Abstract
Intervalence band absorption is discussed as a phenomenon that affects the threshold current of a semiconductor laser. However, calculated values of the absorption coefficient are too small to yield a remarkable effect. The situation is changed if phonon-assisted intervalence band absorption is taken into account. Then the absorption coefficient is considerably enlarged, for instance by more than an order of magnitude in In0.72Ga0.28As0.6P0.4 with a laser wavelength of 1.3 mu m.