When concerned about mismatch in vision chip circuitry, photodetectors
are the last suspects. This is simply because, firstly photodetectors
are generally large, as it is intended to increase the fill-factor and
reduce the noise. Secondly, the processes through which photodetecting
elements produce photocurrent depends mainly on the characteristics of
the bulk semiconductor, which is better controlled than surface
characteristics. For example, typical standard deviation of the output
current of the vertical bipolar transistor in
Figure 7.1, with an area of 100 
is
less than 2%.
In photogates the well-filling process is mainly through diffusion of minority carriers in the substrate. However, the surface states at the Si-SiO2 interface can also contribute to the recombination of stored carriers in the well. The higher mismatch in photogates can be associated with this process.