This work presents a model for describing the shape of the spontaneous emission spectrum from a quantum-well structure. A function is introduced to specify the probability distribution for the effective band gap. Based on this model, the coexisting carrier thermal broadening and effective band gap broadening in the spontaneous emission spectrum can be separated from each other. Applying this model to the spectra of AlGaInP light-emitting diodes reveals that the probability distribution functions are almost Gaussian. Therefore, the emission spectra can be described by an analytical expression with fitted parameters. Possible reasons for this band gap broadening are discussed. The determination of the junction temperatures from the emission spectra and possible deviations of the results thus determined are also elaborated.