0.12 µm Gate Length T-Shaped AlGaAs/InGaAs/GaAs Pseudomorphic High-Electron-Mobility Transistors Fabricated Using a Plasma-Enhanced Chemical Vapor Deposited Silicon–Nitride-Assisted Process

Abstract

A plasma-enhanced chemical vapor deposited (PECVD) silicon–nitride-assisted process has been used to fabricate 0.12 µm T-shaped AlGaAs/InGaAs/GaAs PHEMTs. A two-step SiNx etch was performed to define the T-gate footprint. The SiNx was etched either by dry etching alone or using a combination of wet and dry etching. A structure for the top of the T-gate which consists of a wide head part and a narrow lower layer part has been employed, taking advantage of the large cross-sectional area of the gate and its mechanically stable structure. The gate recessing was carried out in two steps including a wet etching for removal of the damaged surface layer and a dry etching for the narrow recess. The cut-off frequency of the device fabricated by the two-step etch process is higher than that of the device fabricated by dry etching alone. We have observed the increase of the cut-off frequency fT and the maximum oscillation frequency fmax, by 39% and 16%, respectively. This is believed to be due to considerable decreases of the gate–source and gate–drain capacitances, by 25% and 18.3%, respectively. This improvement in RF performance can be understood in terms of the decrease in parasitic capacitances which is due to the use of the dielectric and the gate recess etching method. The minimum noise figure is 1.6 dB, with a 6.9 dB associated gain at 39 GHz.