Surface passivation of InGaP/InGaAs/GaAs pseudomorphic HEMTs with ultrathin GaS film

We report on the successful surface passivation of wide recess InGaP/InGaAs/GaAs pseudomorphic HEMTs with MBE-grown ultrathin GaS film (2 nm) employing a single precursor, tertiarybutyl-galliumsulfide-cubane ([(t-Bu)GaS]/sub 4/). At the recess length of 1.1 /spl mu/m, a GaS-passivated device with a 0.5-/spl mu/m gate length has the maximum transconductance (g/sub m max/) of 347 mS/mm, which is about 40% higher than that of 240 mS/mm for a device without GaS passivation. We found that one of the causes of an increased g/sub m max/ is the decrease of sheet resistance on the recessed surface because GaS passivation has reduced the depletion layer. Meanwhile, the two-terminal gate-to-drain reverse breakdown voltage (BV/sub gd/) was reduced after GaS passivation. The BV/sub gd/ is independent of the recess length between gate and drain (L/sub gd/) for GaS-passivated devices, unlike that for devices without GaS passivation. According to our calculation of the BV/sub gd/ involving the effects of impact ionization and the interface state, the BV/sub gd/ becomes almost independent of the L/sub gd/, when the interface state density (N/sub int/) is below 1/spl times/10/sup 12/ cm/sup -2/. Then, the calculated surface potential at the recess region is less than 0 eV. This result suggests that GaS passivation can remarkably reduce the N/sub int/ at the recess region.