| 摘要 |
An ultra high speed APD capable of realizing reduction in an operating voltage and quantum efficiency enhancement at the same time is provided. Under operating conditions APD, a doping concentration distribution of each light absorbing layer is determined so that a p-type light absorbing layer ( 16 ) maintains a p-type neutrality except a part thereof, and a low concentration light absorbing layer ( 15 ) is depleted. Moreover, a ratio between a layer thickness W<SUB>AN </SUB>of the p-type light absorbing layer ( 16 ) and a layer thickness W<SUB>AD </SUB>of the low concentration light absorbing layer ( 15 ) is determined so that W<SUB>AD</SUB>>0.3 mum and a delay time of an element response accompanying a transit of carriers generated in the light absorbing layer by light absorption takes on a local minimum under a condition that a layer thickness W<SUB>A </SUB>(=W<SUB>AN</SUB>+W<SUB>AD</SUB>) of the light absorbing layer is constant.
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