The reverse current of lateral-type Schottky diodes fabricated on p-type homoepitaxial diamond was analyzed by changing the distance between Schottky and Ohmic electrodes and the metal materials in the Schottky electrodes. The maximum electric field at breakdown was 0.56 MV cm (1) for the Au Schottky contact and less than 0.26 MV cm (1) for the Al Schottky contact. The breakdown voltage depended on the electrode distance when the diamond surface was revealed in vacuum, whereas the Schottky diodes sustained the applied voltage of 500 V, corresponding to 0.69 MV cm (1), after covering of the diamond surface with an insulating liquid. Diamond surface protection is an indispensable technique for fabrication of high-voltage Schottky diodes based on diamond. (C) 2008 Elsevier B. V. All rights reserved.