This article reports the field emission measurements on as grown tetrahedral amorphous carbon (ta-C) and nitrogen incorporated tetrahedral amorphous carbon (ta-C: N) films grown using a pulsed filtered cathodic vacuum arc technique. The effect of varying thickness on field emission in the as grown ta-C films and the effect of varying nitrogen content in ta-C: N films have also been studied. The values of threshold field of emission (E-turn on) increase with decrease of thickness in the as grown ta-C films. Nitrogen incorporation up to 5.2 at. % in ta-C films decreases the value of Eturn on from 9.9 to 5.1 V/mu m and thereafter it starts increasing again. To understand the mechanism of electron emission, a realistic energy band diagram of ta-C: N/n++ Si heterojunction has been proposed from the experimentally measured valence and conduction band offsets, using in situ x-ray photoelectron spectroscopy and optical spectroscopy data already published in DRM 9 (2000) 1148. The data are explained using the Fowler and Nordheim theory. The field emission results obtained reveal that there exists a barrier to emission and the main barrier is at the front surface and this is related to the conduction band offset of the ta-C: N/n++ Si heterojunction. (C) 2008 American Vacuum Society.