Image Number 4 for United States Patent #7651917.
In the present invention, an npn junction is formed by circularly forming a p- type impurity region and n+ type impurity regions on a same single-crystalline substrate as a MOS transistor. Multiple npn junctions are formed apart from each other in concentric circular patterns. With this configuration, steep breakdown characteristics can be obtained, which results in good constant-voltage diode characteristics. Being formed in a manufacturing process of a MOS transistor, the present protection diode contributes to process streamlining and cost reduction. By selecting the number of npn junctions according to breakdown voltage, control of the breakdown voltage can be facilitated.