The invention generally relates to image sensor systems; and in particular, the present invention relates to a digital image sensor including an integrated charge transfer amplifier at each pixel.
Digital photography is one of the most exciting technologies that have emerged in the past years. With the appropriate hardware and software (and a little knowledge), anyone can put the principles of digital photography to work. Digital cameras, for example, are on the cutting edge of digital photography. Recent product introductions, technological advancements, and price cuts, along with the emergence of email and the World Wide Web, have helped make digital cameras the hottest new category of consumer electronics products.
Digital cameras, however, do not work in the way that traditional film cameras do. In fact, they are more closely related to computer scanners, copiers, or fax machines. Most digital cameras use an image sensor or photosensitive device, such as charged-coupled device (CCD) or Complementary Metal-Oxide Semiconductor (CMOS) to sense a scene. The photosensitive device reacts to light reflected from the scene and can translate the strength of that light into electronic signals that are digitized. By passing light through red, green, and blue filters, for example, the intensity of the light can be gauged for each separate color spectrum. When the readings are combined and evaluated via software, the camera can determine the specific color of each segment of the picture. Because the image is actually a collection of numeric data, it can easily be downloaded into a computer and manipulated for more artistic effects.
Digital cameras, however, do not have the resolution attainable with conventional photography. While traditional film-based technology, limited only by the granularity of the chemically based film, typically has a resolution of tens of millions of pixels, image sensors for use in most commercially available digital cameras acceptable to general consumers have a resolution of slightly more than one or two million pixels. Although digital cameras having resolutions of up to six million pixels are available, these high-resolution cameras are prohibitively expensive. Furthermore, the dynamic range of digital image sensors is often not as broad as is possible with film-based conventional photography. This is especially true for CMOS image sensors which, in general, have lower dynamic ranges than CCDs.
FIG. 1 is a block diagram of a digital image sensor as disclosed in U.S. Pat. No. 5,461,425 of Fowler et al. (xe2x80x9cthe ""425 patentxe2x80x9d). As is shown, digital image sensor 10 includes an image sensor core 12 which has a two-dimensional array of pixels. Each pixel 15 of sensor core 12 has a light detecting element (a photodetector or photosensor) coupled to a dedicated A/D converter. Each of the A/D converter outputs a stream of bits representative of the analog output of the associated light detecting element. In other words, the image sensor of the ""425 patent outputs digital image data directly from each pixel. In a digital image sensor such as sensor 10 of FIG. 1, not only does the supporting circuitry for image sensor core 12 become dramatically simplified, there are also numerous advantages provided by the digital image sensor architecture in view of traditional CMOS image sensors. The advantages include better control of operations of the image sensor and far better image quality therefrom.
However, adding a dedicated A/D converter to each of the light detecting elements could introduce some practical problems that may limit the practical application of such digital image sensors. One of the problems is that image sensor core 12 is inevitably larger than it would be without the dedicated A/D converters. If an image sensor is desired to have millions of photodetectors thereon, there would be a large number of dedicated A/D converters, which could take a significant amount of circuit area to implement in the image sensor core. Larger image sensor cores are undesirable because they typically lead to higher manufacturing cost and lower yield. Therefore, designs of digital image sensors having a smaller image sensor core are much more desirable. Further, it is often not feasible to merely reduce the size of the photodetectors to accommodate the dedicated A/D converters in an image sensor of limited size. This is because the sensitivity of the photodetectors could be compromised when the photodetectors are made smaller. This decreased sensitivity leads to a corresponding decrease in the dynamic ranges of the pixels. Therefore, there is a need to improve the sensitivity and dynamic range of a CMOS image sensor while maintaining small device sizes of the image sensor core.
According to the present invention, a digital image sensor includes a sensor array of digital pixels. The sensor array outputs digital signals as pixel data representing an image of a scene. Each of the digital pixels includes a photodetector producing an analog signal indicative of the amount of light impinging on the sensor array. Each digital pixel also includes a charge transfer amplifier coupled to receive the analog signal and amplifying the signal to generate an amplified pixel voltage signal. The digital image sensor further includes analog-to-digital conversion (ADC) circuits located within the sensor array. Each of the ADC circuits is connected to one or more charge transfer amplifiers for converting the amplified pixel voltage signal of each digital pixel to a digitized pixel voltage signal. The sensor array of the present invention is fabricated in an integrated circuit.
The charge transfer amplifier generates a pixel voltage signal having increased voltage magnitude than the voltage signal generated by the photodetector and provides the enhanced voltage value to the analog-to-digital conversion circuit. The integration of a charge transfer amplifier in a digital pixel of the present invention has the effect of increasing the sensitivity level of each of the digital pixels and as a result, provides a digital image sensor with increased sensitivity and dynamic range.
In one embodiment, the photodetector is a photogate and the charge transfer amplifier is implemented as a transfer gate and a floating diffusion as a measuring capacitor.