Abstract: A semiconductor device includes a semiconductor substrate of a first conductivity-type, a buried diffusion layer of a second conductivity-type formed in the semiconductor substrate, a first well of the second conductivity-type having a bottom portion in contact with a top portion of the buried diffusion layer, the first well having an annular shape in a planar view, and a second well of the first conductivity-type formed to be surrounded by the first well. The semiconductor device further includes a diffusion region formed between a first portion of the second well and a second portion of the second well, the diffusion region having an impurity concentration lower than that of the second well, so that a depletion layer formed in the diffusion region can be provided, a transistor formed on the second well to function as an ESD (electro-static discharge) protection element, and an external terminal connected to a drain of the transistor.