Single heterojunction diode lasers in the Pb1−xSnxTe alloy system have been fabricated by low‐temperature vacuum deposition ofn‐PbTe on ap‐Pb0.88Sn0.12Te substrate. The lasers have lower threshold current densities and operate cw at higher temperatures than homojunction devices in this material. At laser threshold the incremental diode resistance drops abruptly from 0.5 &OHgr; to a series resistance limited value of 0.08 &OHgr;, a previously unobserved effect in diode lasers which indicates very high internal quantum efficiency.