ESD protection diode

A semiconductor device according to an embodiment includes a semiconductor layer that has first and second plane and includes first-conductivity-type first semiconductor region, second-conductivity-type second semiconductor region between the first semiconductor region and the first plane, first-conductivity-type third semiconductor region between the second semiconductor region and the first plane and has a lower first-conductivity-type impurity concentration than the first semiconductor region, and second-conductivity-type fourth semiconductor region between the third semiconductor region and the first plane and has a higher second-conductivity-type impurity concentration than the second semiconductor region; a first electrode on a side of the first plane of the semiconductor layer and is electrically connected to the third semiconductor region and the fourth semiconductor region; and a second electrode on a side of the second plane of the semiconductor layer, is electrically connected to the first semiconductor region, and is not electrically connected to the second semiconductor region.