Monolithically Integrated Enhancement-Mode and Depletion-Mode β-Ga₂O₃ MESFETs with Graphene-Gate Architectures and Their Logic Applications

Ultrawide band gap (UWBG) β-Ga₂O₃ is a promising material for next-generation power electronic devices. An enhancement-mode (E-mode) device is essential for designing power conversion systems with simplified circuitry and minimal loss. The integration of an E-mode field-effect transistor (FET) with a depletion-mode (D-mode) FET can build a high-performance logic circuit. In this study, we first demonstrated the realization of an E-mode quasi-two-dimensional (quasi-2D) β-Ga₂O₃ FET with a novel graphene gate architecture via a van der Waals heterojunction. Then, we monolithically integrated it with a D-mode quasi-2D β-Ga₂O₃ FET, achieving an area-efficient logic circuit. The threshold voltage of the n-channel UWBG β-Ga₂O₃ material was controlled by forming a novel architecture of a double-gate graphene/β-Ga₂O₃ heterojunction, where both graphene and β-Ga₂O₃ were obtained by a mechanical exfoliation method. The fabricated double graphene-gate β-Ga₂O₃ metal-semiconductor FET (MESFET) was operated in the E-mode with a positive threshold voltage of +0.25 V, which is approximately 1.2 V higher than that of a single-gate D-mode β-Ga₂O₃ MESFET. Both E-/D-modes β-Ga₂O₃ MESFETs showed excellent electrical characteristics with a subthreshold swing of 68.9 and 84.6 mV/dec, respectively, and a high on/off current ratio of approximately 10⁷. A β-Ga₂O₃ logic inverter composed of E-/D-mode β-Ga₂O₃ devices exhibited desired inversion characteristics. The monolithic integration of an E-/D-mode quasi-2D FET with an UWBG channel layer can pave the way for various applications in smart and robust power (nano) electronics.