CMOS based photodetection systems for biomedical application
Dhanya Balan
s7 AEI
Roll.no.20
College Of Engineering, Trivandrum
2007-11 batch

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Overview
Introduction.
Designs,results and discussions.
Cmos imagers.
Conclusions.
References.

Introduction
Introduction.
Designs,results and discussions.
Cmos imagers.
Conclusions.
References.

CMOS based systems
Cheap
Low power
High integration
Widespread applications

Flourescent imaging
Analysis of fluorescence spectra, resulted from light excitation

A specific fluorophore,depending on its concentration,can have different spectral characteristics
Fluorescence source
Probe introduced into tissue
Natural fluorophores in tissue (Autofluorescence)

Practical issue

The absorption and emission
bands should match available
light sources and detectors
Need to be sure that signals
from other molecules are not
mixed in.

Photodetectors-Background
PMTs
Highest sensitivity, very high gain, high speed
Bulky, costly, high voltages required (1000 VDC)
Low efficiency (10-20%)
Large size prevents use in dense arrays

CCDs
High sensitivity, small pixel size, high FF
Low-noise (passive transfer, no FPN)
Slow (serial readout), high power (high rates) and clock voltages
No easy integration with CMOS circuits

CMOS
Easy to integrate, cheap, low power, high speed readout, window-of-interest
Noisy (FPN)
CMOS Based Photodiodes
Many options for photodiodes
Vertical
Lateral
Avalanche
Combinations

CMOS based photto-transistors
Vertical phototransistor
BJT, open base
Vertical / Lateral Phototransistor
BJT / BJT& MOS hybrid
Open base, Gate to VDD
Vertical / Lateral Phototransistor
BJT / BJT& MOS hybrid
Base and Gate are tied
Base & Gate are floating

Passive pixel system
Pixels with photocurrent integration
Passive pixel sensor(PPS).
Active pixel sensor(APS).
Digital pixel sensor(DPS)
Avalanche Photodiode System.

Active pixel sensor
Simplest and earliest cmos pixel structure
One transistor per pixel
High fill factor
Disadvantages
Less signal to noise ratio

Advantages
Compact, high fill-factor.
High dynamic range
Possible to integrate circuits in the pixel
Low-light level detection
Dark current
Equivalent noise power
Sensitivity
Signal-to-noise ratio

Disadvantages
Noisy(mainly FPN)
Relatively slow operation
Digital pixel sensor
Output of pixel is digital
Applicable for integrated and high speed digital imaging
A/D conversion inside the pixel

Avalanche photodiode
Disadvantages
Lower fill factor
Inherent quantization niose
limited application in ultrasensitive measurements

Conclusion
Chip Layout
3 mm x 3 mm
Analog readout array
Digital readout array
Serial output
Parrel output
16 x 16 (256) pixel array
All digital blocks designed and implemented on a chip
.18micrometer technology

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