To extend the high performances of the GaAs MSM photodetectors with InGaP buffer and capping layers, the (NH4)(2)S-x treatment of InGaP is investigated. The surface states are reduced by sulfur passivation. Whereupon, the performances of InGaP Schottky contact with Ti/Pt/Au metals are improved. The improved dark current and insensitive responsivity with incident optical power are demonstrated by suitable process control of sulfur passivation. (C) 1997 Elsevier Science Ltd.