Disclosed herein is an apparatus comprising: an array of avalanche photodiodes (APDs), each of the APDs comprising an absorption region and an amplification region; wherein the absorption region is configured to generate charge carriers from a photon absorbed by the absorption region; wherein the absorption region comprises a silicon epitaxial layer; wherein the amplification region comprises a junction with an electric field in the junction; wherein the electric field is at a value sufficient to cause an avalanche of charge carriers entering the amplification region, but not sufficient to make the avalanche self-sustaining; wherein the junctions of the APDs are discrete.