The relationship between gain and carrier density is analysed. In the quantum well(QW) lasers, initially, the gain increases rapidly with the carrier density and then starts to saturate. It can be seen that QW lasers have a higher differential gain because of the step-like state density, and that the gain saturates at higher carrier densities because of the constant state density of the lowest subband. It is shown that simple logarithmic gain-carrier density is more accurate than the traditional linearized form for a QW laser.