In high power semiconductor devices where a metallic conductor 5 is bonded to the surface of a body 1 of semiconductor material, there is bonded over the back face of the metallic conductor a body 7 of a material with a similar coefficient of thermal expansion to the semiconductor body, to reduce or prevent thermal damage to the bond. The body 7 may be alumina and restrains the thermal expansion of the copper conductor 5 to prevent thermal fatigue. <IMAGE>
