A model for electron field injection from the n-silicon conduction band (CB) into the nitrogen-diamond CB is presented, The model takes into account the electric field penetration into the silicon base and the disturbance of the electron distribution therein. Allowing for a nonlinear anode voltage dependence of the field in the diamond film (and thus in vacuum too) a good fit to experimental current density-voltage (J-V) characteristics was obtained. The model can also predict the temperature dependence of the emission current in accordance with some existing experimental data.