In recent years, infrared (IR) photodetector has been extensively used and played an important role in environmental control, medical diagnostics, and satellite remote sensing. Therefore, the priority should be given to how to stimulate the development of imaging detection of weak IR signal. Up-conversion IR photodetector has an ability to detect quite weak IR signal in the large plane array focal plane, so it has civil and military significance. However, the poor light extraction efficiency due to total reflection severely restricts the overall efficiency of the up-conversion device, which has become one of the bottlenecks in improving the device efficiency.#br#In this work, we propose that the light-extraction efficiency of up-conversion IR photodetector can be improved by a self-assembled monolayer of SiO2 sphere. Thereby, the up-conversion efficiency can be enhanced. The up-conversion IR photodetector emits the light mainly from the silicon nitride (SiNx) passivation layer. And the hexagonal closely-packed SiO2 sphere monolayer is formed on the SiNx layer. In order to study the effect of the size of nanosphere on the light-extraction efficiency, we prepare the SiO2 spheres with diameters of 300, 450, 750, and 1000 nm respectively.#br#Results indicate that the devices with and without SiO2 nanospheres exhibit similar IR responses and dark currents, while the emission of device with SiO2 spheres obviously increases. And the light extraction efficiency increases up to an optimal level when the average size (750 nm) of SiO2 sphere approximates to the wavelength (770 nm) of light source. Taking into consideration other factors relating to external quantum efficiency, the light extraction efficiency of the device with 750-nm-sized SiO2 spheres on surface increases 2.6 times. In order to explain the physical mechanism for the light-extraction enhancement, we carry out the three-dimensional finite difference time-domain simulation, thereby calculating the transmission spectrum of the device with 750-nm-sized SiO2 spheres. Simulation results show that the incident light beyond critical angle can be partly extracted when the surface of up-conversion IR photodetector has a SiO2 sphere monolayer, leading to an enhanced light-extraction efficiency. So the SiO2 sphere monolayer acts as a two-dimensional diffraction grating, which behaves as a light scattering medium for the light propagating in a waveguiding mode within the up-conversion IR photodetector. Therefore it can be concluded that this is a simple and cost-effective method of improving the efficiency of up-conversion IR photodetector. The finding in this paper can also be applied to improving the light extraction efficiency of other semiconductor devices.