摘要： In order to obtain both the band gap and a?nity of semiconductor by photoelectric e?ect and optical spectra, Lee A. DuBridge`s formula which describes the mathematical relationship between photocurrent and photon energy in metal is extended in semiconductor. Thus cut-o? energy of photon can be ?gured out via this formula, at the same time, the a?nity of semiconductor can be determined from the di?erence between cut-o? energy and band gap detected by optical spectra in semiconductor. The results of application on ?ve semiconductors prove that the method proposed in this paper is credible, the good agreements between experimental light energy-photoelectric yield curves and theoretical curves demonstrate that the classic Lee A. DuBridge`s theory of surface photoemission does an excellent job of ?tting the photo absorption pro?le shapes of semiconductor as an earlier application on metals, these results suggest that the Lee A. DuBridge`s formalism, which is straightforward and physical, may be of signi?cant utility in semiconductor spectroscopy. It is hoped also that the results will encourage a comprehensive theoretical analysis of the applicability of semiconductor. Beside, how the band gap of Cr3+ doped n-type TiO2 is controlled by composition of Ti is express with mathematical way.