Patent Publication Number: US-8975103-B2

Title: CMOS image sensor with wide dynamic range

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 11/646,235, filed Dec. 28, 2006, which claims priority of Korean patent application number 10-2005-0134125, filed on Dec. 29, 2005, each of which is incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a semiconductor device; and, more particularly, to a Complementary Metal Oxide Semiconductor (CMOS) image sensor. 
     Recently, with development of multimedia communication via internet, the demand on digital cameras is dramatically increased. Furthermore, as the supply of the mobile communications terminals such as a Personal Digital Assistant (PDA), an International Mobile Telecommunications-2000 (IMT-2000) cellular phones, and a Code Division Multiple Access (CDMA) terminal is increased, the demand of the small-sized camera modules is increased. Most camera modules basically include an image sensor. Generally, the image sensor is a device to convert optical image into an electrical signal, and a charge coupled device (hereinafter, referred to as “CCD”) and a Complementary Metal-Oxide-Semiconductor (CMOS) image sensor are widely used as the image sensor. 
     The driving method of the CCD is complicated and high power consumption is required. Further, the CCD is required to take lots of mask processes and has the disadvantages in that it is difficult to implement one-chip integration of the signal processing circuits. Recently, the CMOS image sensor has been occupied the attention of the image sensor techniques because it is relatively easy to implement the monolithic integration of the signal processing circuits including a controller and a driver. Moreover, the CMOS image sensor can be manufactured with the cost-effective circuit implementation in the low-power operation, low power consumption, the compatibility with other peripheral circuits and standardized fabrication processes. 
     However, in the CMOS image sensor, the analog signal generated by a light receiving element, for example, a photo diode is generated with various parasitic effects caused by a parasitic capacitance, a resistance, an inconsistency of a semiconductor device characteristic, a dark current leakage and so on. Since the parasitic effects are inevitably generated in the semiconductor device, they bring about degradation of the signal-to-noise of the image data. Therefore, the noise is a critical factor to limit the performance of the CMOS image sensor. 
     The noises caused in the CMOS image sensor can be classified into the kT/C noise related to the image data sampling, the 1/f noise related to a circuit which is used for amplifying the image signal and the fixed pattern noise (hereinafter, referred to as FPN) related to an inconsistency of the signal processing circuits of a sensor. The FPN of them is not very good to a visual since it appears in the picture on which peoples can directly recognize the noise as a strip or perpendicular line. 
       FIG. 1  is a block diagram illustrating a conventional CMOS image sensor having a rectangular unit pixel structure. Referring to  FIG. 1 , in the case where a row address signal is applied to a decoder with a pixel array  10  as the central figure, a row decoder  20  is disposed in one direction of the pixel array  10 , a pixel data output is disposed at right angles with the row decoder  20 , and a column decoder  30  producing a column address signal of the pixel is also disposed centering around the pixel array  10 . 
     The data read-out procedure of the CMOS image sensor in  FIG. 1  will be described. First, data is read out from one pixel by selecting both a first row which is selected by the row decoder  20  and a first column which is selected by the column decoder  30  and, thereafter, the read-out data is amplified. Next, a second row is selected by the row decoder  20 , a second column is selected by the column decoder  30 , and data of another pixel is read out and amplified. In such a manner that the pixel is selected by the row and column decoders  20  and  30 , the whole pixels are read out and amplified one by one. The CMOS image sensor has the various kinds of unit pixels. The general unit pixels are formed in two types of 3-T (transistors) and 4-T structures. The 3-T structure has one photodiode and three transistors and the 4-T structure has one photodiode and four transistors. 
       FIG. 2  is a circuit diagram illustrating the 3-T structure of the CMOS image sensor. Referring to  FIG. 2 , the pixel in the 3-T structure has one photodiode to convert photogenerated charges into an electric signal and three NMOS transistors to process this electric signal. The three transistors are called a reset transistor Rx, a drive transistor Dx and a select transistor Sx, respectively. The reset transistor Rx resets the photodiode connected to a ground voltage by applying a power supply voltage VDD to one end of the photodiode. The drive transistor Dx forms a source follower which acts as a buffer amplifier and the select transistor Sx acts as a switch for addressing the pixel. 
       FIG. 4  is a circuit diagram illustrating the 4-T structure of the CMOS image sensor. Referring to  FIG. 4 , the pixel in the 4-T structure has one photodiode to convert photogenerated charges into an electric signal and four NMOS transistors to process this electric signal. The four transistors are called a transfer transistor Tx, a reset transistor Rx, a drive transistor Dx and a select transistor Sx, respectively. The transfer transistor Tx transfers photogenerated charges in the photodiode to a floating diffusion region FD and the reset transistor Rx sets a potential of the floating diffusion region FD to a predetermined value and then resets the floating diffusion region FD. The drive transistor Dx, as a source follower, acts as a buffer amplifier based on the accumulated charges on the floating diffusion region FD. In similar to the 3-T structure, the select transistor Sx in the 4-T structure acts as a switch for addressing the pixel. 
     The most important difference between the 3-T and 4-T structures is the presence of the transfer transistor Tx and the floating diffusion region FD. The 3-T structure turns on the reset transistor Rx after detecting a reset level of the unit pixel; however, the 4-T structure detects the signal level by turning on the transfer transistor Tx after detecting the reset level by turning on the reset transistor Rx. 
       FIG. 3  is a circuit diagram illustrating a pixel array in which one column line is commonly owned by a plurality of pixels in the 3-T structure. As shown in  FIG. 3 , the unit pixels UP 1  to UPn are commonly connected to one column line and then connected to one load transistor. 
       FIG. 5  is a circuit diagram illustrating the pixel array in which the pixels of the 4-T structure are commonly connected to one column line. Referring to  FIG. 5 , the unit pixels UP 1  to UPn are commonly connected to one column line and connected to one load transistor. 
     As shown in  FIGS. 3 and 5 , in the 3-T and 4-T structures, a plurality of pixels are commonly connected to one column line and connected to one load transistor through the column line. The data are outputted on a column-by-column basis as shown in  FIG. 1 . 
     The CMOS image sensor having these characteristics and configuration has the disadvantage in that the dynamic range of the CMOS image sensor is lower than that of the CCD (approximately, a half (½)). The dynamic range means the rate of the saturation level of the pixel to the signal noise level and it is the significance factor to determine the chroma of the color. 
     If approximately 20 million electrons are detected at the saturation and approximately 40 electrons are detected at the noise, the dynamic range is about 5000 and the noise is −75 dB. Since the variation of the transistors which is employed for converting the photogenerated charges into a voltage level has an effect on the output characteristics of CMOS image sensor, the CMOS image sensor has the disadvantage in the dynamic range, as compared with the CCD. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention are directed to provide a CMOS image sensor capable of improving the dynamic range thereof. 
     In accordance with an aspect of the present invention, there is provided a CMOS image sensor having a pixel array having a plurality of unit pixels each of which includes a photodiode and a fist transistor to act as a source follower buffer amplifier to amplify photogenerated charges accumulated in the photodiode, the CMOS image sensor comprising a second transistor for a buffer amplifier to amplify and output a gate input voltage in the unit pixel, wherein an output signal of the first transistor is applied to a gate of the second transistor. 
     In accordance with another aspect of the present invention, there is provided a CMOS image sensor having a pixel array having a plurality of unit pixels each of which includes a photodiode and a fist transistor to act as a source follower buffer amplifier to amplify photogenerated charges accumulated in the photodiode, wherein the plurality of unit pixels are connected to a common column line, the CMOS image sensor comprising a second transistor having a gate which receives one of output signals from the first transistors in the plurality of unit pixels, wherein the second transistor amplifies the output signal inputted from the gate and outputs the amplified signal to the common column line. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating a conventional CMOS image sensor. 
         FIG. 2  is a circuit diagram illustrating a unit pixel having a 3-T structure of the conventional CMOS image sensor. 
         FIG. 3  is a circuit diagram illustrating a pixel array having a plurality of the unit pixel of  FIG. 2 . 
         FIG. 4  is a circuit diagram illustrating a unit pixel having a 4-T structure of the conventional CMOS image sensor. 
         FIG. 5  is a circuit diagram illustrating a pixel array having a plurality of the unit pixel of  FIG. 4 . 
         FIG. 6  is a circuit diagram illustrating a unit pixel of a CMOS image sensor according to a first embodiment of the present invention. 
         FIG. 7  is a layout of the unit pixel of  FIG. 6 . 
         FIGS. 8A ,  8 B and  9  are timing charts illustrating an operation of the unit pixel of  FIG. 6 . 
         FIG. 10  is a circuit diagram illustrating a pixel array of a CMOS image sensor according to a second embodiment of the present invention. 
         FIG. 11  is a circuit diagram illustrating a pixel array of a CMOS image sensor according to a third embodiment of the present invention. 
         FIGS. 12A to 12C  are a circuit diagram illustrating a pixel array of a CMOS image sensor according to a fourth embodiment of the present invention. 
         FIG. 13  is a circuit diagram illustrating a pixel array of a CMOS image sensor according to a fifth embodiment of the present invention. 
         FIG. 14  is a circuit diagram illustrating a pixel array of a CMOS image sensor according to a sixth embodiment of the present invention. 
         FIG. 15  is a circuit diagram illustrating a pixel array of a CMOS image sensor according to a seventh embodiment of the present invention. 
     
    
    
     DESCRIPTION OF SPECIFIC EMBODIMENTS 
     An object of the present invention is to a CMOS image sensor to improve the dynamic range. 
     Now the preferred embodiments according to the present invention will be described with reference to the accompanying drawings. Since preferred embodiments are provided for the purpose that the ordinary skilled in the art are able to understand the present invention, they may be modified in various manners and the scope of the present invention is not limited by the preferred embodiments described later. 
     Embodiment 1 
       FIG. 6  is a circuit diagram illustrating a unit pixel having the 4-T structure of a CMOS image sensor according to a first embodiment of the present invention and  FIG. 7  is a layout of the unit pixel of  FIG. 6 . 
     Referring to  FIGS. 6 and 7 , in the unit pixel according to the first embodiment 1 of the present invention, driver transistors which act as a buffer amplifier using a source follower are provided at two stages. That is, besides a first driver transistor Dx 1 , an additional driver transistor, a second driver transistor Dx 2 , which is connected between a second power supply voltage VDD 2  and the drain of the select transistor Sx and has a gate to be connected to a source of the first driver transistor Dx 1 , is further provided to the unit pixel. In this configuration, the width of voltage swing of the transistor is increased with wide dynamic range. In order to increase the width of voltage swing, the second power supply voltage VDD 2  is preferably higher than a first power supply voltage VDD 1 . 
       FIGS. 8A and 8B  are graphs illustrating the dynamic range enlarged by the unit pixel having the structure according to the first embodiment of the present invention shown in  FIG. 6 . The graph shown in  FIG. 8A  is an operation waveform of the first drive transistor Dx 1  and the graph shown in  FIG. 8B  is an operation waveform of the second drive transistor Dx 2 . Moreover, the threshold voltage of the second drive transistor Dx 2  is the same as that of the first drive transistor Dx 1 . 
     As shown in  FIG. 9 , the dynamic range is increased when two drive transistors Dx 1  and Dx 2  are used in the invention, as compared with the conventional CMOS image sensor using just one drive transistor Dx 1 . That is, the second drive transistor Dx 2  receives an output signal Vout of the first drive transistor Dx 1  through a gate thereof so that the dynamic range is increased based on such a gate input Vg 2 . 
     Referring to  FIG. 9  showing the operation characteristics of the first and second driver transistors, the dynamic range B of the CMOS image sensor having two diver transistors Dx 1  and Dx 2  is wider than that having just one driver transistor by +α. That is, the first drive transistor Dx 1  receives a gate voltage Vg 1  and outputs an amplified output signal Vout and the output signal Vout from the first driver transistor Dx 1  is inputted into a gate of the second driver transistor Dx 2  as a gate input signal. The final output signal Vout′ is higher than the output signal Vout of the first driver transistor Dx 1  and the saturation caused by the body effect is delayed as shown in  FIG. 9 . 
     Embodiment 2 
       FIG. 10  is a circuit diagram illustrating a unit pixel of the image sensor having the 4-T structure according to a second embodiment of the present invention. Referring to  FIG. 10 , in the unit pixel according to the second embodiment of the present invention, the select transistor Sx is connected between the first and second driver transistors Dx 1  and Dx 2 . That is, the second driver transistor Dx 2  is connected between the second power supply voltage VDD 2  and the output signal Vout and a gate of the second driver transistor Dx 2  is connected to a source of the select transistor Sx. The first driver transistor Dx 1  is connected between the first power supply voltage VDD 1  and the select transistor Sx and the gate of the first driver transistor Dx 1  is connected to the floating diffusion region FD. Other elements in this embodiment are the same as those in the first embodiment of the present invention. 
     Embodiment 3 
       FIG. 11  is a circuit diagram illustrating a unit pixel of the image sensor having the 4-T structure according to a third embodiment of the present invention. Referring to  FIG. 11 , in the unit pixel according to the third embodiment of the present invention, the select transistor Sx is connected between the first power supply voltage VDD 1  and a drain of the first driver transistor Dx 1 . Other elements in this embodiment are the same as those in the first embodiment of the present invention. 
     Embodiment 4 
       FIGS. 12A to 12C  are circuit diagrams illustrating unit pixels of the image sensor having the 3-T structure according to a fourth embodiment of the present invention. Referring to  FIGS. 12A to 12C , the fourth embodiment of the present invention shows the unit pixels when the 3-T structure is respectively applied to the first to third embodiments. 
     Embodiment 5 
       FIG. 13  is a circuit diagram illustrating a unit pixel of the image sensor having the 4-T structure according to a fifth embodiment of the present invention. Referring to  FIG. 13 , being different from the first embodiment having the two-stage structure of the first and second driver transistors Dx 1  and Dx 2 , the unit pixel according to the fifth embodiment of the present invention has a multiple stage Dxn of the driver transistors and uses one power supply voltage as a common drain voltage. 
     Embodiment 6 
       FIG. 14  is a circuit diagram illustrating a unit pixel of the image sensor having the 4-T structure according to a sixth embodiment of the present invention. Referring to  FIG. 14 , being different from the first to fifth embodiments having two stage structure of the first and second driver transistors Dx 1  and Dx 2 , the unit pixel according to the sixth embodiment of the present invention has the second driver transistor Dx 2  every column line in order to reduce the fill factor. That is, each of the unit pixels UP 1  to UPn has one drive transistor Dx 1  and additionally comprises the second drive transistor Dx 2  on a column-by-column basis. This configuration can increase the dynamic range without the reduction of the fill factor. 
     Embodiment 7 
       FIG. 15  is a circuit diagram illustrating the unit pixel of the image sensor having the 4-T structure according to a seventh embodiment of the present invention. Referring to  FIG. 15 , the unit pixel according to the seventh embodiment of the present invention further includes a diode D to carry out the rectification at the gate terminal of the first drive transistor Dx 1  in the unit pixels which are illustrated in the first to sixth embodiments. In similar to the general rectification of a diode, the diode D blocks a negative (−) electric potential and transfers a positive (+) electric potential. In the meantime, the first power supply voltage VDD 1  and the second power supply voltage VDD 2  illustrated in the first to seventh embodiments are 2.5V and 2.8V, respectively. 
     While the present invention has been described with respect to the specific embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.