Protection circuit associated with input terminal of semiconductor device

Claim 1

A protection circuit associated with a terminal for protecting an internal circuit integrated on a semiconductor substrate of a first conductivity type from being damaged by an excess terminal voltage, said semiconductor substrate being covered with an insulating film, said terminal extending across said insulating film, comprising: a) a well of a second conductivity type formed in said semiconductor substrate, said second conductivity type being opposite to said first conductivity type; b) a first impurity region of said first conductivity type formed in said well and located between inner and outer regions of said well, said inner region being located inside of said first impurity region, said outer region being located outside of said first impurity region, said terminal having a trunk portion and first and second branch portions extending from said trunk portion and located over said first impurity region, said first and second branch portions having respective distal ends spaced apart from each other by a distance, said branch portions of said terminal being in contact with said first impurity region through a first contact window formed in said insulating film; c) a second impurity region heavily doped with impurity atoms of said second conductivity type and formed in said inner region of said well; d) a third impurity region heavily doped with impurity atoms of said second conductivity type and formed in said outer region of said well; and e) a current discharge line extending across said insulating film having a trunk portion in contact with said third impurity region through a second contact window formed in said insulating film, and third and fourth branch portions being in contact with said third impurity region through said second contact window and extending so that said third and fourth branch portions substantially surround said terminal, said third and fourth branch portions having respective distal ends facing both sides of said trunk portion, respectively, said current discharge line further having a fifth branch portion extending through said terminal gap and being in contact with said second impurity region through a third contact window formed in said insulating film, a p-n junction being formed between said first impurity region and said well, said p-n junction being broken down upon application of said excess terminal voltage so that current flows from said terminal through said first impurity region, said p-n junction, said well and said second and third impurity regions to said current discharge line.