Silicon photodiodes, quantum efficiency

A p-type electrode is in depletion if a cathodic bias is applied. Illumination generates one electron per absorbed photon, which is collected by the SCR and transferred to the electrolyte. It requires two electrons to form one hydrogen molecule. If the photocurrent at this electrode is compared to that obtained by a silicon photodiode of the same size the quantum efficiencies are observed to be the same for the solid-state contact and the electrolyte contact, as shown in Fig. 4.13. If losses by reflection or recombination in the bulk are neglected the quantum efficiency of the electrode is 1. 

Fig. 9.4. The quantum efficiencies q(%) of the more important radiation detectors. (a) silicon photodiode (b) pholomultiplyer tube with glass (/) and with quart . (2) (c) solid scintillator proportional counter with gas-filled detector...

Semiconductor detectors use the intemal photo effect". That means that the photons generate electron-hole pairs inside the semiconductor. Theoretically the internal photoeffect works with a quantum efficiency of 1. In practice the quantum efficiency of a good silicon photodiode reaches 0.8 around 800 nm. In photodiodes and photoconductors an electrical field separates the electrons and holes, so that a photocurrent flows through the device when it is illuminated. Of course, the photocurrent caused by a single electron-hole pair is far too small to be recorded directly. Single photons can therefore be detected only if the semiconductor detec-... 

Sensor response depends not only on the CFA spectra, but also upon the spectral response of the silicon photodiode. Sensor response is usually characterized by its quantum efficiency (QE), which has units of electrons per incident photon, and so has a maximum of 100 %. Sensor QE is not simply the cascade (wavelength by wavelength multiplication) of the photodiode QE and the color filter transmittance spectra, because the imaging light forms a cone, with different rays traveling different distances through the silicon. 

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