Technological developments in pursuit of recess-free normally-OFF AlGaN/GaN HEMTs

Abstract

Thanks to GaN's high breakdown electric field, high electron mobility and saturation velocity, GaN-based devices are considered promising candidates for power switching and RF power amplifier applications. Among them, the highly conductive two-dimensional electron-gas (2DEG) at the (In,Al,Ga)N/GaN heterostructure enables the power-efficient operation of high-electron-mobility transistors (HEMTs). However, the inherent channel makes GaN HEMTs normally-ON, while the Normally-OFF operation of GaN HEMTs is preferred for various applications. Recess processes for fabricating normally-OFF devices generally compromise devices’ performance. In this thesis, technologies for realizing recess-free normally-OFF GaN HEMTs are developed. Due to the piezoelectric nature of GaN, strain engineering can modulate the electric performance of GaN HEMTs. The local strain engineering based on the SiN_x stress liner is developed. The SiN_x-introduced compression in the gate region increases the threshold voltage (V_TH) and reduces the Schottky gate leakage. As a result, the I_ON/I_OFF ratio of the Schottky gate HEMTs with strain engineering reaches 10⁸. Furthermore, the normally-OFF GaN HEMT using strain engineering is realized, showing the potential application on class-AB or class-B RF power amplifier applications. Schottky gate is inappropriate for normally-OFF GaN HEMTs due to its severe leakage under forward bias. Therefore, as the second part of the thesis, the ozone-based deposition of HfO₂ for viable MIS gate structure is proposed. Replacing the conventional water oxidant with ozone reduces the oxide bulk's oxygen vacancy-related defects, resulting in smaller oxide leakage and higher critical electric field. Furthermore, the more abrupt HfO₂/GaN interface and amorphous HfO₂ bulk form on the ozone pre-oxidized GaN surface. The improved interface translates to the satisfactory electric performance of MIS HEMTs. The high-k nature is also evidently helpful to develop the normally-OFF HEMTs, supported by an analytical model for V_TH of MIS HEMTs. The proposed HfO₂ gate oxide scheme is also integrated with an ultra-thin barrier heterostructure. With the optimized annealing condition, the normally-OFF operation of MIS HEMTs is realized. Also, the electron mobility in the gate region is enhanced after annealing. The high electron mobility in the gate region together with the successful 2DEG recovery by the SiN_x passivation enable the excellent output performance, benchmarked against recess-based devices.

Date of Award	2021
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Man Sun CHAN (Supervisor) & Hongyu Yu (Supervisor)