light emitting devices are requiring greater switching speeds to achieve greater modulation bandwidths . the problems of intrinsic capacitance associated with conventional semiconductor heterojunction devices are reduced by the reduction of pn junction capacitance as well as the use of a semi - insulating blocking layer and a conductive substrate . furthermore , a light absorbing layer is disposed on one side of an unetched portion of the semi - insulating material and an active layer disposed on an opposite side of the unetched portion . also , the interface of the semi - insulating material and the active and absorbing layers are at prescribed angles that reduce back reflections to the absorbing and active layers . this arrangement reduces pumping in the absorbing region and thus reduces the lasing effect , allowing for a stable led . the angle at the interface is determined by having the structure at a predetermined crystallographic direction and having the semi - insulating material etched to reveal a predetermined crystalline plane .