Subthreshold and near-threshold conduction in GaAs/AlGaAs MODFETs

The I-V characteristics of GaAs/AlGaAs MODFETs in the near-threshold and subthreshold regions were investigated. An examination of the long-gate-length (8-μm) devices shows that the source current (I_s) in the subthreshold region has an exponential dependence on the gate voltage over four decades at 300 K with a gate swing S=63 mV/dec and over five decades at 77 K with S=17 mV/dec. A simple analytic model, with diffusion current in the subthreshold region and drift current in the near-threshold region, has been developed to described I_s . The nonlinear charge control is based on a solution of Poisson´s equation with Fermi statistics, including depletion charges in the GaAs buffer layer. The conductance mobility in the whole density range has also been studied and compared with the Hall mobility. It is found that the mobility is constant in the subthreshold region and decreases with decreasing carrier density in the near-threshold region. The decrease is attributed to the carrier density fluctuation effect induced by the randomly distributed Si impurities in the AlGaAs layer