High-Performance In0.5Al0.5As/In0.5Ga0.5As High Electron Mobility Transistors on GaAs

Abstract
InAlAs/InGaAs high electron mobility transistors (HEMTs) lattice-mismatched on GaAs substrates have been fabricated with a 0.6-µ m- thick step-graded In y Al1- y As buffer layer and a low-source-resistance triple cap layer. A peak extrinsic transconductance of 1060 mS/mm was obtained for a device with a 0.16-µ m- long gate due to both the low source resistance of 0.35 Ω mm brought by the triple cap layer and the high quality In0.5Ga0.5As channel brought by the step-graded In y Al1- y As buffer layer. This transconductance is comparable to or higher than that of HEMTs lattice-matched on InP substrates. A maximum oscillation frequency, f max, of 127 GHz and a cutoff frequency, f t, of 47 GHz were obtained for a fully-passivated device with a 0.66-µ m- long gate, giving a high f max/f t ratio of 2.7. This f max is comparable to those of HEMTs lattice-matched on InP with the same gate length.