Avalanche photodiode

An avalanche photodiode comprising:

• a substrate semiconductor layer having a first conductivity type and a uniform impurity concentration;
• a first semiconductor layer formed inside the substrate semiconductor layer, the first semiconductor layer having the first conductivity type and occupying a predetermined region in a lateral direction;
• a second semiconductor layer having a second conductivity type opposite to the first conductivity type and formed inside the substrate semiconductor layer on the first semiconductor layer in contact with the first semiconductor layer;
• a third semiconductor layer having the second conductivity type, the third semiconductor layer being formed inside the substrate semiconductor layer on the second semiconductor layer in contact with the second semiconductor layer and having a lower impurity concentration than the second semiconductor layer;
• a fourth semiconductor layer having the second conductivity type, the fourth semiconductor layer being formed in a shallow portion of the third semiconductor layer and having a higher impurity concentration than the third semiconductor layer;
• a fifth semiconductor layer having the first conductivity type and formed inside the substrate semiconductor layer at a position away from the third semiconductor layer in the lateral direction;
• a sixth semiconductor layer having the first conductivity type, the sixth semiconductor layer being formed in a shallow portion of the fifth semiconductor layer and having a higher impurity concentration than the fifth semiconductor layer;
• a first contact electrically connected to the fourth semiconductor layer;
• and
• a second contact electrically connected to the sixth semiconductor layer,
• wherein the first semiconductor layer is positioned just under the second semiconductor layer and the fifth semiconductor layer in contact with the second semiconductor layer and the fifth semiconductor layer, and an avalanche phenomenon is caused at a junction between the first semiconductor layer and the second semiconductor layer, and
• wherein the first semiconductor layer has a first portion locating just under the second semiconductor layer and a second portion locating just under the fifth semiconductor layer, and the first portion and the second portion are spaced from each other in the lateral direction.