InAlAs/InGaAs high electron mobility transistors (HEMT) with excellent high frequency performance were demonstrated. Device sizes of 0.25×50 &mgr;m and 0.10×50 &mgr;m showed current gain cutoff frequencies of 143 and 205 GHz, respectively. The HEMT structures were grown on a low temperature InAlAs buffer layers designed to eliminate conductive impurity spikes situated at the epitaxial/substrate interface. The highly resistive buffer layer (2×105&OHgr; cm) was obtained at a growth temperature of 475 °C using a combination of trimethylarsenic and arsine as the arsenic sources. ©1995 American Institute of Physics.