Mushroom shaped gates in a dry etched recessed gate process

We have developed a recess gate process to fabricate high electron mobility transistors (HEMTs) of enhancement and depletion type on two inch GaAs wafers. The vertical HEMT structure is grown by molecular beam epitaxy (MBE). The gates are written by e-beam lithography and are recessed by dry etching. Mushroom shaped gates of dimension of 0.4 Mym for Lsubg is equal to 0.1 Mym are used to reduce the gate resistance. The average threshold voltage for D- and E-type FETs is minus500 mV and 100 mV respectively with a standard deviation of equal or smaller than 20 mV over a 2" Wafer with Lsubg is equal to 0.3 Mym.