A semiconductor device includes a semiconductor layer that has a transistor structure including a p type source region, a p type drain region, an n type body region between the p type source region and the p type drain region, and a gate electrode facing the n type body region and a voltage-regulator diode that is disposed at the semiconductor layer and that has an n type portion connected to the p type source region and a p type portion connected to the gate electrode, in which the transistor structure and the voltage-regulator diode are unified into a single-chip configuration.