In the back ground art, lateral pin photodiodes are known. The lateral pin photodiode has a p-type semiconductor layer, an i-type semiconductor layer and an n-type semiconductor layer. The p-type semiconductor layer and the n-type semiconductor layer are disposed parallel to a surface of a semiconductor substrate. The i-type semiconductor layer is interposed between the p-type semiconductor layer and the n-type semiconductor layer.
The lateral pin photodiode detects light to enter into the i-type semiconductor layer.
When the i-type semiconductor layer is a p-type semiconductor layer with a sufficiently low impurity concentration, the n-type semiconductor layer and the i-type semiconductor layer form a pn-junction. A depletion layer of the pn-junction extends to the p-type semiconductor layer side rather than the n-type semiconductor layer side. When light enters into the i-type semiconductor layer, the light is absorbed inside the i-type semiconductor layer to generate carrier.
The carrier which is generated inside the depletion layer flows as drift current. The carrier which is generated outside the depletion layer flows as diffusion current. As a result, a sufficient response speed of the lateral pin photodiode is not obtained.