Hot-electron transport in selectively doped n-type AlGaAs/GaAs heterojunctions

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Abstract
We investigate the high‐field transport of two‐dimensional (2D) electrons in selectively doped AlGaAs/GaAs heterojunctions. The dependencies of mobilities on external applied electric fields are systematically studied by pulsed Hall measurements up to 1.5 kV/cm on various samples with mobilities ranging from 4×104 to 1.3×106 cm2/V s. Measured dependencies of mobilities on electric fields are compared with theories. It was found that the simple theoretical approach based on the electron temperature model is insufficient, and that the measured mobilities are in excellent agreement with the recent theoretical results of Lei et al. [Phys. Rev. B 3 3, 4382 (1986)]. The high‐field descreening effect is found to be more responsible for the reduction of 2D electron mobilities than the electron heating. At very high fields (several kV/cm), drift velocities of electrons determined by two‐terminal I‐V measurements are found to saturate gradually with increasing electric fields. Measured saturation velocities do not depend sensitively on electron densities and low‐field mobilities, and are (2.0±0.2)×107 cm/s at both 4.2 and 77 K, and (1.2±0.1)×107 cm/s at 295 K. The lattice‐temperature dependence of saturation velocities is compared with a crude estimation which takes into account the polar optical phonon scattering, and is found to be in rather good agreement. Consequently, it is clarified that high electron mobility transistors are superior to Si metal‐oxide‐semiconductor field‐effect transistors in the ultimate switching speed at least by a factor of 2.5 at 77 K and 1.8 at 300 K.