Films of vertically aligned carbon nanotubes (CNT's) have recently beeen grown within patterned areas, making them suitable as field emitters in many applications. In this article, we propose and model a nanotriode using CNT's as emitters. The overall device dimensions are around 100 nm. The electric-field distribution in the nanotriode has been computed using the commercial software package Simion 3D 7.0. A special discussion is devoted to the nanotube tip electric-field screening by the neighboring nanotubes in the film. It is outlined that the nanotube film density and the CNT's height distribution can greatly change the local field values. Film electron emission properties are enhanced for regular arrays of parallel nanotubes with the same height, for which an optimum CNT film density has been shown to exist. The CNT emission current has been computed using a recently developed model that takes into account the low dimensionality of the CNT electronic system. The device characteristics have been calculated as a function of the device geometry and its functional parameters. (C) 2003 American Vacuum Society.