In this paper, we present a theoretical study to enhance the performance of SPR biosensor structures based on the Kretschmann configuration. We use a low refractive index CaF2 prism, a gold-silver alloy (Au10Ag90), different protective top layers, and water as a sensing medium (SM). Our results show that a structure with CaF2 prism, Au10Ag90, and SM achieves a sensitivity of 280⁰/RIU. The addition of different cover layers to this structure such as graphene, SiO2, BaTiO3, Fe2O3, and Al2O3 improves the sensitivity to 300⁰/RIU, 340⁰/RIU, 340⁰/RIU, 360⁰/RIU, and 420⁰/RIU, respectively. Finally, we conclude that the structure with Al2O3 has the best simulation results in terms of sensitivity of 420⁰/RIU, then Figure of Merit (FoM) of 83.83/RIU, and Quality Factor (QF) of 176.05/RIU. We also discuss the experimental feasibility of this work. The proposed biosensors can be used in different biological sensing applications and specifically, in the detection of cancer bio-markers in body fluids.