In this paper the flat-band potential of (111) plane, of n-InP, is determinated by two methods, the first is to determine the differential capacitance and the second is to determine the open-circuit potential when the electrode is illuminated with a light of enough intensity. The results obtained from these two methods differ less than 0.14V. Many factors, such as pH, redox couplex, frequency, crystalline plane and surface treatment are investigated. The influence of pH on the flat-band potential is -56V/pH, and four redox couples, i.e. Fe2+/Fe3+, I-/I3-, Fe((CN)6)4-/Fe((CN)6)3-, S2-/(Sn)2- as well as the frequencies do not influence the flat-band potential, the relexation phenomena of the capacitance and the resistance can be explained by the existence of surface states. The crystalline plane and the surface treatment influence the flat-band potential obviously. Finally, the donor concentration and the position of the energy band edge are calculated.

Key words: ELECTRODE, SEMICONDUCTOR MATERIAL, ELECTRODE POTENTIAL