Improvement of AlGaN/GaN HEMT Performance by Using Hybrid Ohmic Source and Drain for Ka-band Applications

In this study, Γ-gate AlGaN/GaN HEMTs featuring a hybrid ohmic source and drain structure were investigated to meet the requirements for high-power and low-noise performance in Ka-band applications. The proposed hybrid metal devices with extension lengths of 0.25 and 0.35 μm and a conventional ohmic device without an extension were fabricated. Experimental results demonstrate that the hybrid metal device with a 0.25 μm extension exhibits an improved drain saturation current (IDSS) of 1087 mA/mm and an enhanced peak transconductance (gm) of 408 mS/mm at VDS = 10 V. The device also achieved a cut-off frequency (fT) of 89 GHz and a maximum oscillation frequency (fmax) of 171 GHz. At 28 GHz, the device shows an output power (Pout) of 2.82 W/mm with power-added efficiency (PAE) of 22.1% and a minimum noise figure (NFmin) of 1.29 dB. These improvements are associated with improved access-region electrical characteristics and reduced parasitic resistance. In contrast, increasing the extension length to 0.35 μm results in a negative Vth shift and degraded gate control under high drain biases. These results indicate that the hybrid ohmic structure with a proper extension length can enhance device performance for millimeter-wave applications.