Modulation-doped In0.5Ga0.5As/In0.5Al0.5As quantum well structures grown by molecular beam epitaxy on GaAs substrates using a relaxed AlGaAsSb buffer showed carrier mobilities of 8500 cm(2)/V s for sheet concentration of 3.5x10(12) cm(-2) at room temperature. The crystallinity of the quaternary buffer layer was verified by x-ray diffractometry. Transistors with 0.25x100 mu m(2) gates demonstrated transconductance values as high as 800 mS/mm. S-parameter measurements revealed a cutoff frequency f(T) of 87 GHz and a maximum oscillation frequency f(MAX) of 140 GHz (both extrinsic values). (C) 1998 American Institute of Physics. [S0003-6951(98)00245-9].