6.5.1 CMOS Image Sensor Architecture

A CMOS image sensor is a mixed-signal circuit that typically includes at least four main

blocks. These blocks are the pixel array (typically disposed of in a 2D configuration),

analog signal processors, row and column selectors, and timing/control units.

Each pixel in the array typically consists of a photodiode that is configured to sense

impinging photons by converting them into photo-generated carriers. These free carriers

are collected by the field across the reverse-biased photodiode, thus creating a photo­

current that is proportional to the light intensity, notwithstanding losses at the interface

due to reflection and the non-unity quantum efficiency of the photodiode. Each pixel in

the array is subjected to a different amount of light, consistent with the scene that is being

imaged by the device. The pixel array is then scanned to reproduce the image based on

the number of carriers that are collected at each pixel.

The row and column selector circuits may be implemented using programmable logic

elements to select any pixel in the array. With this hardware, the array may be rasterized

and reset to produce an image. The timing and control units generate bus access and pixel

readout control signals, and the analog signal processors generally can include circuits for

fixed pattern noise (FPN) mitigation, reset noise mitigation, and other front-end pro­

cessing (e.g., correlated-double sampling, difference double sampling, histogram equal­

ization, etc.). Additionally, the peripheral circuitry can include digital signal processing

modules for on-chip image conditioning. Lastly, an array of microlenses and color filters

may be applied on top of the passivation to increase external quantum efficiency and to

provide color information, respectively.

6.5.2 CMOS Image Sensors in Fluorescence Imaging

In many biomedical and environmental applications, the presence of a target substance, i.e.,

a specific molecule or nucleic acid sequence, is detected through fluorescence sensing. Since

CMOS image sensors offer lower power consumption, on-chip functionality, and high

temporal resolution, they have been used in integrated fluorescence sensors for point-of-

care settings. For instance, Manickam et al. [38] implemented a biochip for DNA and RNA

testing employing a CMOS image sensor. This biochip module is shown in Figure 6.6; it was

FIGURE 6.6

(a) Layout view of a pixel from a CMOS biochip configured for fluorescence-based bioassays. (b) System in­

tegration, showing the readout board, the chip, and the fluidic layer for sample delivery to the analysis sites.

Reprinted with permission from ref. [ 38] Copyright (2017) IEEE.

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