Abstract
Enhancement-mode high electron mobility transistors (EHEMTs) were fabricated on In0.52Al0.48As/In0.53Ga0.47As heterostructures lattice-matched to InP substrates. Vertical scaling of device heterostructures was carried out to realize a positive threshold voltage with Ti/Pt/Au gates. Submicron EHEMTs utilizing Ti/Pt/Au were fabricated and their performances were compared with those of conventional EHEMTs with buried-Pt gates. DC I–V characteristics of both devices exhibited excellent pinch-off characteristics and very low output conductance. Output conductance measured for both devices showed that EHEMTs exhibit smaller kink effects than normal depletion-mode HEMTs (DHEMTs) due to the operating mode of EHEMTs with forward gate bias. EHEMTs with 0.18 µm Ti/Pt/Au gates exhibited a threshold voltage of 100 mV, peak transconductance of 810 mS/mm, and unity current-gain cut-off frequency ( fT) of 150 GHz, while those with buried-Pt gates exhibited a threshold voltage of 300 mV, peak transconductance of 880 mS/mm, and unity current-gain cut-off frequency ( fT) of 100 GHz. Studies on the thermal stabilities of the two types of gate metallization have been carried out. The Ti/Pt/Au gates exhibited better thermal stability than Pt/Ti/Pt/Au gates in terms of variation of threshold voltages and maximum transconductance values at elevated temperatures.
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