Polycrystalline diamond films have been found to display p-type surface conductivity. No bulk impurity is added to the films; the p-type characteristics of the undoped diamond are thought to be due to a surface or near surface hydrogenated layer. Carrier concentrations within the range ${10}^{17}{\text{–10}}^{19}\hspace{0.17em}{\mathrm{cm}}^{\mathrm{-3}}$ have been measured; control over the carrier concentration can be achieved by annealing the “as-grown” films in air. For a given annealing temperature a stable carrier concentration arises. The Hall carrier mobility has been explored and a value of $\text{>70\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{2}}\mathrm{/Vs}$ has been measured for a film with a carrier concentration of ${\text{∼5×10}}^{17}\hspace{0.17em}{\mathrm{cm}}^{\mathrm{-3}},$ the highest reported for polycrystalline thin film diamond and equivalent to boron doped single crystal diamond.

Planar metal–diamond–metal photoconductive devices have been fabricated from free standing large grain (20–30 μm) polycrystalline thin film diamond. An interdigitated electrode design with spacings of 20 μm was used to produce effective UV photodetecting devices at bias values in the range 0.1–10 V. A methane-air treatment has been used to modify the structures such that unprecedented performance characteristics have been recorded (106 higher response to 200 nm than visible wavelengths, <0.1 nA dark currents); spectral features similar to those observed in natural diamond crystals have been observed indicating that the treatment used led to near ideal electronic characteristics from polycrystalline material.