Using coating with a boron nitride (BN) film, we attempted to improve field-emission characteristics of gallium nitride (GaN) cold cathodes. First, we measured the field-emission characteristics of BN/n-Si samples to investigate the electron-emission mechanism of the BN film. We discuss, the electron-emission process of the BN film in terms of the surface roughness dependence of the field-emission characteristics. We suggest that the coating with a BN film thinner than 10 nm is, effective in reducing the turn-on voltage of the electron emission. Second, field-emission characteristics are examined for the hexagonal n-type GaN layers roughened with H-2 plasma treatment. Moreover, nanocoating with a BN film is carried out on the surface of the GaN sample for the BN/GaN sample. We achieved a turn-on electric field as low as 4.6 V/mum. (C) 2003 American Vacuum Society.