Abstract:
An image sensing device, such as a CMOS Active Pixel Sensor device, including an array of pixels. Each pixel has a photoreceptor, a follower transistor connected to said photoreceptor, a select transistor connected to said follower transistor, and a reset transistor which controls applying a reset level to a pixel that is different than the pixel in which said reset transistor is physically located.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional application of U.S. patent application Ser. No. 09/653,527, filed on Aug. 31, 2000, which claims the benefit of U.S. Provisional application No. 60/151,619, filed Aug. 31, 1999, the disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND 
     It is desirable to reduce the power consumption of an image sensor. It may also be desirable to reduce the magnitude, e.g., voltage, of the voltage supply that drives the image sensor. For example, this can allow more flexibility in battery operated applications. 
     Lowering the voltage, however, can lower the dynamic range of the sensor. 
     The voltage can be boosted internally. 
     SUMMARY 
     The present application defines increasing the pixel voltage dynamic range in a photosensor, such as an active pixel sensor. This is done by using two controlling lines to control each pixel. Each pixel line can have its own voltage, thereby enabling applying separate voltages to different parts of the pixel. By selectively controlling the voltages on the different parts, dynamic range boosting can be carried out. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects will be described in detail with reference to the accompanying drawings, wherein: 
         FIG. 1  shows a basic active pixel sensor; and 
         FIG. 2  shows the ways that the control lines are coupled to different parts of the circuits. 
         FIG. 3  shows an active pixel sensor according to an exemplary embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     A single pixel of an active pixel sensor is shown in  FIG. 1 . An array of these active pixel sensor elements can be formed on a single chip and formed using transistors which are compatible with CMOS techniques. A photoreceptor, e.g. a photodiode  100  is formed in the substrate  99 . 
     A first reset controlling line  115  controls a reset transfer gate  105  to reset the charge from the photodiode  100  based on a floating reset diffusion  110 . The diffusion is either floating when gate  105  is off, or connected to line  120 , when gate  105  is on. 
     When gate  105  is off, the value on the floating diffusion  110  represents the charge on the photodiode  100 . This charge level is buffered by a follower transistor  125 , and also switched by an in pixel select transistor  130 . Additional pixels and circuitry may be also placed in the pixel as disclosed in U.S. Pat. No. 5,471,515. 
     All of the elements in this device can be formed from MOS and CMOS transistors. These transistors have a significant threshold voltage between 0.6 and 0.9 volts. The output voltages from the floating diffusion  110 , the source follower transistor  125  and other voltages may be reduced or shifted downward by these thresholds. 
     For a supply voltage of 3.3 volts, the voltage on the floating diffusion may extend between 1.2 volts and 2.7 volts, e.g. the dynamic range may equal 1.5 volts. A boosted reset pulse may be used to increase the floating diffusion level, for example by 0.5 volts. This could correspondingly increase the signal dynamic range. 
     The present application teaches a way to expand dynamic range, maintain low dark current, and provide an operational mode in which quantum efficiency is increased by all the photodiode PN junctions in the pixel being kept near zero potential during the integration time. The improved pixel uses a combination of three different techniques for increasing its performance. 
     A first technique uses in-pixel boosting. In the present technique, the photodiode voltage only increases during the time of integration. 
     The channel of the pixel source follower is filled with charge during reset. The charge dumps from the channel into the drain during the readout time. The readout line is kept grounded during reset. 
     In a typical active pixel sensor circuit, this can result in a large current, since the drain on the source follower shares its VDD with the drain of the reset transistor. 
     The present system may separate the biases to switching elements (e.g. transistors), within a single pixel. This is done by using an additional metal line in each pixel. The circuit as described herein also uses a shared reset/select line which forms a reset for a first line, and a select for a different line. In this way the drain of the reset transistor for a specific pixel is separated from the drain of the source follower transistor for that pixel. By applying pulses to the transistors at different times, the power supplies can be effectively separated. 
     The VDD lines are run horizontally. As described herein, a special dynamic readout regime is used to minimize the DC current along that line, and thereby minimize voltage drop along that line. 
       FIGS. 2 and 3  show an embodiment. An active pixel sensor circuit  200  is used which has special characteristics. 
     As shown in  FIG. 2 , the active pixel sensor circuit  200  has a plurality of bias lines extending through the circuit. In this special circuit, the number of horizontal lines is increased by a factor of two over the prior art active pixel sensor as described in U.S. Pat. No. 5,471,515. This system provides a VDD N−1  line  205  for biasing the follower in each row of pixels and a separate reset/select line for the reset transistor in that row. This compares with prior systems in which the VDD potential was shared among all source follower and reset transistor drains. 
     Each VDD line, such as  205 , is connected to two separate row drivers; here line N−1 and line N. As shown by waveform  209 , this provides the VDD voltage only during the time of the two select pulses, i.e. during almost one row time. During the rest of the frame time, VDD remains grounded. 
     In operation, the VDD N−1  line  205  first rises at  210 . This boosts the voltage on the floating diffusion on line N. This also boosts the reset on line N−1 and also begins the first selecting pulse time period  212 . During the second selecting pulse time  226 , the VDD line raises the level on the source follower  215 . 
     The gate of the source follower  215  for line N−1 is connected to the photodiode  220  for line N. 
     After signal sampling is completed, the voltage on VDD line N−1 drops to 0 at  224  during the reset time for photodiode  220  for line N−1. This means that the floating diffusion for that photodiode  220  will be charged to the reset level when the output column is grounded and the surface potential under the source follower gate is minimum. This may increase the cell capacitance. 
     After reset is completed, the voltage on the VDD line N−1 is raised again to begin the period  226 . The surface potential under the source follower gate is then maximized, thereby minimizing the capacitance of the source follower gate. 
     Reference sampling then occurs during time period  226 . At the end of the reference sampling, the voltage on VDD line n−1 drops down at  227  and remains low for the remainder of the frame period. 
     If the capacitances of the photodiode and of the source follower gate are approximately equal, then the pixel can be boosted by half of the potential swing under the source follower gate. This could reach 1 volt for VDD=3.3 volts and a typical reset boosting. 
     The line  230  shows the photodiode boosting that occurs. During the reset pulse, the output is boosted by an amount  232 . Importantly, the drains of the reset transistor  255  and the source follower  215  for the same photodiode are connected to different VDD lines. The joint VDD contact for the reset transistor drain of one photodiode is connected to the source follower drain of another photodiode. For example,  FIG. 2  shows the gate of source follower  215  being connected to the drain of the reset transistor  225  for a separate line. This layout can save pixel space, provide improved FF, quantum efficiency, and have a relatively small pixel pitch. 
     It could be undesirable to have a steady current from the VDD row driver to ground throughout the horizontal VDD line and vertical output column. In order to avoid this, a special dynamic source follower mode may be used. In  FIG. 2 , the bottom horizontal VLN line  250  is connected to a gate of a current sink transistor  252 . This current sink transistor is turned on to provide a timed pulse (e.g. of 3.3 volts) instead of continuous DC voltage. This causes the transistors which are biased by the voltage line  254 , which includes the transistors  215 ,  216  and corresponding transistors of other pixels, to operate as switches instead of steady state current generators. This also provides two column modes. An “on” mode connects the columns to ground and an “off” mode provides floating columns. In this way, all pixel source followers operate in a dynamic mode. This may increase the output source follower voltage by an extra 0.2 to 0.4 volts. It may keep the output columns at zero voltage for a part of the row period and cause them to float at readout/select time. 
     This system as described above can increase dynamic range, improve quantum efficiency, and reduce power consumption by reduction of the source follower static DC current. 
     Although only a few embodiments have been disclosed in detail above, other modifications are possible. All such modifications are intended to be encompassed within the following claims. 
     For example, other photoreceptors, such as photogates, pinned photodiodes, or other devices could be used. The photogate could require a separate transfer gate to be added.