Diamond films were elaborated in CH4-H-2-N-2 and CH4-CO2-N-2 gas mixtures in a pulsed microwave plasma under the same working conditions, varying the percentage of nitrogen from 0.1 to 5.2%. Raman spectroscopy was used to probe the quality of the film and the incorporation of nitrogen within diamond films was appreciated using cathodoluminescence (CL) spectroscopy. The intensities of peaks due to nitrogen-vacancy pairs showed that the quantity of incorporated nitrogen increased with that of the nitrogen introduced in the gas mixture. Arrhenius plots of the electrical conductivity showed that the incorporation of low quantities of nitrogen in the diamond film led to an increase in its resistivity. It can be speculated that small quantities of incorporated nitrogen balance the natural p-doping of microwave plasma chemical vapor deposition (MPCVD) diamond and, thus, enhance its insulating properties. Nevertheless, as higher quantities of nitrogen were incorporated, the resistivity of the diamond him decreased due to a possible n-doping. The doping hypothesis was confirmed by I-V measurements which exhibited a rectifier behavior of the n-diamond/p-Si heterojunction structure. This behavior was observed only in the case of a CO2-CH4 gas mixture, where a high concentration of nitrogen could be incorporated without affecting the continuity of the film.