The body thickness dependence of impact ionization for a multiple-gate fin field-effect transistor (FinFET) is presented. It is found that the nonlocal effect and series resistance are distinct features of reduced impact ionization in the multiple-gate FinFET, and these effects become more pronounced as the body thickness decreases. The impact ionization constant B-i is newly extracted by considering the series resistance and nonlocal carrier heating effect. A refined analytical substrate current model is developed from the previous model and revamped for multiple-gate devices. The new substrate current model is then compared with measurement data, and good agreement is observed.