The earliest solid-state image sensors with the bipolar and MOS photodiode arrays were invented by Westinghouse, IBM, Plessy, and Fairchild in the late 1960s.
The CCD devices were invented at Bell Laboratory in 1970, and they immediately became the major image devices in the market [1]. Due to the poor performance and large pixel size (for that time) relative to that of the CCD, since the CCD was invented, the CMOS sensor cannot compete with the CCD completely. Therefore, in the early days, the major image technology is Charge-Couple Devices (CCD). The CCD has many advantages, like low noise and high sensitivity. Besides, the circuit architecture of CCD is simple, and the major operation principle is charge transfer.
Therefore, the CCD is still be used in many image application generally today. Until the early 1990s, after the invention of passive pixel sensor (PPS) as shown in Fig. 1.1, the CMOS image sensor was used in the low-end machine vision application [2]. The big push for the development of CMOS sensors came from the introduction of Active Pixel Sensors (APS) in the early ‘90s as shown in Fig. 1.2[3, 4, 5]. It was quickly realized that adding an amplifier to each pixel significantly increases sensor speed and improves its signal-to-noise ratio (SNR), thus overcoming the shortcomings of PPS [6]. Recently, the research has been focusing, mainly, on the improvement of the APS, because APS is the pixel circuit that has shown better performance and flexibility[7].
In order to strongly compete with CCD technology, the aim of researchers has been to obtain higher performance imaging systems based on CMOS technology.
Therefore, there have been several reports on improving the fill-factor (FF) with low power consumption, low voltage operation, low noise, high speed imaging and high dynamic range. After the invention of APS pixel, the performance of CMOS image sensor is increasing rapidly. Accompany with the appearance of 3T and 4T CMOS sensor pixel, CMOS image sensor is gradually replacing the CCD in the commercial and science market. Fig. 1.3 shows the readout circuits of CCD and CMOS sensor.
Many of the difference between CMOS sensor and CCD arise from their difference in the readout architectures. The readout architecture of CCD is described as following:
First, charge is shifted out of the array by vertical and horizontal CCD. Then, the charge is converted into voltage by the source follower, and serially read out. In the CMOS sensor, charge voltage signals are read out one row at a time in a manner similar to a random access memory using row and column select circuits. The advantages comparison of the CCD and CMOS is shown in table I.
The main Advantages of CMOS imagers are:
1. Low power consumption. Estimates of CMOS power consumption range from one-third to more than 100 times less than that of CCDs [8]. Besides, they work at low voltage. CMOS imagers only need one supply voltage, instead of CCDs, which need 3 or 4.
2. Lower cost compared to CCD’s technology.
3. On chip functionality and compatibility with standard CMOS technology. CMOS imagers allow monolithical integration of readout and signal processing electronics.
In 2001, a study for Cross Contamination between CMOS Image Sensor and IC product showed no problems [9]. A sensor can integrate various signal and image processing blocks such as amplifiers, ADCs, circuits for colour processing and data compression, etc. on the same chip.
4. Miniaturisation, although important limitations exist, the level of integration is rather high [10].
5. Random access of image data.
6. Selective read-out mechanism [10,11]
7. High-speed imaging. The flexibility and the possibility to acquire images in a very short period of time [12].
8. To avoid blooming and smearing effects, which aretypical problems of CDD technology [13].
As outlined before, despite these advantages, there are still significant Disadvantages of CMOS image sensors compared to CCD technology. Therefore, these problems need to be solved so that CMOS image sensors can compete in any area.
These disadvantages are:
1. Sensitivity: The basic quality criterion for pixel sensitivity is the product of its Fill Factor and its Quantum Efficiency where Fill Factor is the ratio of light-sensitive area to the pixel’s total size, and Quantum efficiency is the ratio of photon-generated electrons that the pixel captures to the photons incident on the pixel area. It must be pointed out that Active Pixel Sensors (APS) have reduced sensitivity to incident light, due to a limited Fill Factor, hence, less quantum efficiency.
2. Noise: CMOS Image sensors suffer from different noise sources which set the fundamental limits of their performance, especially under low illumination.
3. Dynamic range (DR): Dynamic Range, which is the ratio of the saturation signal to the rms noise floor of the sensor, is limited by the photosensitive-area size, integration time and noise floor.
4. Less image quality than CCD.
According to the advantages and advantages mentioned above, the research of CMOS image sensor mostly focus on the following directions: low noise, high dynamic range, high sensitivity and high fill factor, low power consumption, low voltage operation and high speed imaging. The overall architecture of CMOS image sensor is shown in Fig. 1.4.
Figure 1.1 PPS schematic
Figure 1.2 APS schematic
Figure 1.3 (a) readout architecture of CCD (b) readout architecture of CMOS sensor
Figure 1.4 The overall architecture of CMOS image sensor