Biophotonics_Concepts_to_Applications

and a charge transfer register situated under another opaque gate. During device
operation the light-sensing portion is biased to produce a potential well under it for
storing electric charge. The transfer gate is biased at a voltage V 1 to establish a
potential barrier that confines the photogenerated electrons in the potential well.
When the pixel is illuminated, the charge in the well increases with time and is
proportional to the light intensity. After a specific illumination time (orlight inte-
gration time) the bias V 1 is pulsed low and the transfer gate voltage V 2 is biased
high. This action shifts the accumulated charge to a transfer register potential well
under the transfer gate and from there it is transferred to a readout device that
performs charge-to-voltage conversion. After the charges have been read out, the
bias voltages V 1 and V 2 return to their original values and the cycle starts over. This
process produces a measurable time-dependent output current, which is a propor-
tional to the light intensity falling the pixel.
An example of a 4×4 CCD array is shown in Fig.5.8, which is a 16-pixel
image sensor. Following the specified light integration time, all the photosensing
wells are emptied simultaneously into their associated CCD transfer register (or
shift register) wells. The charges in the shift register wells then are transferred row
by row to the output CCD register array. Thereby each row is sent sequentially to
the output for signal interpretation. This process results in a time-dependent output
current, which is a measure of the light intensity falling on various areas of the CCD
array. Note that in an actual CCD image sensor or camera, the CCD array typically
consists of millions of pixels.

5.4.2 CMOS Array Technology

In contrast to a CCD array in which the contents of the wells are read out row by row
for measurement by an external circuit, in a CMOS array the charge-to-voltage
conversion process occurs inside each pixel. The pixel area is thus more complex. As
Fig.5.9shows, the pixel contains a photodiode for detecting light and electronic

Storage well for Transfer register

photon-induced charges

Transfer

gate

Opaque

Transparent gate gate

Incoming photons

Charge transfer

Fig. 5.7 Operational
structure of one stage of a
CCD array

134 5 Fundamentals of Optical Detectors