Biophotonics_Concepts_to_Applications

conversion and amplification circuitry consisting of four metal-oxide-semiconductor
field-effect transistors (MOSFETs) plus a junction capacitor. Because each pixel has
its own built-in amplifier, the pixel is referred to as anactive pixel. When light is
incident on a pixel, the charges that are built up from the photodetection process are
converted to volts, buffered by a source-follower transistor, and then are transferred
to thin metal output buses through transistor switches that reside inside each pixel to
generate an output video stream.
When light is being detected, the MOSFETs M1, M2, and M3 are turned off and
the capacitor gets charged to nominally 3.3 V. The voltage on the photodiode
depends on the incoming light intensity and the integration time (the time during
which light is measured). Then M1 is turned on to determine the charge on the
photodiode. This action causes electrons toflow from the photodiode through M1,
thereby discharging the capacitor. This process takes about 1μs. After this pro-
cedure, M1 is turned off and the photodiode again detects light. The voltage on the
capacitor represents the light signal, which is measured by turning on M3 and

Transfer gate grid

CCD

registers

Photon

sensing

elements

Output CCD register array Output

Charge flow from CCD registers

Row 1

Row 4

Row 3

Row 2

Fig. 5.8 Illustrative example of a CCD imager using a 4×4 pixel array

M1

M2 M3

M4

Readout

VDD

Out

Photodiode

Junction

capacitor

Reset Select

Incoming

photons

Fig. 5.9 Schematic of the
circuitry in a pixel of a CMOS
array

5.4 Multichannel Detectors 135