Abstract: A semiconductor device including a substrate, a first well, a second well, a gate, a first doped region, and a second doped region. The substrate includes a first conductive type. The first well includes a second conductive type and is formed in the substrate. The second well includes the second conductive type and is formed in the substrate. The gate is formed on the substrate and overlaps the first and the second wells. The first doped region includes the second conductive type. The first doped region is formed in the first well and self-aligned with the gate. The second doped region includes the second conductive type. The second doped region is formed in the second well and self-aligned with the gate. The gate, the first and the second doped regions constitute a transistor.