Patent Application
20110032391
1. Field of the Invention
The present invention generally relates to an image sensor, and more particularly to an image sensor with an active pixel array surrounded by peripheral dummy pixels.
2. Description of the Prior Art
Semiconductor based image sensors, such as charge-coupled devices (CCDs) or complementary metal-oxide-semiconductor (CMOS) image sensors, are widely used in, for example, cameras or camcorders, to convert images of visible light into electronic signals that may be stored, transmitted or displayed thereafter.
The image sensor is generally a mixed-signal system having both analog circuits and digital circuits on a single device. Noise is commonly present in the analog/digital circuits of the image sensor, which noise often may propagate from the analog/digital circuits and disadvantageously affect the imaging area of the image sensor. Another noise source outside of the imaging area is the dark reference pixel array used for obtaining a black level reference. The undesired noise often may be coupled to the imaging area, for example, through the substrate, causing interference. As a result, the performance of the image sensor, for example, measured by a signal-to-noise ratio, can suffer greatly. The effect caused by this interfering or potentially interfering noise becomes more severe when the image sensor employs active pixel sensor (APS) structure in which each pixel contains its own active amplifier.
For the reason that conventional image sensors cannot effectively defend against noise, a need has arisen to propose a novel technique for minimizing an influence of noise on the imaging area (e.g., the active pixel array) of the image sensor.
In view of the foregoing, it is an object of the present invention to provide a scheme to block undesired or potentially interfering noise, originating externally in analog/digital circuitry and/or a dark reference pixel array, from being coupled into the imaging area.
According to one embodiment, an imaging pixel array is surrounded by a ring of non-imaging pixels. External to the ring of non-imaging pixels is a noise source, such as an analog circuit, a digital circuit, or a dark reference pixel array. The embodiment causes unwanted charge originating from the noise source to be drained away by the non-imaging pixels.
According to another embodiment, a dark reference pixel array containing light-shielded pixels is enclosed and protected by the ring of non-imaging pixels.
External to the dummy pixels 10 may, but need not necessarily, be, disposed one or more of an analog circuit, a digital circuit, and/or a dark reference pixel array. The analog circuit(s), digital circuit(s), and/or dark reference pixel array constitutes a noise source that generates undesired or potentially interfering noise in the form of, for example, charge. The noise source is not limited to those mentioned, and, in general, may be any circuit, such as a power circuit, that generates and propagates noise to affect the active pixel 12.
Regarding the active pixel 12, in the present embodiment, the transfer gate TG, controlled under a transfer signal VTG, is used to controllably transfer the integrated light signal at the node PD of the photodiode D. The reset gate RG, when it is turned on by a reset signal (Reset), is used to reset the photodiode D to a reset reference voltage VReset. The output of the transfer gate TG and the output of the reset gate RG are connected together to the floating diffusion (FD) node FD, which is further connected to the gate of the source follower SF, which is utilized to buffer or amplify the integrated light signal of the photodiode D. The output of the source follower SF may be read through a column bus (or a bit line) via a selector SL, when it is turned on by a word line signal WL.
The dummy pixel 10 in the embodiment contains a photodiode D, a transfer gate TG, a reset gate RG, a source follower SF, and a selector SL configured in a way similar to that in the active pixel 12, although they may not necessarily have to be similar to each other. The output of the dummy pixel 10 is floating. Specifically, the selector SL, in the present embodiment, is not connected to the column bus, but is floating. Moreover, the transfer gate TG is always turned on, for example, by connecting its gate to a high voltage ON. The reset gate RG is always turned on, for example, by connecting its gate to a high voltage ON, and its drain is connected to a high voltage V+ in order to constantly reset the photodiode D. As a result, any unwanted collected charge (e−) originating from one or more of the analog circuit, the digital circuit, and the dark reference pixel array (
It is appreciated by those skilled in the pertinent art that the active pixel 12 and the dummy pixel 10 may not necessarily be on the same level in the image sensor. Generally speaking, the undesired noise may be substantially reduced or blocked provided that, by way of example and not limitation, the dummy pixels 10 are located between at least a portion of the active pixel array 12 and the noise source or sources.
According to the embodiments disclosed above, the noise from outside analog/digital circuits and/or a dark reference pixel array may be blocked from being coupled into the imaging area 12. The signal-to-noise ratio may accordingly be substantially increased, and the performance of the image sensor may thus be improved.
According to this embodiment, in addition to the active pixel array 12, the dark reference pixel array 14 is also significantly protected (by the dummy pixel ring 10) from being affected by undesired noise from the noise source such as the analog/digital circuit, even though the pixels of the dark reference pixel array 14 are not effective pixels for the purpose of imaging. Under usual conditions, the dark reference pixel array 14 generates less noise than the analog/digital circuit external to the dummy pixel ring 10.
Although specific embodiments have been illustrated and described, it will be appreciated by those skilled in the art that various modifications may be made without departing from the scope of the present invention, which is intended to be limited solely by the appended claims.
