GaN transistors enable high-performance RF power amplifiers due to their unique capability of simultaneously providing large voltage swing and high operation frequency. Over the past decades, tremendous progress on high-speed GaN transistors has been witnessed featuring operation frequency at several hundred GHz. Aiming to explore the next-generation GaN technology to operate at unprecedented speed without penalty of short channel effects, both novel epilayer structure and advanced transistor architecture need to be engineered together.

To fabricate transistors featuring high transconductance, low parasitic loss, and excellent reproducibility in process to facilitate mass production, several approaches have been employed in this thesis. Compared with the conventional AlGaN/GaN...[ Read more ]

GaN transistors enable high-performance RF power amplifiers due to their unique capability of simultaneously providing large voltage swing and high operation frequency. Over the past decades, tremendous progress on high-speed GaN transistors has been witnessed featuring operation frequency at several hundred GHz. Aiming to explore the next-generation GaN technology to operate at unprecedented speed without penalty of short channel effects, both novel epilayer structure and advanced transistor architecture need to be engineered together.

To fabricate transistors featuring high transconductance, low parasitic loss, and excellent reproducibility in process to facilitate mass production, several approaches have been employed in this thesis. Compared with the conventional AlGaN/GaN heterostructure, the ultra-thin barrier AlN/GaN transistors grown on Si substrate hold great potential for employment in the future deeply-scaled technology. In the first stage of this work, high-performance AlN/GaN metal-oxide-semiconductors high-electron-mobility transistors (MOSHEMTs) were fabricated using high-k Al₂O₃ utilized as both the gate dielectric and passivation layer. The gate leakage current was reduced by four orders of magnitude to 7.6×10^-5 mA/mm and the current collapse was dramatically suppressed in comparison with that of HEMTs counterpart. Regrowth techniques are then incorporated to achieve low contact resistance and robust self-alignment process. Highly-doped n⁺-GaN was selectively regrown as the source/drain to lower the contact resistance and access resistance. As a result, record transconductance (G_m) of 509 mS/mm has been demonstrated for the Enhancement-mode (E-mode) GaN HEMTs on Si substrate. However, the RF performance is greatly limited by the large gate-to-source/drain overlap capacitance in this transistor. To minimize the parasitic capacitance and enhance the f_T&f_max, gate-last self-aligned technology was finally developed to separate the gate and source/drain contacts using a SiN_x supporting layer. In the gate-last devices, the f_T has been improved to be around 40 GHz with a channel length (L_g) of 210 nm. The gate-last process enables the scaling capability in both lateral and vertical dimensions, and can also be adjusted to be compatible with the state-of-the-art Si CMOS fabrication technology.