Abstract: A high power semiconductor heat sink assembly comprising a semiconductor slice or chip and a heat sink structure. The heat sink structure comprises two spaced apart thermally and electrically conductive plates of for example aluminum between which the slice or chip is clamped. A device such as an O-ring is sandwiched between the plates so as to extend around the slice or chip and define a sealed compartment within which the slice or chip is located. An electrically non-conductive thermally conductive material such as a resin comprising alumina fills the space between the plates outside the compartment.