We report electromodulated reflectance studies on the band structure of a dilute-N (∼1%) (GaIn)(NAs)/GaAs/AlAs vertical-cavity surface-emitting laser (VCSEL), as a function of temperature and incidence angle. The wide range of operating temperatures observed for this type of VCSEL (∼360 K here) is due to the reduced temperature variation of the effective band gap of the active (GaIn)(NAs) quantum wells, and broad gain. By comparing lasing properties and band structure we argue that the gain broadening is not simply due to alloy disorder but arises from a recently-proposed intrinsic property of (GaIn)(NAs): the existence of different band gaps for the five possible nearest-neighbor configurations of the N substitutional impurity. © 2001 American Institute of Physics.