High Three-Terminal Breakdown Voltage Exfoliated β-Ga2O3 Nanolayer Field-Effect Transistors with Dual Field Modulating Plates
Authors: Bae Jinho , Kim Hyoung Woo , Kang In Ho , Kim Jihyun 
Published: February 2020

β-Ga2O3 is an intriguing material for high efficiency power devices because of its large direct bandgap (4.85 eV), high breakdown field (~8 MV/cm) and excellent thermal and chemical stability. Baliga’s figure of merit of β-Ga2O3 is 3214.1, superior to those of other materials such as GaN (846.0) or SiC (317.1). Interestingly, although β-Ga2O3 is not a van der Waals material, β-Ga2O3 can be mechanically exfoliated from single crystal substrate into thin layer due to the large anisotropy of the unit cell. The fabrication of high crystal quality β-Ga2O3 nanolayer devices could pave a way for next-generation high-power nanoelectronics devices.

In power device operation, the premature electrical induced by the concentrated electric fields limits the device operation under a high bias voltage, and threaten the reliability of devices. Various techniques such as recess gate or field-plate configurations have been proposed to relieve the electric field concentration and prevent premature breakdown. Adopting gate field plate, source field plate, or multiple field plate greatly enhance the performance of power devices, which can be applied to β-Ga2O3 based nanoelectronic power devices that can unlock the full potential of β-Ga2O3.

Due to the exceptional mechanical and electronic properties of 2D materials, researches about 2D materials has been at the forefront of this area during the last years. Studies on fabrication of functional devices based on 2D materials, including integration with themselves, are exploding. Among 2D materials, hexagonal boron nitride (h-BN) has been used as a dielectric material of 2D devices due to its excellent thermal conduc tivity (1700 – 2000 W/m∙K) and high dielectric constant (8 – 12 MV/cm), as well as defect-free and atomically flat surface, which can easily be obtained through mechanical exfoliation method. In our work, we used h-BN as a gate field plate dielectric layer by precise and selective transfer on β-Ga2O3 channel by using PDMS film. SiO2 dielectric layer was deposited on devices, followed by metal deposition for source field plate. By applying dual field plate structure β-Ga2O3 based power devices can show excellent performance in high voltage condition than conventional metal-semiconductor field effect transistors (MESFETs).

β-Ga2O3 MESFETs with h-BN gate field plate were fabricated by using the β-Ga2O3 and h-BN flakes obtained from respective crystals. Ohmic metal (Ti/Au) was deposited on both end of mechanically exfoliated β-Ga2O3 flakes, followed by precise positioning of exfoliated h-BN flakes on the channel. Top-gate electrode (Ni/Au) was deposited to fabricate β-Ga2O3 MESFETs, with a part of the electrode overlapped with h-BN to achieve a stepped gate field-plate structure. After deposition of SiO2 layer as dielectric layer of source field plate, selective etching of SiO2 layer was performed to construct source field plate structure. Fabricated β-Ga2O3 MESFETs showed excellent n-type DC output and transfer characteristics even after storage for one month in ambient air, which shows excellent long-term air-stability. Three-terminal off-state breakdown voltage of dual field-plated β-Ga2O3
MESFET was measured, which shows improvement in breakdown voltage compared with that of conventional β-Ga2O3 MESFET. The distribution of electric fields was calculated by Silvaco Atlas device simulation framework to study the effect of source field plate and h-BN gate field plate on electric field, which explain the improvement of breakdown voltage in those structure. By adopting dual field plate structure, ease of distribution of concentrated electric field was observed. In this study, we present that the performance of β-Ga2O3 MESFET as a power device can be greatly improved by adopting dual field plate structure, paving a way to the high-power nanoelectronic devices of β-Ga2O3. The details of our work will be discussed in the conference.