheet densities before modulating parasitic charge in the AlGaAs layer. These results indicate that efficient modulation of the current-producing charge is as important as a high electron velocity. (I.E.)