Boron nitride (BN) films are grown on sapphire substrates by plasma-assisted chemical vapor deposition. BN films are doped with sulfur (S). The electrical resistivity of the S-doped BN film is reduced to 10(3) Omega cm, while the electrical resistivity of the undoped BN film is 10(11) Omega cm. It is demonstrated that the negative electron affinity appears on the BN surface. Insertion of a GaN layer between the BN film and sapphire leads to a tight adhesion of the BN film. Cathode and anode electrons are formed on the BN film and the sapphire substrate, respectively, by evaporating Ti and Au. An emission current of 1 mu A is obtained at an electric-field strength of 16 V/mu m for the planar field emitter. An emission current density as high as 0.1 A/cm(2) is detected. It is expected that the planar field emitters can be operated at several tens V with a decreased cathode-anode:spacing and that the present field emitter structure is applicable to a field-emission panel display.