PURPOSE:To form a guard ring whose curvature is relaxed by a method wherein a forbidden-band width in an avalanche multiplication layer is increased to be a stepped shape of two or more steps as the layer becomes more distant from a light-absorbing layer so that an impurity can be diffused deeply. CONSTITUTION:A guard ring 5' completely covers the curvature at an edge part of a stepped-shape p+-n junction, and relaxes the curvature at the edge part of the guard ring 5' to a certain extent; as a result, a breakdown voltage of the guard ring 5' itself is increased. Therefore, a forbidden-band width in an avalanche multiplication layer is increased in a stepped shape of two or more steps as the layer becomes more distant from a light-absorbing layer 3. That is to say, when a p-type impurity is diffused into an n-type conductive region or an ion is implanted, the coupling energy between atoms constituting a semiconductor becomes strong as the forbidden-band width in the n-type conductive region becomes large. Accordingly, the probability to substitute the atoms constituting the semiconductor with the impurity is lowered; the impurity is diffused deeply; the guard ring 5' whose curvature has been relaxed can be formed.
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