We report on demonstrating high performance lateral β-Ga₂O₃ metal-oxide-semiconductor field-effect transistors (MOSFETs) with source-connected field plate (FP) on a thin (150 nm) and highly Si-doped (n = 1.5 × 10¹⁸ cm⁻³) β-Ga₂O₃ epitaxial channel layer grown by ozone molecular beam epitaxy (MBE) on Fe-doped semi-insulating (010) substrate. For a MOSFET with a gate-drain spacing (L_gd) of 25 μm, the three terminal off-state breakdown voltage (V_BR) tested in Fluorinert ambient reaches 2321 V. To the best of our knowledge, this is the first report of lateral β-Ga₂O₃ MOSFET with high V_BR of more than 2 kV and the highest V_BR attained among all the Ga₂O₃ MOSFETs. The breakdown voltages with different L_gd from 5–25 μm ranged from 518–2321V, with a linear trend of increasing breakdown voltage for larger spacing lateral MOSFETs. Combining with high electrical performances and excellent material properties, source-connected FP lateral β-Ga₂O₃ MOSFET implies its great potential for next generation high-voltage and high-power switching devices applications above 2 kV.

The device performance deterioration mechanism caused by the total ionizing dose effect after the γ-ray irradiation was investigated in GaN-based metal-insulator-semiconductor high electron mobility transistors (MIS-HEMTs) for a 5 nm-thick SiN and HfO₂ gate dielectric layer. The γ-ray radiation hardness according to the gate dielectric layer was also compared between the two different GaN-based MIS-HEMTs. Although HfO₂ has exhibited strong tolerance to the total ionizing dose effect in Si-based devices, there is no detail report of the γ-ray radiation effects in GaN-based MIS-HEMTs employing a HfO₂ gate dielectric layer. The pulsed-mode stress measurement results and carrier mobility behavior revealed that the device properties not only have direct current (DC) characteristics, but radio frequency (RF) performance has also been mostly degraded by the deterioration of the gate dielectric quality and the trapped charges inside the gate insulator. We also figured out that the immunity to the γ-ray radiation was improved when HfO₂ was employed instead of SiN as a gate dielectric layer due to its stronger endurance to the γ-ray irradiation. Our results highlight that the application of a gate insulator that shows superior immunity to the γ-ray irradiation is a crucial factor for the improvement of the total ionizing dose effect in GaN-based MIS-HEMTs.

The proton radiation hardness has been investigated in GaN-based MIS-HEMTs with various gate insulating systems. Through the pulsed mode measurements and carrier mobility extraction, we have revealed that the Coulomb scattering generated by the trapped charges inside of the gate insulating layer is a key device performance degradation factor. We also have found out that SiN/Al₂O₃ bi-layer gate insulating system exhibits stronger immunity to the proton radiation compared to the SiN single-layer gate insulating system since the dielectric layer quality of ALD deposited Al₂O₃ is better than that of PECVD deposited SiN layer. Our systematic research emphasizes that to employ an excellent quality dielectric layer such as Al₂O₃ is essential factor for the improvement of the proton radiation hardness in GaN-based MIS-HEMTs.

We report the electrical characteristics of GaN-based high electron mobility transistors (HEMTs) operated on various substrates/films. For the detailed investigation and comparison of the electrical properties of GaN-based HEMTs according to the substrates/films, GaN-based HEMTs were processed using 4-inch sapphire substrates and separated from their original substrates through the laser lift-off technique. The separated AlGaN/GaN films including processed GaN-based HEMTs were bonded to AlN substrate or plated with a 100 µm-thick Cu at the back-side of the devices since AlN substrate and Cu film exhibit higher thermal conductivity than the sapphire substrate. Compared to the sapphire substrate, DC and RF properties such as drain current, transconductance, cut-off frequency and maximum oscillation frequency were improved, when GaN-based HEMTs were operated on AlN substrate or Cu film. Our systematic study has revealed that the device property improvement results from the diminishment of the self-heating effect, increase in carrier mobility under the gated region, and amelioration of sheet resistance at the access region. C(V) and pulse-mode stress measurements have confirmed that the back-side processing for the device transfer from sapphire substrate onto AlN substrate or Cu film did not induce the critical defects close to the AlGaN/GaN hetero-interface.

The impacts of SiN/Al₂O₃ bi-layer passivation on the carrier transport characteristics in GaN-based metal-insulator-semiconductor high electron mobility transistors (MISHEMTs) were studied. Various mechanical stresses, as measured by micro-Ramam spectroscopy, were introduced on the GaN channel according to the different passivation systems. The SiN dielectric layer deposited by plasma enhanced chemical vapor deposition on top of the GaN capping layer resulted in compressive stress. On the other hand, the Al₂O₃ passivation layer deposited by atomic layer deposition on SiN layer generated tensile stress, which compensated the compressive stress produced by the SiN layer. The correlation between the applied mechanical stress induced by the deposited dielectric layers and device performance of the GaN-based HEMT was also investigated. When a slight tensile stress was applied on the GaN channel through the bi-layer passivation, the carrier transfer characteristics were improved in terms of carrier concentration at the AlGaN/GaN interface, as well as carrier mobility and sheet resistance compared to the high compressive stress condition. These results show that the mechanical stress engineering via optimized passivation process is a promising technique for the improvement of the device performance in GaN-based MISHEMTs.

An enhancement-mode AlGaN/GaN metal-insulator-semiconductor high-electron- mobility-transistor was fabricated using a recess gate and CF₄ plasma treatment to investigate its reliable applicability to high-power devices and circuits. The fluorinated-gate device showed hysteresis during the DC current-voltage measurement, and the polarity and magnitude of hysteresis depend on the drain voltage. The hysteresis phenomenon is due to the electron trapping at the Al₂O₃/AlGaN interface and charging times longer than milliseconds were obtained by pulse I-V measurement. In addition, the subthreshold slope of the fluorinated-gate device was increased after the positive gate bias stress because of the two-dimensional electron gas reduction by ionized fluorine. Our systematic observation revealed that the effect of fluorine ions should be considered for the design of AlGaN/GaN power circuits.