The present invention provides a transistor comprising a substrate having a surface; a first deep well region in the substrate; a second deep well region in the substrate, isolated from and encircling the first deep well region; a first well region in the substrate and on the first deep well region; two second well regions in the second deep well region and respectively at two opposite sides of the first well region; a source region in the first well region and adjacent to the surface; two drain regions in the two second well regions respectively and adjacent to the surface; two gate structures on the surface, wherein each of the two gate structures is between the source region and one of the drain regions respectively; and a guard ring in the substrate encircling the second deep well region, and on the periphery of the transistor.