JP2724218B

PURPOSE:To suppress variation in a substrate potential and to improve reliability by switching in two ways the substrate bias voltage limiting level of a substrate bias level limiter at the times of turning ON a power source and normally operating. CONSTITUTION:The output LMT of a substrate bias level limiter 12 becomes 'L', the output P of an initial limiting pulse detector 13 becomes 'H' at the time of turning ON a power source, and the substrate bias limiting level of the limiter 12 is set to V1 lower than the limiting level V2 of the substrate bias at the time of normal operation. If a substrate bias generator 11 is started to be operated, the limiter 12 outputs an 'H' signal LMT when the substrate potential is lowered to the V1. Thus, the generator 11 is stopped, the output P of the detector 13 becomes 'L', and the substrate bias limiting level of the limiter 12 is switched to becomes the V2. Thus, even if the substrate potential is floated by capacitive coupling, the potential is recovered to a predetermined optimized value. Accordingly, variation in the potential is suppressed to improve reliability.