| 摘要 |
A P+layer (3) and an N+layer (1) are provided on the top and bottom surfaces of an N-layer (21), respectively. An electrode (7) is formed on the P+layer (3), while an electrode (8) is formed on the bottom surface of the N+layer (1). In a direction from the electrode (7) to the electrode (8), the area of the cross section of the N-layer (21) is decreased, which cross section is perpendicular to the direction. An N--layer (22) is formed complementarily to the N-layer (21) which is decreased in cross-sectional area. When a potential applied to the electrode (8) is higher than a potential applied to the electrode (7), a depletion layer extends from a PN junction formed by the P+layer (3) and the N-layer (21). Since the impurity concentration of the N-layer ( 21) is lower than that of the P+layer (3), the depletion layer extends substantially to the N-layer (21). The depletion layer extending to the N-layer (21) substantially holds the voltage applied across the electrodes (7) and (8). In order to improve a breakdown voltage, the cross-sectional area of the N-layer (21) is preferably decreased exponentially. A field intensity in the depletion layer extending within the N-layer (21) hardly depends on a distance from the electrode (7) or (8).
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