Abstract: A diode device for an electrostaticdischarge (ESD) protection circuit includes a P-type substrate, a buried N+ heavily doped semiconductor layer implanted in the P-type substrate and bounded by a deep trench isolation, a P well disposed above the buried N+ heavily doped semiconductor layer in the P-type substrate and isolated from the P-type substrate by the deep trench isolation. A P+ doped region, which serves as an anode of the diode device, is located in the P well. A N+ doped region, which serves as a cathode of the diode device, is laterally disposed in the P well and spaced apart from the P+ doped region. The P+ doped region, the buried N+ heavily doped semiconductor layer, and the P-type substrate constitute an open base parasitic PNP bipolar transistor.