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 Schrodinger’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.