A semiconductor device is provided. The semiconductor device is suitable for an electrostatic discharge protection circuit. The semiconductor device includes a gate structure, an N-type source region, an N-type well region, an N-type drain region, and an N-doped region. Wherein, the gate structure comprises a gate and a gate oxide layer. The gate oxide layer is disposed between the gate and a substrate. In addition, the N-type source region is disposed in the substrate at one side of the gate, and the N-type well region is disposed in the substrate at another side of the gate. The N-type drain region is disposed in the substrate between the N-type well region and the gate structure. The N-type drain region has a first toothed part disposed in the N-type well region. The N-doped region is disposed in the N-type well region, and the N-doped region has a second toothed part.