An electrostatic discharge (ESD) protection circuit includes an MOS transistor acting as a trigger for the circuit. A drain region of the MOS transistor is formed by an N-type heavily doped impurity region which overlaps an N-type well region. Further, a P-type heavily doped impurity region is formed in the N-type well region. The N-type and P-type heavily doped impurity regions are electrically connected to an input/output pad. The ESD protection circuit exhibits a reduced input capacitance at the pad, and a reduced breakdown voltage of the MOS transistor.