Aluminum nitride (AIN) thin films with various thicknesses (20-150 nm) are prepared on substrate Si(1 0 0) by radio-frequency (rf) magnetic reactive sputtering in an Ar-N-2, gas mixture. The field emission characteristics of the AIN thin films are measured in an ultra-high vacuum system. They depend evidently on the thicknesses. There is an optimum film thickness for the best field emission characteristics of AIN thin films. A turn-on electric field of 10 V/mu m and the highest emission current density of 284 mu A/cm(2) at an electric field of 35 V/mu m are obtained for the about 44-nm-thick AIN film. The Fowler-Nordheim plots show that electrons are emitted from AIN to vacuum by tunneling through the potential barrier at the surface of AIN thin films. (c) 2004 Elsevier B.V. All rights reserved.