An avalanche photodiode device

An avalanche photodiode (APD) sub-assembly comprising:

• a substrate;
• a n-type contact layer formed on the substrate;
• a p-type contact layer;
• an avalanche layer comprising antimony, the avalanche layer being disposed between the n-type contact layer and the p-type contact layer;
• and
• an absorption layer disposed between the avalanche layer and the p-type contact layer;
• wherein the APD sub-assembly has a multi-stage structure comprising a first stage and a second stage wherein the multi-stage structure comprises a stepped structure, wherein the first stage provides a first step and the second stage provides a second step;
• wherein the first stage comprises the p-type contact layer and the second stage comprises the avalanche layer;
• and
• wherein the first stage has a smaller cross-sectional area than the second stage, the cross-sectional area being perpendicular to a direction from the substrate to the p-type contact layer.