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.