Infrared arrays operate under totally different principles than CCDs. In general, there is substantially less experience with materials which are sensitive to infrared photons than there is with silicon. As a result, the infrared arrays cannot be used as readout registers in the way in which silicon arrays (CCDs) are. Instead, the IR arrays are generally coupled to a silicon array (a multiplexor). For some reason, they are generally not coupled to CCDs, however. Each pixel from the multiplexor array is read out individually, in sequence; charge is not transferred from one pixel to another. In IR arrays, each pixel can be thought of as a capacitor; as photons are detected, charge builds up on the capacitor. The amount of charge on the capacitor can be read out at any time, without affecting the accumulated charge. This leads to so-called nondestructive readouts for IR devices; a given pixel can be read out many times without removing the accumulated charge. The removal charge is done in a seperate reset operation.
Before charge accumulation begins, each pixel is reset to some initial value. Because of thermal noise, however, it is not possible to know precisely what this initial value is from one reset operation to the next. This would introduce a fundamental uncertainty in the total charge measured if one only read each pixel once at the end of the desired integration period. To avoid this, most astronomical IR detectors perform doubly-correlated sampling, in which the array is read shortly after reset (non-destructively) and then again at the end of a specified integration period. The difference between the two readouts give the desired counts per integration period. To lower the effect of readout noise, it is possible that during each of these readouts, the chip is actually read several times; averaging the successive differences reduces the effective readout noise.
PtSi, HgCd, InSb. Typical QE. Subpixel QE.