Origins of high radiative efficiency and wideband emission from InAs quantum dots (QDs) were studied experimentally and theoretically. Radiative efficiency was found to depend strongly on the absence of large nonradiative islands. Theoretical calculations were performed to determine the combined effects of QD size and its fluctuation on the energy range. Good agreement between the theoretical and experimental results was obtained. Recognizing that high output power and large optical bandwidth are key figures of merit for QD superluminescent diodes (SLDs), we believe that the findings from our work will be beneficial for those working on QD-SLDs.