The electrical properties of chemical vapour deposited p-type epitaxial diamond layers are studied in high electric field conditions. The quasi-static current-voltage characteristics have been measured using transmission-line pulse method with 100 ns pulses. Reproducible impurity impact ioni-zation avalanche breakdown occurs at a critical electrical field in the range of 100–200 kV cm À1 depending on the acceptor concentration and temperature, leading to complete ionisation of neutral impurities. The current-voltage characteristics exhibit an S-shape with the bi-stable conduction characteristic of impurity impact ionisation. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4946853] Due to their high ionisation energy, dopants in diamond are only partially ionised at room temperature, e.g., less than 0.1% of boron impurities are ionised in a low-doped p-type diamond. As a matter of fact, the temperature range of incomplete ionisation of boron acceptor, i.e., the freeze out region, extends up to about 1000 K. The consequent high re-sistivity severely limits potential applications of diamond as semiconductor. 1,2 In the freeze out regime, the conductivity can be increased by excitation of free carriers bounded to the neutral impurity centres to conduction/valence band by (1) thermal ionisation rising the temperature, (2) optical ionisa-tion with light irradiation of suitable wavelength, and finally (3) impact ionization of neutral centres by the free carriers under a sufficiently high electric field. Under a high electric field, the free carriers can gain enough energy to be able to ionise neutral impurities by inelastic collisions leading to free carrier multiplication. This process results in nonlinear current voltage characteristics. 3