Avalanche photodiode with low breakdown voltage

A semiconductor avalanche photodiode (APD), comprising:

• a first region of n-type doped silicon above a substrate;
• a second region disposed directly above the first region and further above the substrate, the second region being of silicon, and wherein the second region has a doping concentration of less than about 5×1015 centimeter−3 (cm−3);
• a third region disposed directly above the second region and further above the substrate, the third region being of p-type doped silicon, and wherein the third region has a doping concentration of between about 2×1018 cm−3 and about 3×1018 cm−3;
• and
• a fourth region disposed directly above the third region and further above the substrate, separated from the second region by the third region, and further separated from the first region by the second region and the third region, the fourth region being of germanium.