For the complex light field amplitude of a semiconductor laser an equation of a generalized van der Pol oscillator with a fluctuating driving term is derived from first principles. This equation is shown to be valid for optical band-to-band transitions with and withoutk-selection rule. Neglecting the nonlinearity in the saturation higher than of second order and also the intensity dependence of the dispersion this equation reduces to the standard van der Pol equation with a noisy driving term. From the general equation the linewidth and the noise of the intensity of the laser emission is calculated above and below threshold. The results are in agreement with the experimental data ofArmstrong andSmith.