Charge Injection Devices (CID)

Wednesday, April 25th, 2018 - Light, Photoconductors, Photovoltaic

Charge Injection Devices (CID)

These devices are also based on the MOS capacitor and indeed employ the internal photoeffect in the same way as the Charge Coupled Device (CCD). The essential difference lies in the charge ‘reading’ operation. Unlike the CCD, in fact, the CID do not transfer the charge along to a video amplifier but ‘inject’ it (hence their name) into the substrate by reducing the electrode voltage to zero (i .e. by removing the potential well) thus causing a photocurrent Ip to flow, as shown in Figure 1:

Charge Injection Devices (CID),charge injection device cid detector

Figure 1 Charge Injection Devices principle of operation (after West and Hill)

The photocurrent Ip is, in effect, the MOS capacitor displacement current, as shown in eqn (3 .16):

Charge Injection Devices FormulaSince the MOS capacitor value C and the time interval Δt (necessary to take the electrode voltage from V to 0) are both constants the height of this ‘injected’ photocurrent pulse is proportional to the stored charge q and therefore to the incident irradiation Ein :

Photocurrent FormulaBy monitoring the displacement current peak value during the photo sites scan an analogue signal Vout proportional to the image intensity is obtained, as required. CID solid state cameras have similar features to CCD ones, being based on the same MOS technology, namely stable and accurate geometric configuration with a good input dynamic range and a high unit cost. CID cameras are, however, more tolerant of excess light input levels and suffer from less cross-talk than CCD cameras because of the absence of the charge transfer mechanism.

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