In this study, the structural, optical, and electrical properties of La³⁺ and Nd³⁺-doped calcium orthoborate interlayers in Al/KCaRE(BO3)2/n-Si (RE = La, Nd) photodiode configurations were systematically investigated. The rare-earth-doped KCa(BO3)2 compounds were successfully synthesized, and coated via the spin-coating method, followed by detailed structural and compositional analysis using Fourier-transform infrared spectroscopy (FT-IR), powder X-ray diffraction (P-XRD), inductively coupled plasma mass spectrometry (ICP-MS), and scanning electron microscopy with energy-dispersive spectroscopy (SEM–EDS). FT-IR confirmed the characteristic B–O vibrational modes within the [BO₃]⁻ units, while P-XRD revealed well-defined crystalline phases. ICP-MS analysis verified the elemental stoichiometry in agreement with theoretical values, and SEM–EDS illustrated homogeneous surface morphology. The electrical and photoresponse characteristics of the fabricated heterojunctions were evaluated under variable illumination intensities through current–voltage (I–V) and current–time (I–t) measurements. Key device metrics—including photosensitivity (K), responsivity (R), and specific detectivity (D*)—were extracted to assess photodetector efficiency. Notably, the Al/KCaLa(BO3)2/Si configuration exhibited deviced performance, with higher photocurrent generation, enhanced responsivity, and improved detectivity compared to its Nd-doped counterpart. These findings not only demonstrate the efficacy of rare-earth-doped orthoborate interlayers in silicon photodiodes but also establish a promising platform for the advancement of high-sensitivity optoelectronic devices.