Abstract: A MOS transistor comprising channel stoppers formed of a first polysilicon layer to determine a channel width, and a gate electrode formed of a second polysilicon layer, wherein a bias voltage is applied to the channel stoppers. In a charge detector having a source follower circuit with a drive MOS transistor and a load MOS transistor for converting a transferred signal charge into a signal voltage, the MOS transistor of the invention is used as the drive transistor, and its source output voltage is fed back as a bias voltage to the channel stoppers, thereby minimizing both the DC bias variation in the output voltage of the source follower circuit and the nonuniformity in the conversion efficiency.