There is disclosed the use of a heat pipe type thermal conductive path within a metallic housing such as a transistor can for a highly efficient cooling of high power semi-conductor devices which normally require large heat dissipation. An electrically non-conductive wick structure is provided which is formed, for example, from high purity silica glass cloth in a shape resembling a hollow ''''marshmallow'''' and which forms a liner for the entire transistor can. The wick contacts both the active surface of the semi-conductor device in the bottom of the can and the upper walls of the can. Prior to placing the can upon its mounting base, an appropriate amount of electrically nonconductive, non-polar working fluid such as high purity organic liquid is loaded so that it entirely fills or saturates only the wick like structure. The working fluid held within the wick is thus in immediate contact with the active surface of the semiconducting device. In operation, the surface of the semiconductor device serves as the evaporator section of the closed loop heat pipe. As fluid is caused to evaporate from this region, heat transfer and thus cooling of the device is effected. The vapor thus produced is recondensed over regions of the can which are at slightly cooler temperatures than the semiconductor device. The working fluid vapor thus provides an efficient heat transfer path to the entire radiating surface of the can in order to dissipate the thermal energy of concern.