  Field-effect transistors made of wide-bandgap semiconductors can operate at
high voltages, temperatures and frequencies with low energy losses, and have
been of increasing importance in power and high-frequency electronics. However,
the poor performance of p-channel transistors compared with that of n-channel
transistors has constrained the production of energy-efficient complimentary
circuits with integrated n- and p-channel transistors. The p-type surface
conductivity of hydrogen-terminated diamond offers great potential for solving
this problem, but surface transfer doping, which is commonly believed to be
essential for generating the conductivity, limits the performance of
transistors made of hydrogen-terminated diamond because it requires the
presence of ionized surface acceptors, which cause hole scattering. Here, we
report on fabrication of a p-channel wide-bandgap heterojunction field-effect
transistor consisting of a hydrogen-terminated diamond channel and hexagonal
boron nitride ($h$-BN) gate insulator, without relying on surface transfer
doping. Despite its reduced density of surface acceptors, the transistor has
the lowest sheet resistance ($1.4$ k$\Omega$) and largest on-current ($1600$
$\mu$m mA mm$^{-1}$) among p-channel wide-bandgap transistors, owing to the
highest hole mobility (room-temperature Hall mobility: $680$
cm$^2$V$^{-1}$s$^{-1}$). Importantly, the transistor also shows normally-off
behavior, with a high on/off ratio exceeding $10^8$. These characteristics are
suited for low-loss switching and can be explained on the basis of standard
transport and transistor models. This new approach to making diamond
transistors paves the way to future wide-bandgap semiconductor electronics.
