CRYOGENIC BEHAVIOR OF ULTRASHORT GATE ALGAAS/GAAS AND PSEUDOMORPHIC ALGAAS/INGAAS/GAAS HEMTS

High performance delta-doped double recess 0.1-0.4mum AlGaAs-GaAs S-HEMTs[1] and delta-doped single-recess 0.1-0.7mum AlGaAs/GaInAs/GaAs PM-HEMTs [2-3] are studied from 300K down to 60K. DC and 0.1-40GHz HF measurements followed by electric parameter extractions give extensive evolutions of HEMT electric parameters versus temperature T, gatelength l(g). The results are interpreted in terms of the transition regime from diffusive to strong overshoot/near ballistic carrier transport under the gate. In the longer devices (l(g)=0.4-0.7mum), the latter effects become important at low temperatures and bring frequency intrinsic performance improvement mainly in terms of an increase of maximum intrinsic transconductance g(mo). The shorter devices (0.1-0.2mum) appear to be already well in the overshoot regime at 300K due to the high quality epitaxial layers, and their frequency performance improvement upon cooling comes mainly from a reduction of intrinsic capacitance C(gsint). The relative increase of intrinsic unity current gain cut-off frequency f(tint) upon cooling rates at a similar value of 30-40% for all investigated gatelengths(0.1-0.7mum).