Properties of MODFET's grown on Si substrates at DC and microwave frequencies

We report the first microwave characterization of 1-µm gate GaAs/AlGaAs modulation-doped field-effect transistors (MODFET's) grown on Si substrates by MBE. Maximum transconductances of 170 mS/mm at room temperature were obtained in structures on Si, which compares to values of 200-250 mS/mm for this type of structure on GaAs. At 77 K, no collapse was observed in these structures, and transconductances of 270 mS/mm were obtained. From microwave S-parameter measurements at room temperature, current gain cutoff frequencies of 15 GHz were obtained from these GaAs/AlGaAs MOD FET's on Si, which compares with 12-15 GHz obtained on GaAs substrates. From high-frequency equivalent circuit modeling, very little difference was observed in any of the parameters between growth on Si and on GaAs. This is significant in that it demonstrates the suitability of GaAs on Si for device applications.