Abstract:
A method and apparatus are provided for operation of an image sensor during signal readout. During a reset operation the gate of a reset transistor coupled to the storage node receives a voltage greater than a threshold voltage to produce a reset of the storage node. During a period where photogenerated charges stored at the storage node are read out the gate of the reset transistor receives a voltage V RST     —     LOW  greater than ground, but less than a maximum voltage which can be stored at the storage node.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates generally to imaging devices and more particularly to charge injection suppression in active pixel image sensors. 
       BACKGROUND OF THE INVENTION 
       [0002]    A CMOS imager circuit includes a focal plane array of pixel cells. Each one of the pixel cells includes a photosensor, which may be 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 typically includes an output field effect transistor formed in the substrate and a charge transfer section, typically a floating diffusion node, formed on the substrate adjacent the photosensor connected to the gate of the output transistor. 
         [0003]    The active elements of an individual pixel cell in a CMOS imager circuit perform a number of functions, including: (1) photon to charge conversion; (2) accumulation of image charge; (3) resetting the storage region to a known state before the transfer of charge to it; (4) transfer of charge to the storage region; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel charge. Photo charge may be amplified when it moves from the initial charge accumulation region to the floating diffusion node. The charge is typically converted to a pixel output voltage by a source follower output transistor. 
         [0004]      FIG. 1  is a top plan view of a typical CMOS imager pixel cell  10 .  FIG. 2  is a schematic diagram of the CMOS imager pixel of  FIG. 1 . As is known in the art, a pixel cell receives photons of light and converts those photons into charge carried by electrons. To perform this function, each pixel cell  10  includes a photosensor  20 . The photosensor, which can be a photogate, photoconductor, pinned photodiode, or other photosensitive device, includes a charge accumulation region  30  which accumulates electrons produced by photons of light. 
         [0005]    Each pixel cell also includes a transfer transistor  40  for transferring charge from the charge accumulation region  30  to a floating diffusion region  50 , and a reset transistor  60  for resetting the floating diffusion region  50  to a predetermined charge level, V AA-PIX , prior to charge transfer. The pixel cell  10  also may include a source follower transistor  70  for receiving and amplifying a charge level from the diffusion region  50  and a row select transistor  80  for controlling the readout of the pixel cell  10  contents from the source follower transistor  70 . The reset transistor  60 , source follower transistor  70 , and row select transistor  80  include source/drain regions  120 ,  130 , and  140 . There is also a contact to the gate of the source follower transistor  70 . 
         [0006]    Each pixel cell includes several contacts, such as  90 ,  100 , and  110 , to provide electrical connections for the pixel cell  10 . For example, in the embodiment shown in  FIG. 1 , a source/drain region of the reset transistor  60  is electrically connected to an array voltage source terminal (V AA-PIX ) through contact  100 , the gate of the source follower transistor  70  is connected to the floating diffusion region  50  through contact  90 , and an output voltage V OUT  is output from the pixel cell  10  through contact  110 . 
         [0007]      FIG. 3  illustrates a block diagram of a CMOS imager circuit  190  having a pixel array  200  with each pixel cell being constructed as described above. Pixel array  200  comprises a plurality of pixels arranged in a predetermined number of rows and columns. A plurality of row and column lines are provided for the entire array  200 , selectively activated by the row driver  210  in response to row address decoder  220  and the column driver  260  in response to the column address decoder  270 . Thus, a row and column address is provided for each pixel. The CMOS imager circuit  190  is operated by the control circuit  250 , which controls address decoders  220  and  270  for selecting the appropriate row and column lines for pixel readout, and row and column driver circuitry  210  and  260 , which apply driving voltage to the drive transistors of the selected row and column lines. 
         [0008]    CMOS imager pixels cells and circuits of the type described above are generally known as discussed, for example, in the U.S. Pat. No. 6,140,630, U.S. Pat. No. 6,376,868, U.S. Pat. No. 6,310,366, U.S. Pat. No. 6,326,652, U.S. Pat. No. 6,204,524 and U.S. Pat. No. 6,333,205, assigned to Micron Technology, Inc., which are hereby incorporated by reference in their entirety. 
         [0009]    A problem that causes error in CMOS imager circuit output is the injection of unwanted charges into the data path. In a CMOS imager pixel, there are several common sources of charge injection, including charge-to-voltage conversion error charges, redistribution of transistor channel charges after the transistors are switched off, and redistribution of charges accumulated by charge coupling from the reset transistor  60  gate-source overlap capacitor C OVL . Charge injection will introduce an unknown amount of “noise” charge into the floating diffusion region  50 , decreasing the accuracy of the output signal V OUT . This problem is commonly addressed by using a correlated double sampling (CDS) technique to subtract the noise value from the signal, as described below. 
         [0010]      FIG. 4  shows a timing diagram of a typical CMOS imager circuit performing CDS. In performing CDS, generally, first a reset pulse RST is applied to the gate of the reset transistor  60 , turning on the transistor  60  and charging the floating diffusion region  50  to V AA-PIX  less the voltage drop V TH  of the transistor  60 . Accordingly, the floating diffusion region is set to a known reference value V AA-PIX -V TH . The charge on the floating diffusion region  50  is applied to the gate of the source follower transistor  70  to control the current passing through the row select transistor  80 . Upon the pulse of a signal SHR, a voltage based on the current is stored by sample and hold circuit  265 . After the floating diffusion region  50  has been set and the reference voltage stored, charge collected in the charge accumulation region  30  by the photosensor  20  is transferred from the charge accumulation region  30  to the floating diffusion region  50  by the pulse of a signal TX to the gate of the transfer transistor  40 . Upon the pulse of a signal SHS, the new output charge in the floating diffusion region  50  is translated to an output voltage that is stored in the sample and hold circuit  265 . As shown in  FIG. 3 , the sample and hold circuit outputs two signals, corresponding to the stored sampled vales of the reference value Vrst and the photosensor accumulated charge value Vsig. These two signals are subtracted by a differential amplifier  267  to produce the signal Vrst-Vsig, which represents the amount of light impinging on the pixel less certain unwanted noise charges. This difference signal is digitized by an analog to digital converter  275 . The digitized pixel signals are then fed to an image processor  280  to form a digital image. 
         [0011]    However, not all unwanted charge injection can be subtracted out using CDS. Some of the active elements of CDS may be a source of charge injection themselves. Particularly, as the reset transistor  60  is not an ideal switch, when it turns off some portion of the channel charges will relocate to the floating diffusion region  50 . CDS can also cause other problems by reducing the available voltage swing on the floating diffusion region  50 , thereby increasing lag and reducing the dynamic range of the pixel output signal Vsig. This problem is expected to get worse as developments trend to the scaling of the floating diffusion region  50  area to achieve higher conversion gain. 
         [0012]    Accordingly, it would be advantageous to have an improved image sensor to help suppress charge injection without contributing to lag or reducing the available voltage swing on the floating diffusion region. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The foregoing and other advantages and features of the invention will become more apparent from the detailed description of exemplary embodiments provided below with reference to the accompanying drawings in which: 
           [0014]      FIG. 1  is a top plan view of a typical CMOS imager pixel cell; 
           [0015]      FIG. 2  is a schematic diagram of the CMOS imager pixel cell of  FIG. 1 ; 
           [0016]      FIG. 3  is a block diagram of a typical CMOS imager circuit; 
           [0017]      FIG. 4  is a timing diagram of a typical CMOS imager circuit; 
           [0018]      FIG. 5  is a timing diagram of a CMOS imager circuit operated in accordance with an embodiment of the invention; 
           [0019]      FIG. 6  is an equivalent circuit of a portion of a circuit in accordance with an embodiment of the invention; 
           [0020]      FIG. 7  is a blown-up timing diagram of the RST signal during the beginning of a readout period of a CMOS imager circuit in accordance with an embodiment of the invention; and 
           [0021]      FIG. 8  shows processor system incorporating at least one imager constructed in accordance with an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    In the following detailed description, reference is made to the accompanying drawings, which are a part of the specification and which illustrate various embodiments in which the invention may be practiced. In the drawings, like reference numerals describe substantially similar components throughout the several views. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized, and that structural, logical and electrical changes may be made without departing from the spirit and scope of the present invention. 
         [0023]    The term “substrate” is understood as interchangeable and as including silicon, silicon-on-insulator (SOI) or silicon-on-sapphire (SOS), doped and undoped semiconductors, epitaxial layers of silicon supported by a base semiconductor foundation, and other semiconductor structures. Furthermore, when reference is made to a “substrate” in the following description, previous steps may have been utilized to form regions, junctions or material layers in or on the base semiconductor structure or foundation. In addition, the semiconductor need not be silicon-based, but could be based on silicon-germanium, germanium, gallium arsenide, or other known semiconductor materials. 
         [0024]    The term “pixel” or “pixel cell” refers to a picture element unit cell containing a photo-conversion device for converting electromagnetic radiation to an electrical signal. The embodiments of pixels discussed herein are illustrated and described as employing a four transistor (4T) pixel circuit for the sake of example only. It should be understood that the invention may be used with other pixel arrangements. 
         [0025]    Although the invention is described herein with reference to the architecture and fabrication of one pixel cell, it should be understood that this is representative of a plurality of pixels in an array of an imager device. In addition, although the invention is described below with reference to a CMOS imager, the invention has applicability to any solid state imaging device having pixels. Other solid state imagers may use similar circuits in an output stage or other stage of an imager. Accordingly, the following detailed description is not to be taken in a limiting sense or as limiting to use in CMOS imagers, and the scope of the present invention is defined only by the appended claims. 
         [0026]    Referring back to the timing diagram for operation of a typical CMOS imager, generally, there are three periods of signal timing. The first is the reset period  400 , initiated at t 1  by a pulse of the RST signal, followed by a pulse of the TX signal. The photosensor is reset during the reset period  400 . The second period, the integration period  410 , begins at t 2  on the falling edge of the TX signal, which also marks the end of the photosensor reset period. During the integration period, charges are collected by the photosensor. The integration period overlaps a portion of the third period, which is the readout period  420 , illustrated as time t 4 -t 7  in  FIG. 4 . The falling edge of the TX signal at t 6  marks the end of the integration period. The signals RST, SHR, TX, and SHS are commonly held in a low or ground state for the duration of the integration period until the beginning of the readout  420 , though RST may be held high. 
         [0027]    CDS and the reading of the output of the pixel signal take place during the readout period. CDS is initiated by a pulse on the RST signal from V RST     —     LOW  up to V RST     —     HI  and dropping back to V RST     —     LOW . The rise of the RST signal past the reset transistor  60  threshold voltage marks the beginning of the readout period. Subsequent to this time, marked by t 4  in  FIG. 4 , the reset transistor  60  is in the on state and a channel exists from the source to the drain. As the signal falls from V RST     —     HI  back to V RST     —     LOW , channel charges are injected into both the source and the drain end of the reset transistor  60 . The channel disappears as the signal drops below the threshold voltage and at t 5  the signal reaches V RST     —     LOW , which is ground. During the remainder of the readout period V RST     —     LOW  conventionally remains ground in order to minimize the unwanted charge leakage from floating diffusion region  50  to V AAPIX . The pulse SHR controls the reset signal sample and hold circuit and this denotes the sample and hold period for the reset signal Vrst. The pulse SHS controls the photocharge generated signal sample and hold circuit and this denotes the sample and hold period for the photocharge generated signal Vsig. 
         [0028]      FIG. 5  illustrates a timing diagram of the readout period of a CMOS imager circuit according to an embodiment of the invention. Rather than dropping the RST signal to V RST     —     LOW  ground, V RST     —     LOW  is dropped to a positive voltage, greater than ground but less than a value which would cause the reset transistor to turn on. The maximum value of the V RST     —     LOW  is explained further below. V RST     —     LOW  is maintained at this positive voltage level for the remainder of the readout period. The benefits of this will be made clear below. Prior to and subsequent to the readout period, RST may be held at ground or at V RST     —     HI , where V RST     —     HI  can be a voltage greater than the threshold voltage V T  of the reset transistor and may be set as high as the reset transistor supply voltage V AAPIX  plus the reset transistor threshold voltage V T . Thus, V T &lt;V RST     —     HI ≦V T +V AAPIX . For convenience,  FIG. 5  shows RST oscillating between two levels, V RST     —     LOW  and V RST     —     H . 
         [0029]      FIG. 6  is an equivalent circuit diagram of the CMOS pixel imager cell when the RST transistor  60  is in the off state which further explains the maximum voltage which may be used for V RST     —     LOW  in an exemplary embodiment of the invention to reduce unwanted charge injection. The  FIG. 6  circuit occurs when V RST     —     LOW  drops below V AAPIX +V T . The RST signal is represented as a voltage source V RST , reset transistor  60  gate-drain overlap capacitor as C OVL1 , reset transistor  60  gate-source overlap capacitor as C OLV2 , and floating diffusion region  50  capacitance as C FD . After the reset transistor  60  has switched off, the channel charges will redistribute as charge injection error voltage ΔV INJ  to V RST  and C OVL2 . Additional charge at V RST  is inconsequential, however, additional charge at the gate-source overlap capacitor C OLV2  is a primary contributor to unwanted charge injection in the floating diffusion region  50 . Solving KVL for the circuit for ΔV INJ  will yield the following expression: 
         [0000]      ΔV INJ ≈C OVL2 (V AAPIX +V T −V RST     —     LOW )/(C FD +C OVL1 ) 
         [0030]    Accordingly, it can be seen that there is a range of values of V RST     —     LOW  greater than ground which if applied will lead to a lower ΔV INJ . However, V RST     —     LOW  should not be maintained at a value high enough to risk causing unwanted leakage of the collected charge from the floating diffusion region  50  to V AAPIX . To ensure no lost of collected charge while maintaining a positive voltage, V RST     —     LOW  should not exceed the maximum voltage of the floating diffusion region. Thus, as V RST     —     LOW &lt;V FD  the following equations can be derived: 
         [0000]      V RST     —     LOW MAX =V AAPIX −V SIGMAX ≈V AAPIX −V PIN (C PD /C FD ) 
         [0000]    where V SIGMAX  is the maximum signal voltage from the photosensor  20 , V PIN  is the pinned voltage of the photodiode, representing the maximum charge collected by the photosensor, C PD  is the capacitance of the photosensor and V RST     —     LOW MAX  is the maximum positive value to which V RST     —     LOW  may be maintained during the readout period. For illustrative purposes, in a circuit having values V AAPIX =2.8 v, V PIN ˜1.5 v, C PD =1fF and C FD =2fF, V RST     —     LOW MAX  would be 2.05 volts. 
         [0031]      FIG. 7  illustrates an enlarged view of the pulse of the RST signal during the beginning of the readout period, t 4 -t 5 . The readout period begins at t 4  as RST crosses the reset transistor  60  threshold voltage VT. At t 5  RST has dropped to V RST     —     LOW , in this case, V RST     —     LOW MAX , however it should be clear that V RST     —     LOW  could drop to any level within the shaded region from ground to V RST     —     LOW MAX . Maintaining V RST     —     LOW  within this region will reduce the charge injection, thereby increasing the available voltage swing of the floating diffusion region  50 . Accordingly, the linear full well will be increased, improving the lag performance. In addition, the peak conversion gain is increased due to the non-linearity of the floating diffusion junction capacitance. The positive V RST     —     LOW  also enables excessive charges to escape to V AAPIX  when the floating diffusion region  60  is full, thereby serving as an anti-blooming gate when capturing high contrast scenes. The pixel construction is the same as a conventional pixel, for example, as illustrated in  FIGS. 1 and 2 , or can be other conventional pixel readout structures of other pixel or array designs. However, the controller  250  for generating the signals for array operation provides a reset signal which is at a level above ground and less than or equal to V RST     —     LOW MAX  when the reset transistor is turned off during read out. 
         [0032]    A typical processor based system which may include an imager circuit according to the present invention is a camera  300 , which may be a digital still or video camera, or other type of camera, as shown in  FIG. 8 . It should be noted that the illustration of a camera is not intended to be limiting and that such an imager circuit could be included in any processor system including a computer system, scanner, machine vision, vehicle navigation, video phone, cell phone, personal digital assistant, surveillance system, auto focus system, star tracker system, motion detection system, and other systems employing an imager. 
         [0033]    The illustrated camera system  300 , generally comprises a central processing unit (CPU)  310 , such as a microprocessor for controlling camera operations, that communicates with an input/output (I/O) device  340  over a bus  370 . The imaging device  330 , also communicates with the CPU  310  over the bus  370 . The system  300  also may include random access memory (RAM)  320 , and can include removable memory  360 , such as flash memory, which also communicate with the CPU  310  over the bus  370 . The imaging device  330  may be combined with a processor, such as a CPU, digital signal processor, or microprocessor, with or without memory storage on a single integrated circuit or on a different ship than the processor. 
         [0034]    It should be noted that although the invention has been described with specific reference to CMOS imaging circuits having a photodiode and a floating diffusion region, the invention has broader applicability and may be used in any CMOS imaging apparatus. The above description and drawings illustrate preferred embodiments which achieve the objects, features and advantages of the present invention. It is not intended that the present invention be limited to the illustrated embodiments. Any modification of the present invention which comes within the spirit and scope of the following claims should be considered part of the present invention.