Saturation in the transfer characteristics of (Al,Ga)As/GaAs modulation-doped field-effect transistors at 77 K

Modulation-doped field-effect transistors operating under large forward gate biases at 77 K have been studied both experimentally and theoretically. The theoretical analysis includes the self-consistent solution of Schrödinger’s and Poisson’s equations in the GaAs channel and includes Fermi–Dirac statistics in the GaAs and AlGaAs. A dramatic decrease in transconductance is observed at large forward gate voltages and is explained within the theory as being due to limited two-dimensional electron gas concentration and the gate voltage dependence of the concentration of electrons bound to donors in the AlGaAs. The balance of dynamics of the Fermi level versus conduction-band energy and electron occupation of the relatively deep Si donors lead to the observed saturation in drain current versus gate voltage (transfer) characteristics.