Modulation-dopedIn0.5Ga0.5As/In0.5Al0.5Asquantum well structures grown by molecular beam epitaxy on GaAs substrates using a relaxed AlGaAsSb buffer showed carrier mobilities of8500 cm2/V sfor a sheet concentration of3.5×1012 cm−2at room temperature. The crystallinity of the quaternary buffer layer was verified by x-ray diffractometry. Transistors with0.25×100 &mgr;m2gates demonstrated transconductance values as high as 800 mS/mm. S-parameter measurements revealed a cutoff frequencyfTof 87 GHz and a maximum oscillation frequencyfMAXof 140 GHz (both extrinsic values). ©1998 American Institute of Physics.