In this paper, a thorough characterization of terahertz silicon lasers with respect to doping concentration and operation temperature has been carried out. Several factors limiting the laser operation, such as heating of the laser crystal and absorption by photoinduced free carriers, are discussed. The optimal doping concentration has been determined. The influence of the pump geometry on the laser efficiency has been investigated. It was found that an external uniaxial force applied to the laser crystal lowers the pump threshold and increases the output power.