Basic Principle

The goal of any image sensor is to detect light as efficiently as possible. CCDs and CMOS-based imagers use the photoelectric effect of a semiconductor to convert photons into electrical charges. Unlike CCDs, however, CMOS sensors are not limited to silicon as the detector material. Instead, a number of different materials can be hybridized to a CMOS readout integrated circuit (ROIC) to provide sensitivity for ultraviolet, visible, or infrared light, as discussed in Section 4. The following description of the basic CMOS concept is applicable to both monolithic CMOS detectors and hybrid CMOS detectors.

Figure 1 compares the principle of CMOS sensors to that of CCDs. Both detector technologies use a photodiode to generate and separate the charges in the pixel. Beyond that, however, the two sensor schemes differ significantly. During readout, CCDs shift the collected charge from pixel to pixel all the way to the perimeter. Eventually, all charges are sequentially pushed to one common location (floating diffusion), and a single amplifier generates the corresponding output voltages. On the other hand, CMOS detectors have an independent amplifier in each pixel, also called an active pixel sensor (APS). The amplifier converts the integrated charge into a voltage and thus eliminates the need to transfer charge from pixel to pixel. Instead, the voltages are multiplexed onto a common bus line using integrated CMOS switches. Analog and digital sensor outputs are possible by implementing either a video output amplifier or an analog-to-digital (A/D) converter on the chip.

CCD Approach

CMOS Approach

Pixel

Photodiode

Photodiode Amplifier

* -0-

Charge generation and charge integration

Charge génération, charge intégration and charge-to-voltage conversion

-O^H

Multiplexing of pixel voltages: Successively

Array Readout

Tsd

Charge transfer from pixel to pixel

connect amplifiers to common bus

Sensor Output

Output amplifier performs charge-to-voltage conversion

Various options possible:

- no further circuitry (analog out)

- add. amplifiers (analog output)

- A/D conversion (digital output)

Figure 1. Comparison between the CCD-based and the CMOS-based image sensor approach.

Figure 1. Comparison between the CCD-based and the CMOS-based image sensor approach.

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