This article reports the influence of substrate bias during growth and of hydrogen and nitrogen incorporation on density of states [N (E-F)] and field-emission threshold (Eturn-on) in tetrahedral amorphous carbon (ta-C) films, deposited using an S-bend filtered cathodic vacuum arc process. The variation in negative substrate bias from -20 to -200 V was found to initially lead to a small decrease in N (E-F) and Eturn-on, and a small increase in the emission current density (J) at 12.5 V/mu m in the case of as-grown ta-C films; beyond -200 V substrate bias there is a reversal in the trend. The values of N (E-F)=1.3x10(17) cm(-3) eV(-1), Eturn-on=8.3 V/mu m, and J=6.19 mA/cm(2) were observed at -200 V substrate bias. However at -300 V the properties were not very different from those at -200 V substrate bias and so with a view to use the higher energy, hydrogen and nitrogen incorporation studies were carried out in this condition. It was observed that there was further enhancement in properties with hydrogen and nitrogen incorporation. The best properties measured with in the range of hydrogen and nitrogen incorporation in the present study were N (E-F)=8.0x10(16) cm(-3) eV(-1), Eturn-on=7.6 V/mu m, and J=23.7 mA/cm(2), respectively.