High mobility SiGe/Si n-type structures and field effect transistors on sapphire substrates

SiGe/Si n-type modulation doped field effect transistors (MODFETs) fabricated on sapphire substrates are characterized at microwave frequencies for the first time. The highest measured room temperature electron mobility is 1380 cm/sup 2//V-sec at a carrier density of 1.8/spl times/10/sup 12/ cm/sup -2/ for a MODFET structure. At room temperature, a two finger, 2/spl times/200 micron gate n-MODFET has a peak transconductance of 37 mS/mm at a drain-to-source voltage of 2.5 V and an f/sub max/ of 2.45 GHz. Microwave performance of the transistor improved with decreasing temperatures, with an f/sub max/= 5.25 GHz at 100 K.