High-Mobility P-Channel Wide Bandgap Transistors Based on Hydrogen-Terminated Diamond and Hexagonal Boron Nitride

In this talk, I will present the development of high-performance wide-bandgap p-channel transistors based on heterostructures of hydrogen-terminated diamond and hexagonal boron nitride (h-BN) [1-4].<br/>Diamond field effect transistors (FETs) have attracted interest because of diamond’s exceptional semiconductor properties and their potential applications in power and high-frequency applications [5,6]. Many works on diamond FETs have used hydrogen-terminated diamonds and their p-type surface conductivity induced by surface transfer doping; Electrons in the valence band of diamond are transferred to surface acceptors such as atmospheric adsorbates, acid gases, and high work-function oxides, generating p-type conductivity of holes. Surface transfer doping can create a high density of holes in diamond, but negatively charged surface acceptors act as a source of hole scattering and limit transistor performance. The negatively charged acceptors also cause a positive shift in the threshold voltage, leading to normally-on operation.<br/>In our recent study [4], we fabricated a heterojunction field-effect transistor consisting of a hydrogen-terminated diamond channel and h-BN gate insulator, without relying on surface transfer doping. After the diamond surface was hydrogenated by hydrogen plasma, the diamond was transferred in a vacuum suitcase to an Ar-filled glove box, where the diamond was laminated with a thin crystal of h-BN using the Scotch-tape-exfoliation and dry-transfer techniques. This process reduced the density of atmospheric acceptors substantially. Despite this reduced density of surface acceptors, we observed excellent p-type FET properties. In fact, the transistor has a room-temperature mobility of 680 cm²V^-1s^-1, sheet resistance of 1.4 kΩ, and normalized on-current of 1600 μm mA mm^-1 , which are the best values reported for p-channel FETs made of wide-bandgap semiconductors. Importantly, the transistor also shows a normally-off behavior with a high on-off ratio larger than 10⁸. The FET characteristics were quantitatively explained on the basis of standard FET models. This modeling suggests that the holes in our FETs are generated in an inversion mode by an upward band bending due to gate bias, in the absence of acceptors.<br/>Our findings indicate that the surface transfer doping and surface acceptors, which had been believed to be essential for the surface conductivity of hydrogen-terminated diamond, are unnecessary and even should be removed to improve FET performance. The new approach to making diamond FETs opens a pathway to the development of high-performance wide-bandgap p-channel transistors and complementary circuits.<br/>This work has been carried out in collaboration with Y. Sasama, T. Kageura, K. Komatsu, S. Moriyama, M. Imura, J. Inoue, T. Teraji, S. Sugiura, T. Terashima, S. Uji, K. Watanabe, T. Taniguchi, and T. Uchihashi at National Institute for Materials Science, Japan.<br/>References:<br/>[1] Y. Sasama et al. APL Materials, 6, 111105 (2018).<br/>[2] Y. Sasama et al. Physical Review Materials, 3, 121601(R) (2019).<br/>[3] Y. Sasama et al. Journal of Applied Physics, 127, 185707 (2020).<br/>[4] Y. Sasama et al. arXiv: 2102.05982<br/>[5] M. W. Geis et al. physica status solidi (a) 215, 1800681 (2018).<br/>[6] N. Donato et al. Journal of Physics D: Applied Physics 53, 093001 (2020).