A criterion for mode locking a semiconductor diode laser is proposed. The criterion is based on reducing the difference frequency associated with unequally spaced, longitudinal modes to a value smaller than the mode width narrowed by stimulated emission. An integrated GaAs‐GaAlAs structure that satisfies this condition is described. In addition to providing a more compact structure than the external mirror geometries previously used to generate mode‐locked pulses, the integrated composite resonator avoids the critical alignment problems and subcavity reflection limitations of an external resonator system.