Jihyun Kim

Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics with capabilities beyond existing technologies due to its large bandgap, controllable doping, and the availability of large diameter, relatively inexpensive substrates. These applications include power conditioning systems, including pulsed power for avionics and electric ships, solid-state drivers for heavy electric motors, and advanced power management and control electronics. Wide bandgap (WBG) po

We demonstrated high responsivity metal–semiconductor–metal (MSM) solar-blind photodetectors by integrating exfoliated β-Ga2O3 microlayers with graphene, which is a deep ultraviolet (UV) transparent and conductive electrode. Photodetectors with MSM structures commonly suffer from low responsivity, although they feature a facile fabrication process, low dark current, and fast response speed. The β-Ga2O3 MSM solar-blind photodetectors with graphene electrodes exhibited excellent operating characte

The strong bonding in wide bandgap semiconductors gives them an intrinsic radiation hardness.

Neuromorphic systems, which emulate neural functionalities of a human brain, are considered to be an attractive next-generation computing approach, with advantages of high energy efficiency and fast computing speed. After these neuromorphic systems are proposed, it is demonstrated that artificial synapses and neurons can mimic neural functions of biological synapses and neurons. However, since the neuromorphic functionalities are highly related to the surface properties of materials, bulk materi

We fabricated solar-blind photodetectors based on exfoliated two-dimensional β-Ga₂O₃ flakes, and then systematically characterized their photoresponsive properties. They exhibit extraordinary photoresponsive properties including the highest responsivity among reported semiconductor thin-film solar-blind photodetectors.

Graphene-based, flexible NO(2) sensors on paper substrates exhibited an immediate response (32-39%) once exposed to 200 ppm NO(2) gas under a strain of 0.5%. Chemical vapor deposition-grown graphene with a supporting poly(methyl methacrylate) layer was transferred onto paper substrates, followed by formation of two electrodes using silver paste. Current-voltage characteristics and dynamic sensing response were obtained under both relaxed and strained conditions. We demonstrate a facile method wi

We report defect-engineered graphene chemical sensors with ultrahigh sensitivity (e.g., 33% improvement in NO2 sensing and 614% improvement in NH3 sensing). A conventional reactive ion etching system was used to introduce the defects in a controlled manner. The sensitivity of graphene-based chemical sensors increased with increasing defect density until the vacancy-dominant region was reached. In addition, the mechanism of gas sensing was systematically investigated via experiments and density f

Layered materials separated from each bulk crystal can be assembled to form a strain-free heterostructure by using the van der Waals interaction. We demonstrated a heterostructure n-channel depletion-mode β-Ga₂O₃ junction field-effect transistor (JFET) through van der Waals bonding with an exfoliated p-WSe₂ flake. Typical diode characteristics with a high rectifying ratio of ∼10⁵ were observed in a p-WSe₂/n-Ga₂O₃ hetero

We have demonstrated a β-Ga2O3 metal-semiconductor field-effect transistor (MESFET) with a high off-state breakdown voltage (344 V), based on a quasi-two-dimensional β-Ga2O3 field-plated with hexagonal boron nitride (h-BN). Both the β-Ga2O3 and h-BN were mechanically exfoliated from their respective crystal substrates, followed by dry-transfer onto a SiO2/Si substrate for integration into a high breakdown voltage quasi-two-dimensional β-Ga2O3 MESFETs. N-type conducting behavior was observed in t

Gate-controlled n+p metal–oxide–semiconductor diodes were fabricated in p-GaN using MgO as a gate dielectric and Si+ implantation to create the n+ regions. This structure overcomes the low minority carrier generation rate in GaN and allowed observation of clear inversion behavior in the dark at room temperature. By contrast, diodes without the n+ regions to act as an external source of minority carriers did not show inversion even at measurement temperatures of 300 °C. The gated diodes showed th

The forward current of Pd/GaN and Pt/GaN Schottky diodes is found to increase significantly upon introduction of H2 into a N2 ambient. Analysis of the current–voltage characteristics as a function of temperature showed that the current increase is due to a decrease in effective barrier height through a decrease in metal work function upon absorption of hydrogen. The introduction of 10% H2 into a N2 ambient was found to lower the effective barrier height of Pd on GaN by 50∼70 meV over the tempera

The capacitance–voltage (C–V) characteristics of Sc2O3/p-GaN gate-controlled diodes show unusual hook shapes due to the charging of surface states. From the drain–voltage dependence of the C–V curves, the total surface state density was estimated to be ∼8.2×1012 cm−2 for diodes undergoing an implant activation anneal at 950 °C. The accumulation capacitance showed a significant dependence on measurement frequency and is suggested to result from the presence of an interfacial dielectric between th