| 摘要 |
PROBLEM TO BE SOLVED: To suppress the starting of a parasitic thyristor and improve the latch up breakdown voltage by cutting off trigger currents flowing in a first conductivity type well region and a semiconductor substrate, based on the potential difference between a first and second power sources in the event of a potential variation. SOLUTION: N-channel MOSFETs Qn1 in output buffer circuit cells 20B of a buffer circuit are constituted on a main surface of an SITAP type well region 4 formed on a p- type semiconductor substrate 1. Between the p-type well region 4 and the p-type semiconductor substrate 1, an isolation region 2 for electrically isolating both mutually is constituted. When a noise occurs in a reference power source Vss1 by the operation of the output buffer circuit composed of output buffer circuit cells 20B, trigger currents flowing in the p-type well region 4 and the p- type semiconductor substrate 1 can be cut off through the isolation region 2, based on the potential difference between the reference power sources Vss1, Vss2.
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