Charge-enhancement mechanisms of GaAs field-effect transistors: Experiment and simulation

The charge-collection processes of GaAs field-effect transistors are investigated as a function of the incident laser pulse energy via time-resolved charge-collection measurements and by two-dimensional computer simulation. The measurements and simulations reveal a feature that persists on a time scale of 100 ps, the amplitude of which varies strongly with the injected carrier density (pulse energy). The appearance of this feature is associated with a barrier lowering effect at the source/substrate junction, coupled with the drift-assisted transport of electrons through the substrate to the drain contact. This behavior is similar, but not identical to bipolar-gain models that have been suggested previously. We introduce the concept of ion-track segments and illustrate their utility in interrogating the complex mechanisms of charge collection and enhancement in GaAs FETs.