We achieved the lowest contact resistance between a-IGZO and a metal electrode for >30 GHz operation of an oxide semiconductor device. For high-resolution display and high-speed electronic devices, both bulk and contact resistances need to be reduced. In this study, hydrogen plasma was used to lower the contact resistance significantly by modifying the surface of the a-IGZO thin film. The potential barrier width at the interface was decreased by increasing the carrier concentration, and weak M−OH bonds were sufficiently diffused out with optimized plasma process. The minimum contact resistance was measured to be 1.33 × 10 −6 Ω•cm 2 by the transfer line method, which is the lowest reported value to the best of our knowledge. Utilizing this enhanced contact property between a-IGZO and metal, the metal−insulator−semiconductor varactor was fabricated, and its operating frequency was measured to be higher than 30 GHz.
This paper presents a variable-gain amplifier (VGA) in the 68–78 GHz range. To reduce DC power consumption, the drain voltage was set to 0.5 V with competitive performance in the gain and the noise figure. High-Q shunt capacitors were employed at the gate terminal of the core transistors to move input matching points for easy matching with a compact transformer. The four stages amplifier fabricated in 40-nm bulk complementary metal oxide semiconductor (CMOS) showed a peak gain of 24.5 dB at 71.3 GHz and 3‑dB bandwidth of more than 10 GHz in 68–78 GHz range with approximately 4.8-mW power consumption per stage. Gate-bias control of the second stage in which feedback capacitances were neutralized with cross-coupled capacitors allowed us to vary the gain by around 21 dB in the operating frequency band. The noise figure was estimated to be better than 5.9 dB in the operating frequency band from the full electromagnetic (EM) simulation.
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