A semiconductor device has a p-n-p transistor structure having a collector implemented by a p--substrate, a base formed as an n-diffused region in the surface region of the substrate, and an emitter formed as a p+-diffused region in the first n-diffused layer. The p--substrate and the n-base are maintained at a ground level, while the p+-collector is maintained at a positive potential for biasing the p-n junction formed between the emitter and the base. The bias potential allows the p-n-p transistor structure to operate in its saturation region to activate the base region to define an enlarged carrier-incresed zone. An analog input pad is located within the carrier-increased zone and protected from a noise propagated from a digital circuit section located outside the carrier-increased zone.
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