We report on a barrier height control on electron field emission characteristics of GaN. It has been found that growing an ultra-thin AlN layer is effective to improve the field emission characteristics, lower threshold electric field and higher emission current density, of polycrystalline GaN on Mo. The Fowler-Nordheim plots of GaN/Mo with and without the ultra-thin AlN layer showed that the growth of the AlN layer effectively lowers the barrier height for field emission. The decrease in the barrier height results from the decrease in the effective electron affinity of GaN by the growth of the ultra-thin AlN layer and not from the negative electron affinity of AlN itself. (C) 2004 Elsevier B.V. All rights reserved.