We have studied, by means of simulations and experiments, the interaction between the electromagnetic field, generated by excimer laser, and Si device. This study strictly refers to laser annealing process, recently attracting a broad interest as an alternative thermal treatment. Our numerical methodology is based on coupling the simulation of the electromagnetic field, for the calculation of the heat source distribution, and the simulations of the thermal, phase, and impurity fields. Simulations of laser irradiation in metal-oxide-semiconductor transistor structures are discussed and compared to the corresponding experimental analysis. Our results are useful to understand problematics and perspectives of the laser annealing application in the fabrication of scaled devices.