Abstract:
The image sensor includes an array of pixels. Each pixel has a pinned photodiode which transfers charge via a transfer gate to a floating diffusion, from which output is provided by a source follower. Each column has a voltage supply line and a signal line. Each row has a transfer gate control line, a read/reset control line, and a read/reset voltage line which receives alternately zero volts and a predetermined positive voltage from a decoder circuit.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to solid state image sensors, and is particularly useful in image sensors for use in mobile applications such as cellular phones. The invention also relates to devices, such as mobile phones, digital cameras and optical pointing devices (e.g. computer mouse) incorporating solid state image sensors.  
       BACKGROUND OF THE INVENTION  
       [0002]     Image sensors using pinned photodiode pixels, typically implemented in CMOS architecture, are well known. Such image sensors in many applications have the advantage that both the image sensitive element and the image processing circuitry can be embodied in a single chip which can be manufactured using CMOS techniques.  
         [0003]     However, there is increasing demand for image sensors for use in mobile applications such as mobile phones where power supplies are very limited. Specifically, as discussed in more detail below, many mobile devices have a battery supply at 2.4V whereas prior art pinned photodiode image sensors require a voltage of about 3V. This can be met by providing a charge pump, but at the expense of increasing the circuit complexity and cost. Moreover, the charge pump will typically require a storage capacitor which is too large to be provided on-chip, and thus an off-chip component is required.  
       SUMMARY OF THE INVENTION  
       [0004]     The present invention seeks to provide an image sensor which at least addresses the above described problems.  
         [0005]     The invention provides a solid state image sensor comprising an array of pixels arranged in rows and columns, each pixel having a photodiode connected to a floating diffusion via a transfer gate, the floating diffusion providing an output to a column output line via a source follower. Each row has a transfer gate control line. The image sensor includes at least one read/reset voltage supply line connected to the floating diffusion via a read/reset transistor. The or each read/reset voltage supply line is driven by a decoder circuit at alternately a predetermined reset voltage and a substantially lower voltage.  
         [0006]     The invention further provides a digital camera, a mobile telephone, and a computer pointing device, each having such an image sensor. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     Embodiments of the invention will now be described, by way of example only, with reference to the drawings, in which:  
         [0008]      FIG. 1  is a schematic diagram of one pixel of a prior art four-transistor pinned photodiode image sensor;  
         [0009]      FIG. 2  is a schematic diagram showing a prior art image sensor incorporating an array of pixels as in  FIG. 1 , and showing the power supply arrangement;  
         [0010]      FIGS. 3   a  and  3   b  are schematic diagrams of one pixel an image sensor according to the invention;  
         [0011]      FIG. 4  is a timing diagram showing timing in the pixel of  FIG. 3 ;  
         [0012]      FIGS. 5   a - 5   c  are schematic diagrams showing an image sensor incorporating an array of pixels as in  FIG. 3 ;  
         [0013]      FIG. 6  is a schematic diagram showing voltage conditions in the pixel of  FIG. 3  after reset and during readout; and  
         [0014]      FIG. 7  is a schematic diagram illustrating an alternative embodiment. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]     Referring initially to  FIG. 1 , in a prior art arrangement a pixel  10  of an image array has three horizontal lines: a reset line RST(j), a transfer gate line TG(j), and a read line RD(j), and two vertical lines: a reset voltage line VRT and a signal line Vx. However, the reset potential of the floating diffusion FD is VRT, which is also the source follower supply voltage. This causes a problem that the reset potential of FD cannot be generated separately from the source follower supply voltage. In this case, VRT must supply current (typically a few mA) to bias the source followers at a sufficiently high voltage to cause complete charge transfer from the pinned photodiode without lag; typically the required voltage is around 3V. In an image sensor for mobile applications the supply voltage to the chip is often only 2.4-2.5V, and thus the 3V supply must be generated from a charge pump.  
         [0016]     Referring to  FIG. 2  which shows a prior art circuit in greater detail, an array of pixels  10  is supplied at 3V at  12  from a battery supply Vdd of 2.4V via a charge pump  14  and a voltage regulator  16  controlled by a bandgap voltage  18 . The load current required to be delivered by the charge pump  14  determines the size of the pump capacitors, clock frequency, and efficiency. Often this necessitates a storage capacitor  20  of a size which can only be provided off-chip, which increases the system physical size and cost.  
         [0017]     Referring to  FIGS. 3   a  and  3   b , there is shown the operating principle of a single pixel  30  in one embodiment of the present invention. The pixel  30  operates as a pinned photodiode pixel using three transistors M 1 -M 3 , the floating diffusion being shown as a capacitor FD. The pixel  30  has vertical VRT and Vx lines, and horizontal read reset line RD/RST(j)k and transfer gate line TG(j). The floating diffusion FD is reset to the voltage on a further horizontal line VRST(j). This voltage is time multiplexed between VRST and a much lower voltage (from zero to a few hundred mV above ground) by a Y-decoder circuit  32 .  FIG. 3   a  illustratively shows the Y-decoder circuit  32  as an analog multiplexer.  FIG. 3   b  illustratively shows the circuit  32  in the form of a level shifter.  
         [0018]     The timing of the pixel  30  is illustrated in  FIG. 4 . To deactivate the pixel in row j the sensing node FD is set at 0V (or other low voltage) when the pixel is not being read, switching the source follower transistor M 3  off. This is done by RD/RST(j)=0 and VRST(j)=0, rstb(j)=hi, which will set VRST(j)=0V. The transistor M 2  conducts the FD to VRST(j)=0V, maintaining the source follower transistor M 3  in the off state. RD/RST(j) and rst(j) are maintained in these states by the Y-decoder  32  for all rows of pixels (j) which are not being read.  
         [0019]     To read out a row of pixels, rst(j) is asserted setting VRST(j)=VRST. Since RD/RST(j) is normally still asserted (pixel is not being read beforehand) the FD is set to VRST by M 2 . The high voltage on the gate of M 3  causes current to flow producing a voltage Vx which is Vt below the voltage on M 3 . All other M 3  in pixels on the same column have a gate voltage of 0V and so are not conducting.  
         [0020]     The reset and signal voltages from the pixel are sampled by signals CDS 1  and CDS 2  at the base of the column. The RD/RST(j) line falls to 0V causing charge injection and reset coupling to produce a small decrease in the voltage on FD. The voltage on Vx follows this change and is sampled by CDS 1  on a column capacitor. This sample is the black reference sample from the pixel.  
         [0021]     The signal sample from the pixel is produced by pulsing TG(j) low then high to transfer charge from the pinned photodiode to the floating diffusion FD. This produces a variable downwards voltage swing on FD proportional to the amount of light charge integrated on the photodiode. The column voltage Vx follows the change in FD and is sampled by CDS 2  onto a second column sampling capacitor. The sampled voltages are subtracted to remove kT/C noise introduced when releasing FD from reset.  
         [0022]     It is noted that the pixel in  FIG. 3  can be realized with three horizontal lines (RD/RST(j), VRST(j), and TG(j)) and two vertical lines (VRT and Vx). Using this three transistor pixel with horizontal VRST allows the constraints on VRT and VRST to be separated. Referring to  FIGS. 5   a - 5   c , VRT can be supplied directly by an input battery voltage AVDD=2.5V via a voltage regulator  34 . VRST still has to be typically 3V which can be produced by a charge pump  36  supplying a voltage regulator  38 .  
         [0023]     The load on VRST is purely capacitive. A row of FD capacitances must be charged on the readout of each video line; this typically amounts to 3 pF every 50 μs. No DC current is involved. This makes it possible to use a charge pump with on-chip components of reasonable size, typically with a storage capacitor of about 20 μF which can readily be provided on-chip.  
         [0024]      FIG. 6  illustrates the voltage conditions on M 3  during and after reset. The bias conditions on M 3  make it possible for the drain voltage to be lower than the gate voltage provided the Vgs drop of M 3  is sufficiently large. Since body effect has a significant effect, the Vgs drop is around 1V.  
         [0025]      FIG. 7  shows an alternative embodiment, in which a number n of photodiode sites are coupled to a single floating diffusion FD and share source follower and reset transistors M 3  and M 2 . The technique of sharing transistors is effective in reducing the proportion of non-photosensitive circuitry area to photodiode area for maximum light gathering capacity and minimum pixel size.  
         [0026]     The invention thus makes it possible to dispense with off-chip storage elements. Further, the invention allows a pinned-photodiode arrangement which requires only three (or fewer) transistors per pixel. It will be appreciated that the solid state image sensor of the invention can be incorporated in a number of devices.  
         [0027]     As one example, the sensor may form part of a digital camera in which a camera lens focuses a desired image onto the image plane of the image sensor. In another use, the image sensor may be incorporated in a mobile phone having the customary receiving and transmitting means for digital signals. A further use is in an optical mouse, or pointing device, for use with a computer. The optical mouse has a housing which provides a button surface and which contains an illumination source, a lens, and a solid state image sensor. The lens provides optical transmission of highlights, produced by the illumination source on a mouse mat or equivalent to the image sensor.  
         [0028]     The invention is particularly suited to such applications, where efficient power supply arrangements are desirable to minimize current draw and battery size, and to prolong battery life.