Titanium dioxide (TiO2) is one of the excellent materials used for photodetector applications due to its high photoactive nature. In this research, highly sensitive ultra-violet (UV) photodetectors with high detectivity and external quantum efficiency (EQE) are developed by depositing Nb-doped TiO2 (TNO) films onto a Si/SiO2 substrate at different Nb concentrations. The doping effect of Nb on the electronic structure and photodetector performance of TiO2 films has been investigated. The charge compensation mechanism due to Nb-doping has been explained using X-ray photoelectron spectroscopy. Lower doping concentrations of Nb5+ result in creating a Ti vacancy and an excess electron per Nb atom, contributing to the enhanced photocurrent. The metal-semiconductor-metal (M-S-M) structure of the device generates an ohmic-contact and the Si/SiO2 layer assisted toward reducing the surface recombination. All these factors cause an enhancement in the photocurrent, detectivity, and EQE of the TNO photodetector. Finally, the optimized TNO film grown using 1.0 at.% Nb shows improved photodetector properties and is suitable for ultra-sensitive UV photodetector applications.