We have grown InxGa1-xAs modulation doped field effect transistors (MODFETs) using novel buffer layer schemes on GaAs substrates with x values up to 0.5. The MODFET active layers were grown at substrate temperatures between 500 and 520-degrees-C and characterized by x-ray diffraction, photoluminescence, and Hall effect measurements. MODFET devices with 0.15-mu-m gate length were fabricated, and dc and rf tested. Our work has indicated that the electron mobility in the MODFETs is a function of the growth mode, i.e., two-dimensional (2D) layer-by-layer versus three-dimensional island growth. MODFET performance, the ability to maintain a streaky reflection high-energy electron diffraction pattern, as well as morphology had a very strong correlation. With these observations in mind we used a buffer layer scheme to give 2D growth fronts for InxGa1-xAs with x values up to 0.5, which is a modification of the buffer scheme described in K. Maezawa and T. Mizutani, IEEE Trans. Electron Devices 37, 1416 (1990). For In0.34Ga0.66As channel MODFETs we observed a room-temperature electron mobility of 6620 cm2/V s with a 2D carrier density of 5.9 x 10(12) cm-2 (double-doped channel), a dc transconductance of 800 mS/mm and a rf gain of 13.5 dB at 35 GHz for a 0.15-mu-m gate length.
