Leakage current for MOSFET in off-state is one of the serious problems in charge-based analog circuits under low power supply. To suppress the leakage current, a method that a slight voltage is applied to source to accomplish reverse bias between source and bulk is proposed. The proposed bias condition, also other bias conditions, is analyzed by injection carrier density in p-n junction and surface carrier concentration in MOS diode in four-terminal MOSFET. Leakage current is modeled by combining the characteristics of p-n junction with MOS diode in MOSFET. The characteristics of MOSFET fabricated with a standard 0.18µm n-well CMOS technology are measured to investigate the basic principle. Measured leakage current fits to the theoretical leakage current exactly. The proposed slight bias to source terminal in MOSFET is proved most efficient to reduce the leakage current. Based on the proposed source bias condition, MOSFET switches with low leakage current under a single power supply are proposed.