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The impacts of SiN/Al 2 O 3 bi-layer passivation on the carrier transport characteristics in GaN-based metal-insulator-semiconductor high electron mobility transistors (MISHEMTs) were studied. Various mechanical stresses, as measured by micro-Ramam spectroscopy, were introduced on the GaN channel according to the different passivation systems. The SiN dielectric layer deposited by plasma enhanced chemical vapor deposition on top of the GaN capping layer resulted in compressive stress. On the other hand, the Al 2 O 3 passivation layer deposited by atomic layer deposition on SiN layer generated tensile stress, which compensated the compressive stress produced by the SiN layer. The correlation between the applied mechanical stress induced by the deposited dielectric layers and device performance of the GaN-based HEMT was also investigated. When a slight tensile stress was applied on the GaN channel through the bi-layer passivation, the carrier transfer characteristics were improved in terms of carrier concentration at the AlGaN/GaN interface, as well as carrier mobility and sheet resistance compared to the high compressive stress condition. These results show that the mechanical stress engineering via optimized passivation process is a promising technique for the improvement of the device performance in GaN-based MISHEMTs.

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A W-band dual-channel receiver is presented for the frequency-modulated continuous wave radars. In order to improve the isolation between two RF channels, a compact active power divider is implemented by a dual-output local oscillator (LO) amplifier. The temperature-compensated circuit also mitigates the gain variation of the receiver according to the ambient temperature. The fabricated receiver integrated circuit shows a conversion gain of 16 dB, a noise figure of 6.4 dB, and an input 1 dB compression point of −13.4 dBm at RF frequency of 76.5 GHz and LO input power of −1 dBm. The channel-to-channel isolation is >33 dB and the conversion gain variation is as low as −0.031 dB/°C between −40 and 120°C. This is the first temperaturecompensated multi-channel receiver demonstration in W-band.