In this study, transient leakage current caused by a parasitic bipolar junction transistor (BJT) in nanowire-type gate-all-around metal-oxide-semiconductor field-effect transistors is physically modeled for circuit design. The model considers the majority carrier concentration in the body, which is modulated by the gate-to-body bias. The parasitic BJT gain is dependent on the majority carrier concentration, which exceeds the body doping concentration in transient conditions. Three-dimensional technology computer-aided design simulation is performed to verify the model. The model accurately predicts the transient leakage current according to various structural parameters.