a p + layer and an n + layer are provided on the top and bottom surfaces of an n - layer , respectively . an electrode is formed on the p + layer , while an electrode is formed on the bottom surface of the n + layer . in a direction from the electrode to the electrode , the area of the cross section of the n - layer is decreased , which cross section is perpendicular to the direction . an n -- layer is formed complementarily to the n - layer which is decreased in cross - sectional area . when a potential applied to the electrode is higher than a potential applied to the electrode , a depletion layer extends from a pn junction formed by the p + layer and the n - layer . since the impurity concentration of the n - layer is lower than that of the p + layer , the depletion layer extends substantially to the n - layer . the depletion layer extending to the n - layer substantially holds the voltage applied across the electrodes and . in order to improve a breakdown voltage , the cross - sectional area of the n - layer is preferably decreased exponentially . a field intensity in the depletion layer extending within the n - layer hardly depends on a distance from the electrode or .