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Deep quantum well In0.8Ga0.2As/AlGaAsSb MHEMTs on GaAs are described. The step-graded AlGaAsSb strain-relief buffer layer provided a high-quality surface for growth of the MHEMT layers. AlGaAsSb barrier layers offer flexibility in choosing the channel composition and the barrier height. Typical Hall mobilities were 11,000 cm2/V-sec at 300 K for carrier concentrations of 2.4×1012 cm-2. Extrinsic DC transconductance of 820 mS/mm was obtained for an MHEMT with a 0.15 μm×64 μm gate. Typical extrinsic unity current gain cutoff, ft, was 173 GHz with maximum frequency of oscillation, fmax, of 474 GHz. Aside from layer growth, the MHEMTs were fabricated using only small changes from conventional GaAs PHEMT processing. This technology promises affordable production costs for high performance millimeter-wave low noise amplifiers.