As a new type of low-light night vision imaging device technology, electron-bombarded complementary metal-oxide-semiconductor (EBCMOS) technology can realize photoelectric conversion, electric signal enhancement, digital processing, and target output below an illumination of 10-4 lx. It has the advantages of a small size, light weight, high gain, low noise, and fast response. Therefore, it has wide application prospects in military equipment, astronomical observation, remote-sensing mapping, and space detection. In the EBCMOS working process, the photoelectrons generated from the photocathode by the external photoelectric effect are accelerated by the negative high voltage between the photocathode and the surface of the electron-sensitive CMOS and bombard the P-type semiconductor substrate to obtain the gain from the secondary electrons in the multiplier layer. Because of the concentration difference of the minority carriers in the P-type substrate, the secondary electrons diffuse to the pixel region, are collected by the photodiode in the active pixel circuit, and are finally read out by the MOS transistor amplification circuit. Therefore, to improve the gain characteristics of EBCMOS devices, the design and optimization of the structural parameters of EBCMOS substrates and the building of corresponding theoretical models are important issues for researchers. In this study, secondary electron charge collection in EBCMOS substrates under different doping modes and structural parameters was investigated, laying a theoretical and technical foundation for the preparation of high-gain EBCMOS electron multiplier layers.
According to carrier transport theory and the Monte Carlo simulation algorithm, a theoretical model of the entire electronic trajectory of an EBCMOS substrate was established. Electron charge collection in the electron multiplier layer under uniform and gradient doping of the P-type substrate was simulated, and transport calculation models of photogenerated electrons and multiplier electrons in the proximity region of the EBCMOS were established. Various EBCMOS structural models were designed to simulate the electronic motions under the condition of different doping concentrations, substrate thicknesses, proximity distances, and gradient doping structures, and the influence of different structural parameters on the electron charge collection of the electronic multiplier layer was analyzed.
For a uniformly doped substrate, with an increase in doping concentration, the recombination rate of the carrier increases, the lifetime of minority carriers decreases, and the number of secondary electrons collected in the pixel region decreases, which causes the charge collection efficiency to decrease continuously (Fig. 4). When the substrate doping concentration reaches 1019 cm-3, the charge collection efficiency approaches 0—that is, the secondary electrons are completely recombined. As the thickness of the substrate increases, the diffusion range of the secondary electrons increases (Fig. 5), and the scattering radius of the secondary electrons collected in the pixel region increases, which is not conducive to improving the charge collection efficiency (Fig. 6). Therefore, a thinner P-type substrate treatment is necessary to obtain a higher charge collection efficiency. As the proximity distance between the cathode surface and the EBCMOS substrate increases, the initial energy obtained by the incident electrons decreases, and the number of secondary electrons that generate multiplication decreases, thereby reducing the number of electrons collected in the pixel area and reducing the charge collection efficiency (Fig. 8). When the substrate is divided into two sections for gradient doping, the range of secondary electron diffusion in the diffusion and depletion regions is obviously reduced, indicating that the electron focusing effect of gradient doping is better than that of uniform doping (Fig. 10). The built-in electric field distribution generated by gradient doping can provide an additional drift speed for secondary electrons in the direction of their movement, shortening the diffusion time of electrons in the diffusion region and obtaining a higher charge collection efficiency. The charge collection efficiency can reach a maximum of 86.28% when the width of the surface heavily doped region is 2 μm.
Based on the carrier transport mechanism in semiconductor physics and the Monte Carlo algorithm, the electronic trajectory of incident optoelectrons in EBCMOS is theoretically simulated. The electronic trajectory in the device is determined based on the simulation results, and the factors affecting the efficiency of charge collection are analyzed. The results show that the charge collection efficiency of the EBCMOS increases with the decreases in substrate doping concentration, substrate thickness, and proximity distance. The gradient doping of the substrate clearly improves the charge collection efficiency. The optimized gradient doping structure model achieves a charge collection efficiency of 86.28%. The results provide theoretical support for the fabrication of high-gain EBCMOS devices.
