  Surface electronic properties of undoped hydrogen terminated diamond covered
with adsorbates or in electrolyte solutions are summarized. The formation of a
conductive layer at the surface of diamond is discussed based on Hall effect,
conductivity, contact potential difference (CPM), scanning electron microscopy
(SEM), and cyclic voltammetry data applied on homoepitaxially grown CVD diamond
films with atomically smooth hydrogen terminated surfaces. Due to electron
transfer from valence band states into empty states of the electrolyte
(transfer doping), a highly conductive surface layer is generated. Holes
propagate in the layer with mobilities up to 350 cm2/Vs. The sheet hole density
is in the range 10^11 to 5x10^12 cm^-2, and dependents on pH of the
electrolyte. Numerical solutions of the Schrodinger and Poisson equations
reveal a 2D density of state (DOS) distribution. This has been utilized to
manufacture ion-sensitive field effect transistors (ISFET). The drain source
conductivity is pH dependent, with about 66 mV/pH. Application of potentials
larger than the oxidation threshold of +0.7 V (pH 13) to +1.6 V (pH 1) gives
rise to strong leakage currents and to partial surface oxidation.
