In this study, the effect of barrier thickness on the DC and RF performances of AlGaN/GaN high electron mobility transistors (HEMTs) on silicon for millimeter wave applications is experimentally investigated. GaN HEMT devices with different barrier thicknesses were fabricated and characterized. While the device with the thinnest barrier exhibited the highest extrinsic transconductance (gm) resulting from the shortest gate-to-channel distance, such configuration suffered from the lowest unit current-gain cut-off frequency (fT) due to the increase of the total gate capacitance. Moreover, degradation in the output power was observed for devices with thinner barriers. Such degradation was related to the severe knee walkout as evidenced from drain-lag characterization.

We investigated the tri-gate InxGa1-xAs high electron mobility transistors (HEMTs) with different indium compositions for low noise applications. The tri-gate configuration in this work exhibits better device performance of the transconductance (gm), the current gain cutoff frequency (fT), the maximum oscillation frequency (fmax), and the minimum noise figure compared with the conventional planar structure due to the improved gate controllability. The indium contents of 20%, 40%, and 100% tri-gate devices were also fabricated for comparison. The resistances and leakage current were measured to investigate the effect between the indium compositions and the noise figure. Experimental results showed that the tri-gate In0.4Ga0.6As mHEMT with neither the highest transconductance nor the lowest gate leakage current exhibited the lowest minimum noise figure (NFmin) of 2 dB when operating at 50 GHz. The study reveals that device architecture and indium composition are important factors in the determination of noise performance.