  The atomic layer deposition(ALD) coating lengthens the lifetime of
microchannel plates(MCP), which are used as the electron amplifier of the
photomultiplier tubes(PMT). In the Jinping Neutrino Experiment, the newly
developed 8-inch MCP-PMT achieves high collection efficiency by coating with
high secondary emission materials. The resulting single electron response(SER)
charge distribution deviates from the Gaussian distribution in large charge
regions.To understand the nature of the jumbo-charged SER, we designed a
voltage-division experiment to quantify the dependence of the MCP gain on the
energy of incident electrons. Combining the relationship with the Furman
probabilistic model, we reproduced the SER charge spectra by an additional
amplification stage on the input electrode of the first MCP. Our results favor
a Gamma-Tweedie mixture to describe the SER charge spectra of the MCP-PMTs.
