Wide bandgap GaN-based heterojunction devices have successfully entered the market of RF/microwave power amplifiers for military radar systems and commercial wireless base-stations. They are also being intensively investigated worldwide as high-voltage power switches and rectifiers for next generation electric power converters, promising performance far superior to that achievable with the current, mainstream silicon technology. The readily available high-quality heterojunctions in a group III-nitride material system provide unique advantages, including high electron density and mobility that yield high current density and low channel resistance, both of which are highly desirable for power switches with low on-resistance and high power handling capability.

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Wide bandgap GaN-based heterojunction devices have successfully entered the market of RF/microwave power amplifiers for military radar systems and commercial wireless base-stations. They are also being intensively investigated worldwide as high-voltage power switches and rectifiers for next generation electric power converters, promising performance far superior to that achievable with the current, mainstream silicon technology. The readily available high-quality heterojunctions in a group III-nitride material system provide unique advantages, including high electron density and mobility that yield high current density and low channel resistance, both of which are highly desirable for power switches with low on-resistance and high power handling capability.

Recently, In_0.17AI_0.83N/GaN has attracted extensive attention because of its unique material properties over other GaN-based heterostructures. With 17% composition of Indium In_0.17AI_0.83N/GaN is a lattice-matched heterojunction in which the inherent stress can be eliminated. Such a stress-free property bodes well for excellent device reliability, especially for high voltage applications. In addition, the In_0.17AI_0.83N/GaN is able to produce record high density of two-dimensional electron gas (2-DEG) due to strong spontaneous polarization, which allows the device to deliver high current density and low on-resistance. Despite the above advantages, up to now the breakdown voltage (V_BD) of In_0.17AI_0.83N/GaN devices is still limited by the threading dislocations in the In_0.17AI_0.83N barrier layer. This thesis focuses on the breakdown voltage improvement of In_0.17AI_0.83N/GaN transistors by using novel device design and fabrication technologies, including Schottky contacts and a high-k gate dielectric.

The research work in this thesis consists of three parts: 1)In_0.17AI_0.83N/GaN Schottky source/drain (SSD) HEMTs and Schottky-source/Ohmic-drain (SSOD) HEMTs; 2) Schottky source/drain Al₂O₃/In_0.17AI_0.83N/GaN metal-insulator-semiconductor HEMTs (MISHEMTs); and 3) steep subthreshold swing in the Al₂O₃/In_0.17AI_0.83N/GaN SSD MISHEMTs. Firstly, effective suppression of electron injection into the buffer under the Schottky source has been achieved in the SSD In_0.17AI_0.83N/GaN HEMTs, yielding a breakdown voltage V_BD of 605 V -- a 230% improvement compared to the conventional Ohmic source/drain HEMTs. SSOD HEMTs featuring Schottky-source/Ohmic-drain further have reduced R_sp,on by 12.8%, leading to a low R_sp,on of 3.49 mΩ·cm². The good thermal stability of the Schottky contacts and the proposed devices are also validated by high temperature measurement. Secondly, the Al₂O₃/In_0.17AI_0.83N/GaN SSD MISHEMTs are developed for low gate leakage, yielding a low off-state leakage current of 100 pA/mm and high ON/OFF current ratio of 10¹⁰. Last, we report the steep subthreshold swing (SS) observed in the SSD MISHEMTs. At room temperature, the steep SS is as low as 6.6 mV/dec, which is significantly lower than the thermal limit (60 mV/dec) imposed on conventional FETs. The mechanism for the steep SS has been proposed to be based on the dynamic discharging process of the interface states at the Al₂O₃/In_0.17AI_0.83N interface and the resultant positive feedback in the drain current during the channel turn-on process. The model has been validated by temperature-dependent characterization.