An electrostatic discharge (ESD) protection circuit includes a substrate, and a plurality of unit bipolar transistors formed in the substrate. Each of the plurality of unit bipolar transistors may include a first-conductivity-type buried layer formed in the substrate, a first-conductivity-type well formed over the first-conductivity-type buried layer, a second-conductivity-type well formed in the first-conductivity-type well, a first-conductivity-type vertical doping layer vertically formed from the surface of the substrate to the first-conductivity-type buried layer so as to surround the first-conductivity-type well, and a first-conductivity-type doping layer and a second conductivity-type doping layer formed in the second-conductivity-type well. The first-conductivity-type doping layer of any one of the adjacent unit bipolar transistors and the first-conductivity-type vertical doping layer of another one of the adjacent unit bipolar transistors may be connected to each other.