Page 223 - Organic Electronics in Sensors and Biotechnology

200 Cha pte r S i x

Light

– + – Gate electrode

-- SiO 2
-
p-Si

Collected electrons
(a) (b)

FIGURE 6.5 (a) Typical charge-coupled device. (b) Schematic of a light-sensitive
pixel; electrons and holes are photogenerated in the p-Si layer and driven to the
p-Si/SiO interface and ground electrode, respectively. The gate electrode of
2
the pixel is held at a higher potential than the gate electrodes of the adjacent
pixels, and so electrons are trapped at the p-Si/SiO interface and accumulate in
2
direct proportion to the cumulative photon exposure.

coating of silicon oxide (the insulator), on top of which sits an array
of conductive polysilicon ‘‘gate’’ electrodes (the ‘‘metal’’) (Fig. 6.5b).
By setting the potential of a given gate electrode positive with respect to
its immediate neighbors, a potential well is constructed in which pho-
togenerated electrons can collect. The positive charge on the gate
electrode drives (intrinsic) holes away from the semiconductor/oxide
interface, creating a depletion zone. When the pixel is illuminated, elec-
trons and holes are generated in the depletion zone and driven in oppo-
site directions by the electric field—electrons to the semiconductor/oxide
interface and holes to the ground rail. The electrons accumulate at the
interface in direct proportion to the number of incident photons.
Hence, if the charge on the photocapacitor is measured after a given
time, the integrated photon count can be determined. This is done by
shuttling the charges pixel by pixel toward a single charge-to-voltage
amplifier which processes the data from each pixel in sequence.
The major cause of noise in CCDs is thermal generation of electron-
hole pairs which causes additional electrons to accumulate at the
semiconductor/oxide interface. The rate of photogeneration must
substantially exceed the rate of thermal generation if a reliable mea-
surement is to be obtained, and for this reason a thermoelectric
cooler is frequently employed to reduce the temperature of the
CCD.
The optimum detector for a given application is governed by
many considerations, including spectral response, sensitivity,
dynamic range, speed, active area, and cost. PMTs offer high levels of
virtually noise-free gain and are therefore the preferred choice for
ultralow light level detection. They are also currently the only choice