This website has an interesting demo at the bottom, where you can specify different parameters to create different curves for the quantum efficiency of a solar cell!
Frog vision has a quantum efficiency of ~30%, which is about double human vision! Because of this, frogs can detect even just a single photon of light. Source: https://www.sciencedaily.com/releases/2015/01/150120084545.htm
Here's some more background info about Eric Fossum's invention of the CMOS sensor at Caltech's JPL: https://spinoff.nasa.gov/Spinoff2017/cg_1.html Fossum reduced the signal noise in earlier imagers using the intra-pixel charge transfer technique with correlated double sampling by measuring a pixel’s voltage both before and after an exposure. CMOS is small and low-power compared to CCD.
Fossum is currently working on the new Quanta Image Sensor at Dartmouth, which is the successor of CCD and CMOS. It would "cram a billion pixels, each designed to sense a single photon, into an array no larger than those in current CMOS imagers, significantly enhancing low-light sensitivity." Counting every photon that arrives is the ideal case, as Prof. Ng mentioned in lecture. Here is a paper describing a technique for improving the signal-to-noise ratio of the reconstructed image by spatial-temporally varying the threshold: https://ieeexplore.ieee.org/document/8169093 Each sensor has many single photon detectors, and the number of arriving photons trigger a binary response if above a certain threshold, which is typically threshold of 1.
How are electrons replenished on the silicon surface after electrons are removed? Is there a maximum quantum efficiency? If so, when each photodiode is reset, does the rate at which it resets vary depends on how many electrons were removed in the last receipt of photons?
University of Washington has a video demo showing the process they use to get EQE measurements from a photovoltaic device: https://www.youtube.com/watch?v=PGq4xpnNuCs
This article shows the design of various image sensors and how the different components relate to the sensors' quantum efficiency values.