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]₄). At the recess length of 1.1 μm, a GaS-passivated device with a 0.5-μm gate length has the maximum transconductance (g_{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_{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_gd) was reduced after GaS passivation. The BV_gd is independent of the recess length between gate and drain (L_gd) for GaS-passivated devices, unlike that for devices without GaS passivation. According to our calculation of the BV_gd involving the effects of impact ionization and the interface state, the BV_gd becomes almost independent of the L_gd, when the interface state density (N_int) is below 1×10¹² cm^-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_int at the recess region.