Charge-coupled devices (CCDs) have been the mainstay of conventional imaging circuits for converting images in the form of light energy into electrical signals. A CCD generally includes a plurality of MOS capacitors and each MOS capacitor is adjacently disposed to each other. Charge carriers are stored on one of the MOS capacitors and then transferred to another MOS capacitor next to the MOS capacitor with the charge carriers stored therein. CCDs typically output pixel-by-pixel analog signals. CCD technology has been incrementally engineered to provide high quality images with low visible noise (granular distortion visible in the image). CCDs imagers have a greater sensitivity to light and have better dynamic range than CMOS imagers, and therefore yield superior quality images. CCDs are also capable of large formats with small pixel size, and produce less noise (visual artifacts). A charge coupled device (CCD) has a high dynamic range and a low dark current. The sophistication of the current technology of a charge coupled device allows the charged couple device to become the most popular image sensing device. As a result of these advantages, CCDs are the preferred technology for high end imaging applications. A CCD also has various disadvantages, such as complicated drive mode, high power consumption, difficulty of realization of a signal processing circuit in a chip for the CCD due to undesirably large number of mask processes. The manufacturing for a charge coupled device is rather special. The price for a CCD is therefore very high. Since CCDs are fabricated in a non-standard technology, other analog and digital camera functions such as A/D conversion, image processing and compression, control and storage cannot be integrated with the sensor on the same chip and must be implemented using several other chips. Such implementation can be quite expensive because of the specialized processing involved for CCDs. Moreover, the driver requires a high voltage operation, leading to the problems of high power dissipation and inability of random access of memory. CCDs must transfer an image by line charge transfers from pixel to pixel, requiring that the entire array be read out into a memory before individual pixels or groups of pixels can be accessed and processed. CCDs may also suffer from incomplete charge transfer from pixel to pixel during charge transfer, which results in image smear. Additionally, a CCD has a disadvantage in that it is difficult to make a compact-size product, due to the difficulty in integrating various circuits such as a controlling circuit, a signal processing circuit, analog/digital converting circuit and so on into a single chip.

Because of the inherent limitations and expense of CCD technology, CMOS imagers have been increasingly used as low cost imaging devices. A complementary metal oxide semiconductor (CMOS) image sensor is a device that converts an optical image to an electrical signal using a CMOS manufacturing technology, which employs a switching scheme of an MOS transistor for transporting photo-electric charges from a photodiode to an output node as well as detection of an output signal at the output node. The CMOS technology provides the possibility of integrating image sensing and digital signal processing on the same chip, resulting faster, smaller, less expensive, and lower power image sensing devices. The advantages of CMOS image sensors over CCD image sensors are well known. The primary advantages of CMOS imagers are their relatively low cost generally resulting from the use of standard, high-volume CMOS processes and their ability to be integrated with native CMOS electronics for control and image processing. CMOS image sensors can be manufactured by means of an ordinary CMOS process, are characterized by low power consumption, low voltages and a low cost. The CMOS image sensor can be operated by a single power voltage source. So, the CMOS image sensor has the advantages of lower power consumption and smaller size, as compared with the CCD image sensor. A CMOS image sensor has the characteristics of high quantum efficiency, low read noise, high dynamic range and random access, and it is fully compatible with the manufacturing for a CMOS device. A CMOS image sensor can combine with other control circuit, A/D converter and several signal processing circuits on a single wafer to achieve the so-called system on a chip (SOC). On-chip integration of electronics is particularly advantageous because of the potential to perform many signal conditioning functions in the digital domain as well as to achieve a reduction in system size and cost. A fully compatible CMOS sensor technology enabling a higher level of integration of an image array with associated processing circuits is beneficial in many digital imaging applications such as, for example, cameras, scanners, machine vision systems, vehicle navigation systems, video telephones, computer input devices, surveillance systems, auto focus systems and star trackers, among many others.

In general, A CMOS image sensor comprises a pixel section where unit pixels including a photodiode are arranged like a matrix, a scanning circuit for scanning unit pixels in order, and a correlated double sampling (CDS) circuit for processing signals output from the pixel section. The CMOS image sensor is composed of a photo-sensing means for sensing light and a CMOS logic circuit for processing sensed light into electrical signals to make them as data. A CMOS imager circuit includes a focal plane array of pixel cells, each one of the cells including either a photogate, photoconductor or a photodiode overlying a substrate for accumulating photo-generated charge in the underlying portion of the substrate. A readout circuit is connected to each pixel cell and includes at least an output field effect transistor formed in the substrate and a charge transfer section formed on the substrate adjacent the photogate, photoconductor or photodiode having a sensing node, typically a floating diffusion node, connected to the gate of an output transistor. A CMOS image sensor may include at least one electronic device such as a transistor for transferring charge from the underlying portion of the substrate to the floating diffusion node and one device, also typically a transistor, for resetting the node to a predetermined charge level prior to charge transference. A charge transfer device can be included as well and may be a transistor for transferring charge from the photoconversion device to the floating diffusion region. Imager cells also typically have a transistor for resetting the floating diffusion region to a predetermined charge level prior to charge transference. The output of the source follower transistor is gated as an output signal by a row select transistor. A photodiode is formed in the light sensing region, and transistors or other devices may be formed in the peripheral circuit region, thereby forming a semiconductor structure. An interconnect structure including a plurality of insulating layers and metal lines is formed over the semiconductor structure to interconnect the photodiode, transistors and other devices. A CMOS imaging system typically includes the sensor core and various ancillary circuits which dynamically amplify the signal depending on lighting conditions, suppress noise, process the detected image and translate the digitized data into an optimum format.

CMOS pixel arrays form the core of a CMOS image sensor. CMOS pixel arrays are based on either active or passive pixels. CMOS imagers include an array of photo-sensitive diodes, one diode within each pixel. A pixel is an individual picture element. In solid-state imagers, a pixel refers to a discrete photosensitive cell that can collect and hold a photocharge. Photocharge is a phenomenon in which silicon exposed to photons results in the release of charge carriers. Each pixel in a CMOS imager has a radiation sensitive element with each radiation sensitive element connected to an amplifier. Passive pixels typically use a simpler internal structure, which does not amplify the photodiode's signal within each pixel. Active pixels typically include amplification circuitry in each pixel. A CMOS pixel which converts an optical image into an electronic signal with an arrangement of having an amplifier attached to each radiation-sensitive element is called an active pixel. In a CMOS imager, the active elements of a pixel cell perform the necessary functions of photon to charge conversion, accumulation of image charge, transfer of charge to the floating diffusion node accompanied by charge amplification, resetting the floating diffusion node to a known state before the transfer of charge to it, selection of a pixel for readout; and output and amplification of a signal representing pixel charge. The active pixels in a CMOS imager can be arranged in a matrix form and be utilized to generate video signals for video cameras, still photography, or anywhere incident radiation needs to be quantified. In the field of imaging, complimentary metal oxide semiconductor (CMOS) active pixel image sensors have made considerable inroads into applications served by charge coupled imaging devices. The photosensitive element of a CMOS imager pixel is typically either a depleted p-n junction photodiode or a field induced depletion region beneath a photogate or photoconductor. In general, a unit pixel in CMOS image sensor includes one photodiode (PD) and four NMOS transistors. The four transistors include a transfer transistor for transferring the photo-electric charges generated from the photodiode to a floating sensing node, a reset transistor for discharging the charges stored in the floating sensing node to detect subsequent signals, a drive transistor serving as a source follower, and a selection transistor for switching and addressing.

A complementary metal oxide semiconductor (CMOS) image sensor converts an optical image to an electrical signal using a CMOS manufacturing technology, which employs a switching scheme of an MOS transistor for transporting photo-electric charges from a photodiode to an output node as well as detection of an output signal at the output node. A CMOS image sensor senses an image using a photodiode and a MOS transistor in each pixel for detecting light signals in a switching mode. When an incident radiation interacts with radiation sensitive element in a CMOS imager, charge carriers are liberated and can be collected for sensing. The number of carriers generated in pixel is proportional to the amount of the incident light impinging on the radiation sensitive element and the sensitivity of radiation sensitive element to light. The electronic signal generated by pixel 100 in a CMOS imager is then read directly on an x-y coordinate system. In a CMOS image sensor, a pixel detects an image by accepting light rays in an area on which a photodiode is formed. A photodiode image sensor device is the most commonly used device for detecting images. A typical photodiode image sensor device comprises a reset transistor and a light sensor region formed by a photodiode. The photodiode utilizes a diode depletion region formed across a p-n junction. The photodiode receives light and outputs an electrical charge corresponding to the amount of light received. The electric charge is converted into current or voltage levels defining an image which is stored in digital memory. A photodiode produces electrical signals by generating electron hole pairs (EHPs) by means of incident light rays through a p-n junction. CMOS transistors co-located in each picture element (pixel) select, amplify and transfer the photodiode signals. The amount of incident light which is detected by the photodiode is computed by subtracting the voltage that is transferred from the capacitor from the reset voltage level on the photodiode.

Developments in CMOS image sensor technology are paving the way for a new generation of digital imaging products with broad consumer applicability. CMOS active pixel sensors (APS) exploit the mature CMOS industry and can compete with charge coupled devices for low power, high levels of integration and functionality. A fully compatible CMOS sensor technology enabling a higher level of integration of an image array with associated processing circuits is beneficial in many digital imaging applications such as, for example, cameras, scanners, machine vision systems, vehicle navigation systems, video telephones, computer input devices, surveillance systems, auto focus systems and star trackers, among many others. Business and industrial applications include videoconferencing, machine vision, medical instrumentation, broadcasting and video-based information display or acquisition for real estate, insurance and other business segments. Such a CMOS imaging system-on-chip requires lower voltages and dissipates less power than a CCD-based system with supporting camera system. Visible imaging systems implemented in CMOS significantly reduce video camera cost and power by efficiently combining the image sensor with the ancillary components including drive electronics and output signal conditioning electronics. These improvements translate into smaller camera size, longer battery life, and applicability to many new products.