Abstract:
A new kind of pixel is formed of two floating diffusions of different sizes and different conductivity type. The two floating diffusions have different image characteristics, and hence form a knee-shaped slope.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims benefit of U.S. Provisional application No. 60/124,153, filed Mar. 8, 1999. 
     
    
     BACKGROUND 
       [0002]    Active pixel sensors are described in U.S. Pat. No. 5,417,215. Higher charge and efficiency from these devices is desirable. In addition, different parameters and operations can benefit from different kinds of samples. For example, a short sampling period can provide the highest amount of dynamic range, while a longer sampling period can provide better resolution. Fossum and Yadid-Pecht have described one such system and “Wide Intrascene Dynamic Range CMOS APS Using Dual Sampling, IEEE Transactions On Electronic Devices, volume 44 page 1721-1723, October 1997. In that system, two signals are obtained using two different integration intervals. 
       SUMMARY 
       [0003]    The present application teaches a new pixel design with dual floating diffusion regions, each of which is separately controlled. The two regions collectively provide dual integration, but do so in a way that increases sensitivity, allows dual dynamic range, and also provides multiple junctions for improved photocarrier detection. 
     
    
     DETAILED DESCRIPTION  
       [0004]      FIG. 1  shows a first embodiment which is formed by a more standard CMOS fabrication process. There is a relatively large floating diffusion capacitance, which can tend to reduce the charge conversion gain. However, this system may be easier to make due to its use of a standard CMOS process, with the floating diffusion being on the surface. 
         [0005]    An N-type well  100  is formed in the P-type substrate  105 . A first floating diffusion region  110  is a P-type floating diffusion region formed on the surface, i.e., its top surface close to or touching the active oxide region  102 . The P-type floating diffusion region  110  is connected to a P-type output transistor  115  and a P-type reset transistor  120 . The reset transistor  120  connects to a P+-type diffusion region  125  which is biased, for example, to the voltage level of the drain voltage. 
         [0006]    The second floating diffusion region  130  is an N-type floating diffusion region. Note that the second floating diffusion region  120  takes up a much smaller area then the first floating diffusion region, e.g. one fifth as much area. 
         [0007]    The second floating diffusion region is N-type, and is connected to an N-type output transistor  135 . An NMOS reset transistor  140  connects the floating diffusion region to N+ diffusion region  145 , which can be connected to a supply voltage level. In this way, a P-type region is formed extending from the edge of the P-type floating diffusion  110  to the edge of the P-type diffusion  125 . The N-type region, starting at floating diffusion  130 , is separate from the P-type region. In addition, the N-type region can surround virtually the entire active P type region, and all of the P+ region. 
         [0008]    This new pixel design needs two separate reset control lines, one for the NMOS reset transistor  140  and the other for the PMOS reset transistor  120 . One column can be used for both output signals, e.g. with two select control lines. Alternately, two output columns can be used with one select line. 
         [0009]    Note that since the size of the two different floating diffusion regions is different, they will store different amounts of charge. Therefore, the floating diffusion  110  can store more charge then the floating diffusion  135 . Different integration periods for these two diffusion floating regions allow a flexible saturation exposure for each element. It also facilitates obtaining a highlighted compression knee sloped light transfer curve. 
         [0010]    A second embodiment is shown in  FIG. 2 . In this embodiment, the P type diffusion region  200  is formed below the surface of the N-well  202 . An overlying N region  215  is formed above the floating diffusion, covered by the active oxide. The N-well  202 , in this embodiment, is arranged to be fully depleted. A second smaller floating diffusion region  205  is connected to the buried floating diffusion  200 , and is connected to output transistor  115 . In this way, there are three superimposed PN junctions: A first junction between the overlying N region area  215  and the buried floating diffusion  200 . Another PN junction is formed between the bottom of the floating diffusion  215  and the N region  202 . The third PN junction is between the fully depleted N region  202 , and the P type substrate  105 . The capacitance for the floating N type diffusion can be reduced by this structure. 
         [0011]    Although only a few embodiments have been disclosed and detailed above, other modifications are possible. All such modifications are intended to be encompassed within the following claims, in which: