For avalanche photodiodes(APDs), low operation voltage is required for integrated circuit stability and low power consumption. In this paper,a model for InAlAsSb separate absorption, charge,andmultiplication(SACM)APDisestablished. Togethighergainatlowerreversedbiasvolt-age without sacrificing the operating voltage range,a high/low band gap multiplication layer is adopt-ed. Theeffectsofthethicknessanddopingconcentration ofthemultiplicationlayeronthe dark-current and the break-down voltage have been investigated. By optimization of the doping concentration,the break-down voltage and punch-through voltage can be decreased simultaneously.