Ultrawide-Bandgap Nanoscale p-n Junction Field Effect Transistors of GaN / β-Ga2O3 for High Power Operation
Authors: Bae Jinho , Choi Yong ha , Kim Jihyun 
Published: May 2020

β-Ga2O3 is emerging as a next-generation ultrawide bandgap materials for high efficiency power devices, owing to its large direct band gap (4.85eV), high breakdown field (~ 8 MV / cm) and excellent thermal∙chemical stability. The Baliga’s figure-of-merit (BFOM) of β-Ga2O3 is 3214.1, which is superior to other materials such as GaN (846.0) or SiC (317.1). In addition to superior characteristics, the availability of large crystalline substrates with controllable carrier concentrations from semi-insulation to n-type is responsible for many of the recent improvements that will utilize β-Ga2O3 as near future power device material. However, potential of β-Ga2O3 has not yet been fully explored because of the low thermal conductivity of β-Ga2O3 and absence of p-type doped β-Ga2O3. To overcome this limitation, integration with p-type materials with excellent thermal conductivity is being studied.

Although β-Ga2O3 is not a van der Waals material, β-Ga2O3 can be mechanically exfoliated into a thin layer due to the large anisotropy of the unit cell from the single crystal substrate. The mechanically exfoliated flakes with high crystallinity have no significant strains due to lattice mismatch with the underlying substrate, which has great advantages in integration with other substrates and miniaturization of devices. Based on these advantages, various studies have been conducted to fabricate nanoscale devices that utilize the excellent properties and overcome the disadvantages of β-Ga2O3, through integration of exfoliated flakes with p-type substrates such as diamond and SiC. In our work, nanoscale junction field effect transistors (JFETs) were fabricated by utilizing p-n junction between β-Ga2O3 flakes and p-type GaN substrate.

β-Ga2O3 / GaN JFETs were fabricated by using the β-Ga2O3 flakes exfoliated from single-crystal bulk substrate. Dielectric layer between GaN substrate and β-Ga2O3 metal electrode were defined by hydrogen silsesquioxane (HSQ), followed by e-beam patterning. β-Ga2O3 flakes were precisely transferred on HSQ pattern by PDMS-assisted dry transfer method. Ohmic metal of GaN (Ni/Au) and β-Ga2O3 (Ti/Au) was deposited by e-beam evaporator, followed by rapid thermal annealing to confirm ohmic contact of GaN and β-Ga2O3, respectively. Fabricated β-Ga2O3 / GaN JFETs showed outstanding electric properties with high rectification ratio between p-n junction of GaN and β-Ga2O3. Fabricated devices showed excellent n-type DC output and transfer characteristics even after few weeks, which shows excellent long-term air-stability. Robustness and stability of fabricated devices under harsh conditions were confirmed by operating devices under various temperature. Thermal distribution and electric field simulation of fabricated JFETs were performed to confirm the effect of GaN substrate in thermal issues. In this study, we present the way to integrating β-Ga2O3 devices with p-GaN, resolving issues of thermal conductivity and p-type doping without lattice mismatch or defects. The details of our work will be discussed in the conference.