The dual junction (tandem) cell structure (InGaP/GaAs) is depending on Indium gallium Phosphides (InGaP) as the upper cell, Gallium Arsenide (GaAs) as the lower cell, and behaves as tunnel junction (TJ). The structure of the (InGaP/GaAs) dual junction cell was simulated in this work using SILVACO program to obtain a high solar cell efficiency. Firstly, the effect of doping concentration and thickness of window layer of the upper cell was investigated on the (InGaP/GaAs) tandem cell performance. Then the GaAs /GaAs (TJ) is replaced by an InGaP/GaAs (TJ).A comparison of performance parameters between the two types of tunnel diode (GaAs / GaAs) and (InGaP/GaAs) was studied. The parameters that have been compared are open-circuit voltage (VOC), efficiency (η), the short circuit current density (JSC), and Fill Factor (FF). Quite high operating factors for tandem cell are achieved by taking into account the crucial number of cells as well as improving layer parameters of the layers. All calculations and simulations of tandem cell are performed with the typical AM1.5 solar spectrum light intensity of 1-sun at room temperature (300 K). Lastly, the findings illustrate that the optimum properties of the suggested tandem cell are efficiency (η) = 34.37 percent, VOC = 2.449 V, JSC = 21.69 mA/cm 2 , FF=89.33 percent.