We present an experimental and theoretical study ofn‐type Hg1−xCdxTe photoconductors in which a large band‐gap alloy was grown on top of a smaller band‐gap active region and contacts were made to the larger gap material. The larger band‐gap material causes an energy barrier to holes which decreases the rate at which they reach the high recombination region of the metal‐semiconductor interface. As a result, this heterojunction contact greatly reduces the effects of carrier sweepout on device performance and leads to much higher detector responsivities. Experimental results in a symmetric device with a cutoff wavelength of 7.8 &mgr;m at 77 K show responsivities in excess of 106V/W and detectivities close to the background limited value and nonsaturation of responsivity with bias voltage. In an asymmetric device, in which only one heterojunction contact was used, an order of magnitude increase in responsivity was observed when the heterojunction contact was biased to attract minority carriers, compared with the opposite bias polarity. A theoretical model of the heterojunction contact photoconductor is presented. Calculated results are in good agreement with experimental results. The results of the calculation suggest that the optimum compositional difference &Dgr;xof the two layers should be &Dgr;x∼0.04, and that the thickness of the large band‐gap region should be 2–3 &mgr;m.