Inverted heterojunction photodiode

A focal plane comprising:

• a plurality of mesa photodiodes on a PbTe substrate of n-type conductivity, and wherein each of said mesa diodes comprises:
• an epitaxial buffer layer of Pb.sub.(1-x) X.sub.x Te of n-type conductivity on said substrate, wherein X is a Group VI element selected from the group consisting of S and Se and wherein x is a value greater than 0 but less than 0.1;
• an epitaxial active layer of Pb.sub.(1-y) Sn.sub.y Te on said buffer layer, wheren y is a value greater than 0 but less than 0.3, said active layer forming a heterojunction between said active layer and said buffer layer;
• a p-n junction located outside said buffer layer in either said active layer or in said heterojunction, the n side of said p-n junction being adjacent said buffer layer;
• and
• electrical contact means coupled to said substrate and to said active layer.

A photodiode comprising:

• a substrate comprising a PbTe crystal of n-type conductivity;
• an epitaxial buffer layer of Pb.sub.(1-x) S.sub.x Te of n-type conductivity on said substrate, wherein x is a value greater than 0 but less than 0.1;
• an epitaxial active layer of Pb.sub.(1-y) Sn.sub.y Te on said buffer layer, wherein y is a value greater than 0 but less than 0.3, said active layer forming a heterojunction between said active layer and said buffer layer;
• a p-n junction located outside said buffer layer in either said active layer or in said heterojunction, the n side of said p-n junction being adjacent said buffer layer;
• and
• electrical contact means coupled to said substrate and to said active layer.

A photodiode comprising:

• a substrate comprising a PbTe crystal of n-type conductivity;
• an epitaxial buffer layer of Pb.sub.(1-x) Se.sub.x Te on n-type conductivity on said substrate, wherein x is a value greater than 0 but less than 0.1;
• an epitaxial active layer of Pb.sub.(1-y) Sn.sub.y Te on said buffer layer, wherein y is a value greater than 0 but less than 0.3, said active layer forming a heterojunction between said active layer and said buffer layer;
• a p-n junction located outside said buffer layer in either said active layer or in said heterojunction, the n side of said p-n junction being adjacent said buffer layer;
• and
• electrical contact means coupled to said substrate and to said active layer.

A method of fabricating a semiconducting device comprising the steps of:

• providing a substrate comprising PbTe crystal of n-type conductivity;
• epitaxially growing a buffer layer of Pb.sub.(1-x) X.sub.x Te n-type conductivity on said substrate, wherein X is a Group VI element selected from the group consisting of S and Se and wherein x is a value greater than 0 but less than 0.1;
• epitaxially growing an active layer of Pb.sub.(1-y) Sn.sub.y Te on said buffer layer, wherein y is a value greater than 0 but less than 0.3, said active layer forming a heterojunction between said active layer and said buffer layer;
• annealing said substrate, buffer layer, and active layer at approximately 450.degree. C. for approximately 20 hours to form a p-n junction located outside said buffer layer in either said active layer or in said heterojunction, the n side of said p-n junction being adjacent said buffer layer;
• and
• depositing electrical contact means coupled to said substrate and to said active layer.