A semiconductor structure includes a first P-well, a first P-type diffusion region, a first N-type diffusion region, a second P-type diffusion region, and a first poly-silicon layer. The first P-type diffusion region is deposited in the first P-well and coupled to a first electrode. The first N-well is adjacent to the P-well. The first N-type diffusion region is deposited in the first N-well. The second P-type diffusion region is deposited between the first P-type diffusion region and the first N-type diffusion region, which is deposited in the first N-well. The second P-type diffusion region and the first N-type diffusion region are coupled to a second electrode. The first poly-silicon layer is deposited on the first P-type diffusion region.