Solar cells have been through many development phases. Silicon is a one of many important materials in solar cell manufacturing. One of silicon solar cell applications which can produce high efficiency is Heterojunction with Intrinsic Thin-layer (HIT) crystalline silicon solar cell. Another alternative of silicon solar cell applications is Heterojunction with Compound Thin-layer (HCT) crystalline silicon solar cell where silicon is coupled with III-V compound semiconductor. n-AlGaAs is used as an alternative from n-AlAs on Heterojunction with Compound Thin-layer crystalline silicon solar cell. Compared to AlAs, lattice constant of AlGaAs is more suitable to the silicon. To increase the efficiency of solar cell the step grading method is used for AlxGa1-xAs layer on the front surface. The optimization of step grading AlxGa1-xAs layer was done by using the wxAMPS software as simulation tool. Simulation results show that HCT crystalline silicon solar cell produce the 16.64 mA/cm2 short circuit current density (Jsc), the 1.05 V open circuit voltage (Voc), the 0.95 fill factor, and the 16.64% efficiency. The power it produces can be used as power source for HGM-111 biosensors.