Avalanche photodiode

An avalanche photodiode comprising:

• a first semiconductor layer of a first conductivity type formed on a substrate of the first conductivity type;
• a second semiconductor layer of a second conductivity type opposite from the first conductivity type, the second semiconductor layer being formed under the first semiconductor layer;
• a third semiconductor layer of the first conductivity type formed in a shallow portion of the first semiconductor layer on the substrate, the third semiconductor layer having a higher concentration than an impurity concentration of the first semiconductor layer;
• a fourth semiconductor layer of the first conductivity type formed in a region of the first semiconductor layer immediately below the third semiconductor layer;
• a first contact electrically connected to the first semiconductor layer;
• a second contact electrically connected to the second semiconductor layer;
• and
• an electrode formed, with an insulating film provided therebetween, on a region that is on a lateral side of the first semiconductor layer on the substrate and that is between the first contact and the second contact,
• wherein an impurity concentration of the fourth semiconductor layer is higher than the impurity concentration of the first semiconductor layer and lower than an impurity concentration of the third semiconductor layer,
• an impurity concentration of the second semiconductor layer is higher than the impurity concentration of the first semiconductor layer and lower than the impurity concentration of the third semiconductor layer,
• the second semiconductor layer does not contact the insulating film, and
• the electrode is in direct contact with the insulating film.