Realize the population inversion, meet the threshold condition and resonant condition. The first condition of produce light of stimulated emission of radiation is a population inversion, is to put in the valence band in the semiconductor electron to the conduction band. In order to obtain ion inversion, usually adopt the heavily doped p-type and n-type p-n junction, so, under the effect of applied voltage, the inversion occurs near the junction area ion - high in the Fermi level below the EFC is stored in the conduction band electrons, and in low Fermi level above EFV of storage of the holes in the valence band. A necessary condition for the population inversion is the laser, but not sufficient condition. To produce laser, have minimal loss of cavity, the cavity is the main part of the two parallel mirror each other, activate the material stimulated radiation emitted from the light bounces back and forth between the two mirrors, constantly bringing new stimulated radiation, make its have been enlarged. Only stimulated radiation amplifier gain greater than the various losses inside the laser, which satisfy a certain threshold conditions: