Resolution characteristics of transmission-mode exponential-doping GaN photocathodes

According to the electron transport equation for the transmission-mode uniform-doping GaN photocathode, we have obtained the electron transport equation for the transmission-mode exponential-doping GaN photocathode. And then through solving this equation, the expression of modulation transfer function for an exponential-doping GaN photocathode is determined. Subsequently, the resolution characteristics of transmission-mode exponential-doping and uniform-doping GaN photocathodes are calculated and comparatively analyzed. Simultaneously, the quantum efficiencies of both GaN photocathodes are given. These calculated results show that the exponential-doping structure can upgrade remarkably not only the resolution but also the quantum efficiency of a negative electron affinity GaN photocathode, compared with the uniform-doping structure. This upgradation differs from the approach for high resolution by shortening the thickness of emission layer T-e and the diffusion length of electron L-d or by increasing the recombination velocity of back-interface S-i,, which leads to a low quantum efficiency. Furthermore, the upgradation of resolution and quantum efficiency for a transmission-mode exponential-doping GaN photocathode result mainly from the facilitation of the electron transport and constraint of the lateral diffusion by the built-in electric field.