  Field-effect transistors made of wide-bandgap semiconductors can handle high
electric power in a small space and have been of increasing importance in power
and high-frequency electronics. An obstacle to advancing the field is the poor
performance of hole-based p-channel transistors compared with that of their
electron-based n-channel counterparts. This problem stems from the low mobility
of holes, and it has constrained the production of energy-efficient
complimentary circuits with integrated n- and p-channel transistors. The
present paper reports fabrication of a p-channel wide-bandgap transistor with
an exceptionally high hole mobility of $680$ cm$^2$V$^{-1}$s$^{-1}$, consisting
of a hydrogen-terminated diamond channel and hexagonal boron nitride (h-BN)
gate insulator. A high-quality h-BN/diamond heterostructure, with a reduced
density of acceptor states near the diamond surface, suppresses hole scattering
and thereby enables the device to have the highest mobility, lowest sheet
resistance ($1.4$ k$\Omega$), and largest on-current ($1800$ $\mu$m mA
mm$^{-1}$) among p-channel wide-bandgap transistors. Importantly, the
transistor also shows normally-off behavior, with a high on/off ratio exceeding
$10^8$. These characteristics are all desirable for switching high powers with
low losses. This study opens a pathway to the development of high-mobility
p-channel wide-bandgap transistors with great potential for compact, fast, and
energy-efficient electronic devices.
