Backside−illuminated Pb1−xSnxTe heterojunction photodiode

The need for a long−wavelength infrared detector that makes more efficient use of the available radiation has led us to construct a backside−illuminated Pb1−xSnxTe heterojunction in which the radiation passes through a transparent substrate and is absorbed in the active region behind a p−n junction. This device permits a selectivity in the spectral bandwidth of the device due to the short−wavelength cut−on properties of the transparent PbTe substrate or an epitaxial Pb1−xSnxTe filter layer. Additionally, an effective increase in optical area is achieved because the radiation incident upon the sloping sides of the mesa is internally reflected into the junction region due to the index of refraction mismatch between air and PbTe; thus with this design the optical area which gives rise to the signal current is larger than the electrical area which is the source of noise. Laboratory measurements on devices of this construction verify these properties and show that a peak Dλ greater than 1011 cm Hz1/2 W−1 has been obtained at an operating temperature of 77 K.