A mesa-structure avalanche photodiode in which a buffer region in the surface of the mesa structure effectively eliminates the sharply-angled, heavily doped part of the cap layer that existed adjacent the lightly-doped n-type multiplication layer and p-type guard ring before the buffer region was formed. This reduces electric field strength at the ends of the planar epitaxial P-N junction and prevents edge breakdown in this junction. The lateral extent of the guard ring is defined by a window formed in a masking layer prior to regrowth of the guard ring. This guard ring structure eliminates the need to perform additional processing steps to define the lateral extent of the guard ring and passivate the periphery of the guard ring.