In this work, a broadband Nb2O5/n-Si Schottky photodetector was fabricated and systematically investigated for both self-powered and bias-assisted photodetection across the ultraviolet (UV), visible (Vis), and near-infrared (NIR) spectral regions (351–1000 nm). X-ray diffraction (XRD) results of Nb2O5 confirmed a phase-pure orthorhombic Nb2O5 with preferred orientation along the (180), (001), and (200) planes with an average crystallite size of 32.81 nm. Scanning electron microscopy (SEM) micrographs revealed a densely packed nanoparticle morphology with an average particle size of 269.11 nm. Under self-powered operation (0 V), the photodetector demonstrates stable broadband response, generating photocurrents of 1.11 × 10−6 A at 351 nm (UV), 1.28 × 10−6 A at 600 nm (visible), and reaching a maximum of 1.65 × 10−6 A at 900 nm (NIR). Correspondingly, the photosensitivity exceeds 106 at 351 nm, increases to 121.96 at 600 nm, and peaks at 157.08 at 900 nm, while the specific detectivity reaches 1.09 × 1010, 1.25 × 1010, and 1.61 × 1010 Jones, respectively. Wavelength-dependent measurements under reverse bias further demonstrate strong UV–Vis–NIR sensitivity, with responsivity increasing from 0.092 mA/W at 351 nm to 0.441 mA/W at 600 nm, and reaching 0.529 mA/W at 1000 nm, while the external quantum efficiency (EQE) rises from 33.99% at 351 nm to a maximum of 95.36% at 600 nm, remaining above 68% in the NIR region. Under an applied bias of +5 V, measured under simulated solar illumination with a power density of 100 mW/cm2, the device performance improves markedly, achieving a high photocurrent of 1.33 × 10−2 A, a responsivity of 16.96 A/W, a low noise-equivalent power of 3.8910−13 W Hz−1/2, and a specific detectivity exceeding 3.07 × 1012 Jones. These results demonstrate that the Nb2O5/n-Si photodetector offers broadband sensitivity and excellent performance under both self-powered and biased conditions, making it a promising candidate for low-power and high-sensitivity optoelectronic applications.