Patent Application
20060103749
The present invention relates to image sensors, and more particularly, to an image sensor that uses pixels that can vary their capacitance based upon the intensity of incident light.
Image sensors have become ubiquitous. They are widely used in digital still cameras, cellular phones, security cameras, medical, automobiles, and other applications. The technology used to manufacture image sensors, and in particular CMOS image sensors, has continued to advance at great pace. For example, the demands of higher resolution and lower power consumption have encouraged the further miniaturization and integration of the image sensor.
As the pixels become smaller, it becomes more difficult for the pixel to output a signal of adequate strength that can be easily deciphered by downstream signal processing. Moreover, there are demands on the image sensor to perform over a large range of lighting conditions, varying from low light conditions to bright outside sunlight. This is generally referred to as having a large dynamic range. Still, because of the decreasing size of the pixel, as described below, the dynamic range of the pixel may be limited.
In the following description, numerous specific details are provided to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well known structures, materials, or operations are not shown or described in order to avoid obscuring aspects of the invention.
Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrase “in one embodiment” or “in an embodiment” in various places throughout the specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.
In operation, during an integration period (also referred to as an exposure period), the light sensing element 101 generates charge which is held at the light sensing element 101 because transfer transistor 201 is off. After the integration period, the transfer transistor 201 is turned on to transfer the signal to the floating node A. After the signal has been transferred to floating node A, the transfer transistor 201 is turned off again for the start of a subsequent integration period. Thus, as seen, the transfer transistor 201 turns on and off periodically to transfer signal from each integration period to the floating node A.
The signal on the floating node A is then used to modulate the amplification transistor 105. Finally, a row select transistor 107 is used as a means to address the pixel and to selectively read out the signal onto a column bit line 109. After readout through the column bitline 109, the reset transistor 103 resets the floating node A to a reference voltage, in this particular embodiment, Vdd.
In general, in the 4T pixel design, the floating node A is designed to be relatively small. Floating node A is designed to be relatively small in order to achieve high transfer or conversion gain. However, in high illumination conditions, the amount of charge (signal) produced by the light-sensing element 101 may be greater than the capacity of the floating node A. This will result in saturation of the floating node A and reduced dynamic range, as well as reduced signal-to-noise ratio (SNR).
Turning to
Under high illumination light conditions, the auxiliary transistor 203 is turned on during the readout operation. This, in effect, increases the capacitance of the floating node A. However, in low-light conditions, the auxiliary transistor 203 is turned off and the floating node A maintains its relatively small capacitance.
Turning to
Next, at box 303, the illumination level determined at box 301 is compared to a threshold value. The threshold value is the trigger for turning on the auxiliary transistor 203. The precise point where the threshold is set may be made variable depending upon design considerations, parameters, and characteristics of the image sensor, and may even be adjustable at the discretion of the user of the image sensor.
If the illumination level determined at box 301 is higher than the threshold, then at box 307, the auxiliary transistor 203 is turned on during operation. However, if the illumination level is lower than the threshold, then at box 305, the auxiliary transistor 203 is turned off.
In a 4T pixel, in order to obtain high transfer gain, the floating node A is designed to be relatively small and have a relatively small capacitance, for example, on the order of 2 femtofarads. If the image sensor and pixels use a one-volt signal range, this means that the maximum number of electrons (Qmax) that can be held at the floating node A is about 12,500. Considering “shot noise,” this results in a maximum signal-to-noise ratio of 112. However, in many illumination conditions, a light-sensing element, such as a pinned photodiode, may generate more signal (electrons) than Qmax.
In order to address this issue, in accordance with the present invention, an auxiliary capacitance (in this embodiment the auxiliary transistor 203) is switched on during operation where there is relatively high ambient illumination. This results in the capacitance on the floating node A to increase, i.e., the sum of the “4T normal” floating node (2 femtofarads) and the capacitance provided by the auxiliary transistor 201.
Returning to
Assuming that the amount of charge collected by the light sensing element 101 is Q and the auxiliary transistor 203 is off, then the voltage change (ΔV1) at floating node A is given by:
ΔV1=Q/(CV1+CF)
Similarly, assuming that the amount of charge collected by the light sensing element 101 is Q and the auxiliary transistor 203 is on, then the voltage change (ΔV2) at floating node A is given by:
ΔV2=Q/(CV2+CF)
Thus, for example, if CV1 is approximately equal to 0.1CF and CV2 is approximately equal to 1.2CF, then ΔV1=Q/1.1CF and ΔV2=Q/2.2CF. It can be seen that the voltage change (conversion gain) is about twice as high when the auxiliary transistor 203 is turned off then when the auxiliary transistor 203 is turned on. In other words, during low light conditions, there is higher conversion gain.
Note that in
In the above description, the capacitance of the floating node A is increased upon detection of a high level of illumination. The converse arrangement may also be implemented. In other words, in yet another alternative embodiment, “normal” operation may have the capacitance at floating node A be relatively high. When there is detection of a low level of illumination, the capacitance at floating node A may be decreased to improve conversion gain. This decrease may be done be turning off the auxiliary capacitance. These embodiments are considered equivalent since the same function, manner, and result is accomplished.
Thus, as seen from the description above, a pixel and image sensor formed in accordance with the present invention has two modes of operation: a high illumination mode and a low illumination mode. The present invention switches on an auxiliary capacitance at the floating node based upon the amount of illumination on the image sensor. The amount of illumination on the image sensor can be determined in a variety of ways, and any method for determining the level of illumination could easily be applied to the present invention. Once the level of illumination is determined, a decision is made by comparing the level of illumination to a threshold whether to switch on the auxiliary capacitance (for high illumination) or not switch on the auxiliary capacitance (for low illumination).
The active pixels described above may be used in a sensor array of a CMOS image sensor 1101. Specifically,
The sensor array 1103 portion may be, for example, substantially similar to the sensor arrays portions of image sensors manufactured by the assignee of the present invention, OmmiVision Technologies, Inc., of Sunnyvale, Calif., as model numbers OV5610 or OV7640, except that the pixels are replaced with the active pixels disclosed herein.
The description of the invention in this application as set forth herein is illustrative and is not intended to limit the scope of the invention. Variations and modifications of the embodiments described herein are possible, and practical alternatives to, or equivalents of the various elements, the embodiments are known to those of ordinary skill in the art. These and other variations and modifications of the embodiments disclosed herein may be made without departing from the scope and spirit of the invention.
From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.
