TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Components: The p-n junction is located within two depletion lengths of the heterojunction. The offset of the p-n junction from the heterojunction is provided by an intermediate layer located between the absorber layer and the emitter layer. The intermediate layer includes a graded layer having a material gradation from GaAs at a side closer to the absorber layer, to the different material of the emitter layer, and a back window layer not having the gradation and having an approximately uniform composition of the different material. The material of the emitter layer is aluminum gallium arsenide (110). The optoelectronic semiconductor device further comprises: an negative (n)-metal contact disposed on a front or back side of the device; a positive (p)-metal contact disposed on the back side of the device, where the front side is disposed over the back side; an anti-reflection coating disposed over the n-metal contact or on a front side of the device; and a cell disposed between the front and back sides, and including an n-type film stack disposed over a p-type film stack, where the n-type film stack comprises an n-type contact layer comprising gallium arsenide disposed over an n-type front window disposed over the absorber layer comprising n-type gallium arsenide and disposed over the intermediate layer comprising n-type aluminum gallium arsenide, the p-type film stack comprises the emitter layer comprising p-type aluminum gallium arsenide disposed over a p-type contact layer comprising p-type gallium arsenide, the n-metal contact is coupled to the absorber layer, and the p-metal contact is coupled to the p-type contact layer. The graded layer is located adjacent to the absorber layer, and the back window layer is located between the graded layer and the emitter layer. The intermediate layer includes a material gradation of aluminum (30%), GaAs (70-100%), in a direction of the emitter layer to the absorber layer. The emitter layer is located closer than the absorber layer to a back side of the device, so that single carrier transport is provided in the device. In the device, a majority of the absorber layer is outside of a depletion region formed by the p-n junction, which is located within two depletion lengths of the heterojunction.