The differential gain which is an important parameter for modulation dynamics in semiconductor lasers is evaluated experimentally by measuring the gain coefficient and the carrier lifetime in GaAs/AlGaAs double‐heterostructure (DH) lasers, quantum well (QW) lasers, andp‐modulation‐doped quantum well (p‐MDQW) lasers. The results indicate that the differential gain of the QW laser is 2.4 times as high as that of the DH laser, which is consistent with the theory. In addition, it is found that improvement of the differential gain using thep‐MDQW structure is not so large as that expected by the theory. This result suggests that enhanced energy broadening due to the reduction of the equivalent dephasing time &tgr;eqin, which includes both the dephasing time &tgr;indue to the intraband relaxation and the band tailing effects, significantly affects the gain spectra in thep‐MDQW lasers, which is confirmed by the measurement of the spontaneous emission spectra.