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The studies of quantum efficiency, electronic energy distribution and stability are highly concerned in the application
of Negative electron affinity (NEA) gallium nitride (GaN) photocathodes while the resolution of photocathodes are
concerned rarely. The resolutions of some image intensifiers are smaller than computational value partly because of
ignoring the resolution of photocathodes. To a certain extent, the resolutions of image intensifiers are influenced by
photocathodes. Electronic transverse diffusion is the main cause of decreasing the resolution of photocathodes whereas
the exponential-doping structure can reduce its influence. In this paper, the resolution characteristics of photocathodes
have been studied by using the modulation transfer function (MTF) method. The MTF expressions of transmission-mode
exponential-doping photocathodes have been obtained by solving the two-dimensional continuity equations. According
to the MTF expressions, the resolution characteristics between exponential-doping and uniform-doping GaN
photocathodes are calculated theoretically and analyzed comparatively. At the same time, the relationships between
resolution and thickness of the emission layer Te, electron diffusion length LD are researched in detail. The calculated results show that, compared with the uniform-doping photocathode, the exponential-doping structure can increase the
resolution of photocathode evidently. The resolution of exponential-doping GaN photocathode is improved distinctly
when the spatial frequency varies from 400 to 800 lp/mm. The MTF characteristics approach gradually when f increases
or decreases. Let f =600 lp/mm, the resolution increases by 20%-48% approximately. The constant built-in electric field
for exponential-doping GaN photocathode can increase the resolution of photocathode. The improvement of resolution is
different from decreasing Te, LD or increasing the recombination velocity of back-interface which are at the cost of reducing the quantum efficiency of photocathode. Therefore, the MTF expressions of transmission-mode
exponential-doping photocathode play a positive role in improving the resolution of ultraviolet detector and optimizing
the structural design of GaN photocathode.
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