We experimentally investigated the mechanisms ruling the electro-optical performance of a light intensity modulator, based on the light dispersion/absorption effect by free carriers. Comparing the analyses obtained with different microelectronic standard tools we obtained experimental bi-dimensional maps of carriers in the device. Our results are in full agreement with theoretical distributions, with optic transmission loss measurements and light intensity images by an IR camera. We compared emission microscopy analysis to electro-optical measurements to fully investigate the device dynamic performances. We experimentally discriminated the two phenomena regulating the carrier distribution in the device: the generation/recombination processes and the electrical drift of carriers, at low and high electrical modulation frequencies, respectively. The observed threshold of 500 kHz between the two regimes is in full …