The present invention relates to a buried mesa semiconductor device such as a laser diode with reduced leakage currents past the heterojunction, and to a method of forming such a device. The method comprises the steps of: growing a semiconductor wafer (1) with a plurality of layers including a substrate (2) and an active layer (6); depositing a mask (52) on the wafer (1) which defines one or more mesa regions (51); etching the wafer (1) to form a mesa (51) above the substrate (2), growing one or more current confining layers (56A, 59A) to cover the mesa sides (53, 54); removing part of the mask (52) along a part (64, 65) of the top adjacent the mesa sides (53, 54); growing a leakage current confining layer (68A) on a previous current confining layer (59A) so that the leakage current confining layer (68A) extends to overlie partly (64, 65) the mesa top adjacent said mesa sides (53, 54).
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