NOVELTY - Optoelectronic semiconductor device, comprises: an absorber layer made of gallium arsenide (GaAs) (108) and having only one type of doping; an emitter layer located closer than the absorber layer to a back side of the device, where the emitter layer made of a different material than the absorber layer and having a higher band gap than the absorber layer; a heterojunction (154) formed between the emitter layer and the absorber layer; and a positive-negative (p-n) junction (152) formed between the emitter layer and the absorber layer at a location offset from the heterojunction.    USE - The optoelectronic semiconductor device is useful as a single carrier transport solar cell and a photovoltaic cell (claimed).    ADVANTAGE - The optoelectronic semiconductor device: is cost effective, flexible and has increased efficiency; and has heterojunctions with reduced dark current, improved voltage productions, and improved radiative recombinations as compared to homojunctions of the conventional photovoltaic materials.    DETAILED DESCRIPTION - Optoelectronic semiconductor device comprises: an absorber layer made of gallium arsenide (GaAs) (108) and having only one type of doping; an emitter layer located closer than the absorber layer to a back side of the device, where the emitter layer made of a different material than the absorber layer and having a higher band gap than the absorber layer; a heterojunction (154) formed between the emitter layer and the absorber layer; and a positive-negative (p-n) junction (152) formed between the emitter layer and the absorber layer at a location offset from the heterojunction, where the p-n junction causing a voltage to be generated in the device in response to the device which is exposed to light at a front side of the device. An INDEPENDENT CLAIM is also included for forming an optoelectronic semiconductor device, comprising: forming an intermediate layer (114) on an absorber layer, where the intermediate layer having the same type of doping as the absorber layer and includes the different material of the emitter layer; and forming an emitter layer on the intermediate layer, where the emitter layer is located closer than the absorber layer to a back side of the device.    DESCRIPTION OF DRAWING(S) - The figure shows a cross-sectional view of the photovoltaic unit.    Gallium arsenide (108)    Aluminum gallium arsenide (110)    Intermediate layer (114)    Positive-negative junction (152)    Heterojunction (154)