Current Transport in Modulation-Doped AlxGa1-xAs/GaAs Heterojunction Structures at Moderate Field Strengths

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Abstract
Current vs. electric field characteristics of single and multiple period modulation-doped GaAs–Al x Ga1-x As heterostructures are reported for lattice temperatures in the range of 10–300K. Measurements were performed at electric fields up to 2 kV/cm which is below the field range where non-linear effects due to real-space transfer or κ-space transfer (Ridley-Watkins-Hilsum (RWH) mechanism) are expected to occur. The basic heterostructure included an undoped “intrinsic” layer of Al x Ga1-x As between each doped (∼1×1017 cm-3) Al x Ga1-x As layer and undoped GaAs layer. The low field mobilities were found to decrease sharply with increasing electric field at low temperatures, but remained nearly constant at higher temperatures. The low field mobilities of the single period structures showed significant improvement with increasing AlAs mole fraction in the range 0.16≤x≤0.36. The increased mobility enhancement occurs in these structures due to the virtual elimination of electron scattering by increasing the spacing between the donors and electrons with the undoped AlGaAs layers. The current-field characteristics showed this mobility enhancement to increase, reach a maximum and then decrease with increasing undoped layer thickness. At a field strength of 2 kV/cm, modulation-doped single period (x=0.36) structures showed an improvement in current of four times at 78 K and 1.6 times at 300 K, compared to bulk GaAs with identical electron concentration. If used for normally-off field effect transistors (FETs) a speed improvement of 60% at 300 K and 400 % at 78 K can be obtained as compared to conventional normally-off GaAs FETs.