In a field emission display panel, a triode structure is often used to obtain high brightness and low driving voltage. This article studies the under-ate triode structure with different dielectric layer thicknesses. To increase the emission current, some holes are designed on the cathode electrode. In this structure, the fringe field has been increased effectively and the current density emitted from the cathode surface is more uniform. Apart from this, a focus electrode is used to improve the focus performance of the electron beam in an under-gate structure. The spot size and current density distribution on the screen have been optimized with the different focus voltage. Consequently, a good resolution and high brightness are obtained. (c) 2006 American Vacuum Society.