  Electrically-conducting diamond is a promising candidate for next-generation
electronic, thermal and electrochemical applications. One of the major
obstacles towards its exploitation is the strong degradation that some of its
key physical properties - such as the carrier mobility and the superconducting
transition temperature - undergo upon the introduction of disorder. This makes
the two-dimensional hole gas induced at its surface by electric field-effect
doping particularly interesting from both a fundamental and an applied
perspective, since it strongly reduces the amount of extrinsic disorder with
respect to the standard boron substitution. In this short review, we summarize
the main results achieved so far in controlling the electric transport
properties of different field-effect doped diamond surfaces via the ionic
gating technique. We analyze how ionic gating can tune their conductivity,
carrier density and mobility, and drive the different surfaces across the
insulator-to-metal transition. We review their strongly orientation-dependent
magnetotransport properties, with a particular focus on the gate-tunable
spin-orbit coupling shown by the (100) surface. Finally, we discuss the
possibility of field-induced superconductivity in the (110) and (111) surfaces
as predicted by density functional theory calculations.
