Abstract:
A photosensor includes a plurality of photosensitive regions including a first photosensitive region connected to a first voltage reference, and at least one additional photosensitive region. A signal collector is connected to the first photosensitive region. At least one switching device is for switching the at least one additional photosensitive region between the first voltage reference and a second voltage reference that is less than the first voltage reference, and for reversibly connecting the at least one additional photosensitive region to the signal collector so that the photosensor is variably responsive to different light levels.

Description:
FIELD OF THE INVENTION 
   The present invention relates to photosensors, and more particularly, to a photosensor in which its response may be varied in accordance with different lighting conditions. 
   BACKGROUND OF THE INVENTION 
   It is often desirable for a photosensor array to contain at least one photosensor capable of operating under a variety of different lighting conditions (e.g., high light levels) and poorly lit conditions (e.g., low light levels). This feature is typically provided by changing the exposure time of the photosensor. For example, under bright lighting conditions, the exposure time of the photosensor is reduced to prevent the photosensor from saturating. 
   Since the present invention relates to the response of a photosensor under different lighting conditions, it is useful at this point to briefly review a number of the operating characteristics of photosensors in reference to  FIGS. 1 and 2 . 
   The full well capacity of a photosensor array refers to the total amount of charge that can be stored in any one of its photosensors before overflowing into adjoining photosensors. Accordingly, the full well capacity of a photosensor array is dependent upon the physical size of its photosensors. 
   Dynamic range is defined as the maximum signal strength achievable by a photosensor array divided by the noise in the array. The maximum achievable signal strength by a photosensor array is determined by the full well capacity of the array. The noise in the photosensor array is the sum of dark and read noise components. Accordingly, the dynamic range of a photosensor array can be described by equation (1) below. 
   
     
       
         
           
             
               
                 
                   Dynamic 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   Range 
                 
                 = 
                 
                   
                     Full 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     well 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     capacity 
                   
                   
                     
                       Dark 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       Noise 
                     
                     + 
                     
                       Read 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       Noise 
                     
                   
                 
               
             
             
               
                 ( 
                 1 
                 ) 
               
             
           
         
       
     
   
   Thus, one way of increasing the dynamic range of a photosensor array is to increase its full well capacity. 
   Responsivity is a measure of the effectiveness of a photosensor in converting incident electromagnetic radiation into electrical current or voltage, and is inversely related to the capacitance of the photosensor. 
   There are two main sources of noise in a photosensor array, namely photon shot noise and read noise. 
   Photon shot noise results from natural fluctuations in the number of photons detected by a photosensor, and is caused by the quantum statistical nature of photon emission and detection. Thus, photon shot noise imposes a fundamental limit on the responsivity of a photosensor array insofar as it determines the minimum noise level achievable therein. 
   Photon shot noise is governed by Poisson statistics and is described by the square root of the flux (F) of radiation incident on the photosensor (e.g., 1M photogenerated electrons corresponds with a photon shot noise of √{square root over (10 6 )}, or 10 3 , and a signal to noise ratio (SNR) of 10 3 ). 
   From the above, it can be seen that increasing flux density (F) reduces the relative fraction of photon shot noise. However, as will be recalled, the full well capacity of a photosensor array provides an upper limit on the number of photons that can be integrated in its photosensors. Thus, any attempt to increase the flux density of the radiation incident on a photosensor and the detection thereof (to reduce the relative effect of photon shot noise) needs to be accompanied by an increase in the full well capacity of the photosensor array. This is typically achieved by including a large storage capacitor in each photosensor. 
   However, having a large full well capacity can present problems at low light levels. In particular, since the voltage output (V) of a photosensor is inversely related to its capacitance (C) (i.e., V=Q/C), the inclusion of a large capacitance in a photosensor reduces the output voltage swing from the photosensor. 
   Read noise is a combination of system noise components inherent in the conversion of photogenerated charge carriers into a measurable signal; processing of the resulting signal; and analog-to-digital (A/D) conversion thereof. 
   Thus, read noise determines the lower detection limit of a photosensor. Unfortunately, one of the major components of read noise is reset noise, which depends on the capacitance of the photosensor. Accordingly, any attempt to reduce the read noise of a photosensor (by reducing its full well capacity) conflicts with increasing dynamic range and reducing photon shot noise. 
   U.S. Pat. No. 5,296,698 describes a photosensing device that uses the “varicap” effect to vary the response of a (single) photodiode by changing the voltage applied across its PN junction. However, the photosensing device in U.S. Pat. No. 5,296,698 is a lateral device, which is not suitable for use in photosensor arrays. 
   Referring to  FIG. 1 , in a 3T photosensor  10 , photogenerated charge is collected by photodiode  12  and converted to a voltage (Vpd) on the gate of a sense transistor M 1 . Increasing (or decreasing) the light-sensitive area of the photosensor  10  increases (or decreases) the photogenerated charge collected therein, and the photosensor&#39;s capacitance at the same rate. Hence, the voltage swing in a 3T photosensor  10  remains constant. 
   Referring now to  FIG. 2 , a 4T photosensor  14  separates the photocharge collection and sensing areas. In particular, the 4T photosensor  14  relies on a charge transfer from the charge collection capacitor Cpd to the sensing capacitor Csn. While this approach works well for small (&lt;5 μm) photosensors, it is not as effective for larger (≧30 μm) photosensors. 
   U.S. Pat. No. 6,801,258 describes a photosensor array that is particularly suited for detection at low light levels. Under these conditions, the dominant source of noise in the photosensor array is read noise. However, averaging N signals from the array results in a √{square root over (N)} reduction of its noise. 
   In summary, prior art photosensors are characterized by having either a low capacitance or a high capacitance. Photosensors with a low capacitance typically have high sensitivity but increased photon shot noise. In contrast, photosensors with a high capacitance typically have low photon shot noise but low sensitivity. However, the lower sensitivity of such photosensors means that read noise provides a more significant contribution to the overall noise of the photosensor. Consequently, the relative noise level of the photosensor is increased. 
   SUMMARY OF THE INVENTION 
   In view of the forgoing background, an object of the invention is to provide a variably responsive photosensor. 
   This and other objects, advantages and features in accordance with the invention are provided by a photosensor comprising a plurality of photosensitive regions, with a first one of the photosensitive regions being connectable to a first potential source and signal collection means or signal collector. The photosensor may further comprise switching means or a switch adapted for switching the remaining photosensitive regions between the first potential and a second potential that is lower than the first potential, and for reversibly connecting the remaining photosensitive regions to the signal collection means so that the photosensor is variably responsive to different light levels. 
   The second potential may be ground. The switching means may be operable such that under high light levels the remaining photosensitive regions are connected to the first potential and the signal collection means so that the signal collected at the signal collection means is from all of the photosensitive regions. 
   The switching means may be operable so that under low light levels all of the remaining photosensitive regions are connected to the second potential and disconnected from the signal collection means so that the signal collected at the signal collection means is substantially from the first photosensitive region. 
   The switching means may also be operable so that at intermediate light levels between the high light levels and low light levels some of the remaining photosensitive regions are connected to the second potential and disconnected from the signal collection means so that the signal collected at the signal collection means is from the first photosensitive region and from the remaining photosensitive regions connected to the signal collection means. 
   The photosensitive regions may comprise N well and P-type epitaxial junctions. Alternatively, the photosensitive regions may comprise P well and N-type epitaxial junctions. 
   The photosensitive regions may comprise N+ and P− junctions. Alternatively, the photosensitive regions may comprise P− and N+ junctions. The switching means may comprise an NMOS transistor. Alternatively, the switching means may comprise a PMOS transistor. 
   A second aspect of the invention is directed to a method of varying the response of a photosensor in accordance with incident light levels comprising the steps of dividing the photosensing area of a photosensor into a plurality of photosensitive regions, connecting a first one of the photosensitive regions to a first potential and to signal collection means of the photosensor, and connecting the remaining photosensitive regions to the first potential and to the signal collection means in the event of the incident light being at a high light level. The method may further comprise disconnecting the remaining photosensitive regions from the signal collection means and the first potential, and connecting the remaining photosensitive regions to a second potential that is lower than the first potential in the event of the incident light being at a low light level. 
   A third aspect of the invention is directed to an optical mouse comprising data transmission means or a transmitter, movement controlling means or a controller and a variably responsive photosensor as discussed above. 
   A fourth aspect of the invention is directed to a digital camera comprising focusing means or a focuser, data processing means or data processor, and a variably responsive photosensor as discussed above. 
   A fifth aspect of the invention is directed to an x-ray sensor comprising a variably responsive photosensor as discussed above. 
   For purposes of discussion, the photosensor will be referred to a variable response photosensor. The variable response photosensor may comprise at least one switchable photosensitive site per photosensor. This allows a single photosensor to operate under either high light levels (with low photon-shot noise) or low light levels (with low readout-noise). 
   The underlying principle of the variable response photosensor is the splitting of the large photosensor into a plurality of smaller photosensitive regions. In particular, the variable response photosensor may be designed such that when operating under high light levels all the photosensitive regions collect photocharge, but under low light levels only some of the photosensitive regions collect photocharge. 
   The variable response photosensor may connect unused photosensitive regions to a lower potential (e.g., ground). This causes photogenerated electrons to be preferentially drawn to the positively charged collection node. In effect, this causes the reduction in collection area (resulting from the use of a smaller number of photosensitive regions) to be less than the coinciding reduction in photosensor capacitance. 
   In general, the variable response photosensor may increase the signal generated in a photosensor by a factor of M. In other words, the variable response photosensor may enable the SNR of the photosensor to increase by M. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a circuit diagram of a conventional three transistor (3T) photosensor according to the prior art; 
       FIG. 2  is a circuit diagram of a conventional four transistor (4T) photosensor according to the prior art. 
       FIG. 3  is a circuit diagram of a first embodiment of the variable response photosensor according to the present invention; 
       FIG. 4  is a plan view of a pixel employing a first embodiment of the variable response photosensor shown in  FIG. 3 ; 
       FIG. 5  is a cross-section through the pixel of  FIG. 4  along the line A-A′; 
       FIG. 6  is a cross-section through the pixel of  FIG. 4  along the line A-A′ (as shown in  FIG. 5 ), when exposed to high light levels; 
       FIG. 7  is a cross-section through the pixel of  FIG. 4  along the line A-A′ (as shown in  FIG. 5 ) when exposed to low light levels; 
       FIG. 8  is a plan view of a pixel comprising the first embodiment of the variable response photosensors according to the present invention; 
       FIG. 9  is a circuit diagram of a second embodiment of the variable response photosensor comprising four different photosensitive regions; and 
       FIG. 10  is a plan view (layout view) of a pixel comprising the second embodiment of the variable response photosensors of  FIG. 9 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   TABLE 1 lists the operating parameters of the second embodiment of the variable response photosensor of  FIGS. 9 and 10 . 
   
     
       
             
             
             
             
             
             
             
             
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
               Light 
                 
                 
               Active 
                 
                 
                 
                 
                 
                 
             
             
               Level 
               Capacitance 
               Sensitivity 
               Regions 
               HL1 
               LH1 
               HL2 
               LH2 
               HL3 
               LH3 
             
             
                 
             
           
           
             
               High 
               High 
               Low 
               1, 2, 3, 4 
               1 
               0 
               1 
               0 
               1 
               0 
             
             
               Medium 
               Medium 
               Medium 
               1, 3, 4 
               0 
               1 
               1 
               0 
               1 
               0 
             
             
               High 
               High 
               Low 
             
             
               Medium 
               Medium 
               Medium 
               1, 4 
               0 
               1 
               0 
               1 
               1 
               0 
             
             
               Low 
               Low 
               High 
             
             
               Low 
               Low 
               High 
               1 
               0 
               1 
               0 
               1 
               0 
               1 
             
             
                 
             
           
        
       
     
   
   The following description will provide a brief overview of the concept underlying the variable response photosensor. This will be followed by a more detailed explanation of a first embodiment of the variable response photosensor comprising only two photosensitive regions. In particular, the discussion will focus on the structure of the first embodiment of the variable response photosensor and its operation with reference to the electric fields developed therein. The detailed description will finish with a description of a second embodiment of the variable response photosensor comprising multiple photosensitive regions. 
   The following descriptions are based on an embodiment comprising a P-type semiconductor substrate with N-type semiconductor implants. However, it will be understood that this embodiment is used for exemplary purposes only and should in no way be interpreted as limiting the present invention to this embodiment. In particular, it will be understood that the present invention could be equally implemented with a photosensor comprising an N-type semiconductor substrate and P-type semiconductor implants. 
   The underlying principle of the variable response photosensor is the splitting of a large photosensor into a plurality of smaller photosensitive regions. In particular, the variable response photosensor is designed such that when operating under high light levels all the photosensitive regions collect photocharge, but under low light levels only some of the photosensitive regions collect photocharge. 
   When a small number of photosensitive regions are used to collect photocharge (i.e., under low lighting conditions), the light-collection area of the photosensor is reduced. Furthermore, as will be recalled from the background to the invention, the full well capacity of a photosensor array is dependent on the physical size of its photosensors. Accordingly, the use of a smaller number of photosensitive regions to collect photocharge also reduces the capacitance of the photosensor. In particular, the reduction in the photosensor&#39;s capacitance is approximately matched by the reduction in the number of photoelectrons collected therein. 
   The capacitance of the entire body of photosensitive regions in the variable response photosensor is denoted by C all . Further, the number of photons incident on the variable response photosensor is denoted by N e     —     photo , and the charge of an individual photogenerated electron is denoted by e. In this case, if all the photosensitive regions in the variable response photosensor are used to collect photocharges (i.e., under high light levels), the voltage swing of the variable response photosensor (ΔV pd1 ) can be described by equation (2) below. 
   
     
       
         
           
             
               
                 
                   Δ 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     V 
                     
                       pd 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       1 
                     
                   
                 
                 = 
                 
                   
                     
                       N 
                       e_photo 
                     
                     · 
                     e 
                   
                   
                     C 
                     all 
                   
                 
               
             
             
               
                 ( 
                 2 
                 ) 
               
             
           
         
       
     
   
   Similarly, if only half of the photosensitive regions in the variable response photosensor are used to collect photocharges (i.e., under low light levels) the voltage swing of the variable response photosensor (ΔV pd2 ) can be described by equation (3) below. 
   
     
       
         
           
             
               
                 
                   Δ 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     V 
                     
                       pd 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       2 
                     
                   
                 
                 = 
                 
                   
                     
                       
                         
                           N 
                           e_photo 
                         
                         · 
                         
                           1 
                           / 
                           2 
                         
                       
                       ⁢ 
                       e 
                     
                     
                       
                         C 
                         all 
                       
                       · 
                       
                         1 
                         / 
                         2 
                       
                     
                   
                   = 
                   
                     
                       
                         
                           N 
                           e_photo 
                         
                         · 
                         e 
                       
                       
                         
                           C 
                           all 
                         
                         · 
                       
                     
                     = 
                     
                       Δ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         V 
                         
                           pd 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           1 
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 3 
                 ) 
               
             
           
         
       
     
   
   Hence, the voltage swings under different lighting conditions are the same and there is no apparent benefit in reducing the number of photosensitive regions in the variable response photosensor. 
   To overcome this problem, the variable response photosensor connects unused photosensitive regions to a lower potential (e.g., ground). This causes photogenerated electrons to be preferentially drawn to the positively charged collection node. In effect, this causes the reduction in collection area (resulting from the use of a smaller number of photosensitive regions) to be less than the coinciding reduction in photosensor capacitance. 
   In other words, if Q part  and Q full  are respectively used to denote the photocharge collected by a portion of and the entirety of the variable response photosensor, and C part  and C full  are used to respectively denote the capacitance of a portion of, and the entirety of the variable response photosensor, then the connection of unused photosensitive regions in the photosensor to a lower potential results in equation (4) below. 
   
     
       
         
           
             
               
                 
                   
                     Q 
                     part 
                   
                   
                     Q 
                     full 
                   
                 
                 &gt; 
                 
                   
                     C 
                     part 
                   
                   
                     C 
                     full 
                   
                 
               
             
             
               
                 ( 
                 4 
                 ) 
               
             
           
         
       
     
   
   Referring to  FIG. 3 , a variable response photosensor  20  comprises a first photosensitive region (R 1 ) represented by the diode D R1  and its intrinsic capacitance C R1  and a second photosensitive region (R 2 ) represented by diode D R2  and capacitor C R2 . Transistor M 5  is used to separate the first and second photosensitive regions (R 1 , R 2 ) and transistor M 6  is used to ground any unwanted photosensitive regions. The sense node of the variable response photosensor  20  is the gate of transistor M 1 . 
   At high light levels, the voltage of a first line (HL) is high (e.g., 1.8V or 3.3V) which turns on transistor M 5  and the voltage of a second line (LH) is low (i.e., 0V) which turns off transistor M 6 . As transistor M 5  is turned on, the voltage of the first and second photosensitive regions (R 1 , R 2 ) are equal (V R1 =V R2 ) and the net capacitance of the sense node (M 1 ) is C R1 +C R2 . Furthermore, a photocharge is generated and collected from the first and second diodes (D R1  and D R2 ). 
   At low light levels, the voltage of the first line (HL) is low (i.e., 0V) which turns off transistor M 5 . Similarly, the voltage of the second line (LH) is high (e.g., 1.8V or 3.3V) which turns on transistor M 6 . As transistor M 5  is turned off, the capacitance of the sense node is C R1  only and any photogenerated charge is collected from diode D R1  only. 
   A first embodiment of the variable response photosensor comprising two photosensitive regions will now be discussed. In terms of structural aspects, the isolation, at low light levels, of the diode and capacitance of the second photosensitive region (i.e., D p2 /C pd2 ) from the sense node (M 1 ) by the transistor M 6  is best explained by examining an exemplary pixel employing the variable response photosensor in  FIG. 4  whose internal semiconductor structure is shown in  FIG. 5 . It will be noted that the position and size of transistors M 1  to M 6  depicted in  FIG. 4  are provided for illustration purposes only. In particular, the actual size and layout of the transistors may be altered to satisfy individual system performance requirements (e.g., pixel size, noise, speed, power consumption, etc.). 
   Referring to  FIG. 4 , let a pixel  30  comprise a first variable response photosensor (PD 1 ) and a second variable response photosensor (PD 2 ). Let the first variable response photosensor (PD 1 ) be divided into two photosensitive regions (PD 1a  and PD 1b ). Similarly let the second variable response photosensor (PD 2 ) be divided into two photosensitive regions (PD 2a  and PD 2b ). It will be understood that each of photosensitive regions PD 1a  and PD 2a  correspond to photosensitive region R 1  in  FIG. 3 . Similarly, it will be understood that each of photosensitive regions PD 1b  and PD 2b  correspond with photosensitive region R 2  in  FIG. 3 . Accordingly, each of the photosensitive regions is arranged so that no R 2 -type photosensitive region is adjacent to another and so that no R 1 -type photosensitive region is adjacent to another. 
   To simplify the figures, the (typically metal) connections between the regions (PD 1a , PD 1b , PD 2a  and PD 2b ) and the connections (typically metal) between the variable response photosensors (PD 1 , PD 2 ) are not shown. The number of photosensitive regions in either the first or second variable response photosensors (PD 1 , PD 2 ) can be greater than or equal to two. In fact, as will be shown below, it is advantageous to have a large number of photosensitive regions in a given variable response photosensor. 
   Referring to  FIG. 5 , the photosensitive regions (R 1  and R 2 ) PD 1a  and PD 2b  are formed by N− (N Well) P− epitaxial junctions. N Well structures are preferred as they are deeper and collect more charge. Furthermore, the lower doping profile of N Well structures reduces dark current. However, N Well structures are typically less precisely implanted than other structures. In view of the practical problems with implanting N Well structures, such structures need to be manufactured larger. Consequently, if the variable response photosensor is to be used in a smaller pixel (e.g., &lt;12 μm), it may be preferable to use N+/P− epitaxial diodes. 
   Returning to  FIG. 5 , N+ implants form the transistors (M 1 -M 6 ). These structures are implanted with much greater accuracy than the afore-mentioned N Well structures. Consequently, the N+ implants (i.e., of the transistors) can be made smaller than the N Well structures of the photosensitive regions (R 1  and R 2 ) PD 1a  and PD 2b . 
   Preferably, epitaxial material  32  is deposited on a (P++) substrate  34 . However, the epitaxial layer  32  is not essential. In particular, in the absence of the epitaxial layer  32 , the photosensitive regions (R 1  and R 2 ) PD 1a  and PD 2b  could be formed from an N+/P− junction.  FIG. 5  shows P− between photosensitive regions (R 1  and R 2 ) PD 1a  and PD 2b . This is a common implementation, but if epitaxial material  32  is available, it is preferred over the P-material as the lower doping of the epitaxial material  32  extends the electric field in the photosensitive regions and increases the collection area therefore. 
   The transistor M 6  is shown to the right of the photosensitive regions (R 1  and R 2 ) PD 1a  and PD 2b .  FIG. 5  shows the polysilicon gate of the transistor M 6  for illustration purposes only. Whether or not this would appear in a given cross-section of a pixel would depend on the precise layout of the pixel. 
   P+ implants provide the electrical ground for the pixel. Typically, there will be a small implant in each pixel. For efficient operation, the distance between the P+ implants should not be greater than 100 μm (this distance is determined by the diffusion constant of silicon). For pixels that are larger than this, it is important to have a ground connection close to the source of M 6 . This ensures that M 6  is fully turned on (signal LH is set high) during low light levels and that the ground potential is distributed to the PD 2  nodes. 
   Having discussed the underlying structure of a photosensor array comprising the variable response photosensor, it is useful to discuss the operation of the variable response photosensor and the development of the electric fields therein. The variable response photosensor has two modes of operation: high light level operations and low light level operations. Both of these operational modes will be described in more detail below. 
   With high light levels a high capacitance is required to maximize the number of electrons that can be stored (i.e., full well capacity) and to reduce photon shot noise. Referring to  FIG. 6  together with  FIG. 3 , as the voltage on the HL line is set high, transistor M 5  is fully turned on and the two photosensitive regions (R 1  and R 2 ) PD 1a  and PD 2b  are held at similar potentials. As a result, both photosensitive regions (R 1  and R 2 ) PD 1a  and PD 2b  are reversed biased and two depletion regions α and β (associated with photosensitive regions PD 1a  and PD 2b  respectively) are produced which extend into the epitaxial layer  32 . 
   Charge carriers generated in the photosensitive regions (R 1  and R 2 ) PD 1a  and PD 2b  by the incident light randomly move (by diffusion) through the silicon of the variable response photosensor  30  until they reach a depletion region. At this point, the electrical fields associated with the depletion region specifically draw and accelerate the movement of the charge carriers across the depletion region. This results in a photocurrent that is collected and transmitted to external circuitry. 
     FIG. 6  shows the depletion region α of PD 1a  touching the depletion region β of PD 2b . This represents an ideal situation, which ensures that photogenerated charges in both photosensitive regions are collected. Unfortunately, depending on the manufacturing of a pixel (e.g., design rules which increase the separation of the wells, high doping levels that reduce the electric fields associated with a depletion region, or insufficient voltage across the depletion region) it may not be possible to achieve such proximity between the depletion regions α and β. 
   Referring to  FIG. 7 , under low light levels there is not enough light to saturate the variable response photosensor  30 . Nonetheless, the objective remains to produce the maximum possible voltage swing from the variable response photosensor  30 . As will be recalled from the earlier overview, it is not sufficient to reduce the photosensor&#39;s capacitance (by effectively reducing its photosensitive area) because the number of photons (and hence photogenerated electrons) collected is reduced proportionately. Instead, as will be recalled, the variable response photosensor overcomes this problem by connecting unused photosensitive regions to a lower (e.g., ground) potential. For example, referring to  FIG. 7 , photosensitive region PD 2b  is connected to a lower potential than PD 1a  and only one depletion region is formed in the variable response photosensor  30  (depletion region α around PD 1a ). 
   To better explain the low light operation of the variable response photosensor  30  it is useful to examine the paths of three photogenerated electrons (PE 1 , PE 2  and PE 3 ) as shown in  FIG. 7 . PE 3  is quite distant from depletion region α and is unlikely to be influenced by the electrical fields associated with α. Thus, PE 3  will diffuse in random fashion through the silicon of the photosensor  30  and will probably end up at the ground node (P+ implant) of the photosensor  30  whereupon PE 3  will be lost. 
   PE 1  is proximal to the depletion region α. Thus, the electric field associated with α will accelerate PE 1  towards the positive potential of PD 1a , from which PE 1  will be collected. PE 2  is not quite as close to the depletion region α as PE 1 , but is not as distant therefrom as PE 3 . Consequently, PE 2  will diffuse in the silicon of the photosensor  30  and will probably reach the depletion region α whereupon it will be accelerated towards PD 1   a.    
   From this it can be seen that the collection area of PD 1a  is, in effect, greater than its implanted (i.e., electrical/capacitive) area. This gain can be improved by increasing the number of PE 2  type photoelectrons collected by minimizing the distance such photoelectrons have to diffuse in the silicon of the variable response photosensor  30 . In particular, the gain in the collection area of the variable response photosensor  30  can be improved by making its collection areas (photosensitive regions) as small as possible and increasing their number, as shown in  FIG. 8 . The minimum size of a collection area is usually limited by design rule limitations on well size. 
   A downside of the variable response photosensor is that while its large number of small photosensitive regions has a similar area to a single larger photosensor, the set of small photosensitive regions have a larger side-wall capacitance than the single larger photosensor. Nonetheless, although there is an increase in the capacitance and dark-current in the variable response photosensor (compared with a conventional photosensor), the above-mentioned gain in sensitivity of the variable response photosensor more than compensates for this. 
   A second embodiment of the variable response photosensor comprising multiple photosensitive regions will now be discussed. The previous discussions have related to a variable response photosensor employing two types of photosensitive regions (R 1  and R 2 ) for use under two different lighting conditions. However, it is possible to extend the variable response photosensor, for example, to work with even lower light levels. 
     FIG. 9  shows a further embodiment of the variable response photosensor  40 , comprising four different photosensitive regions R 1 , R 2 , R 3  and R 4 . Referring to  FIG. 9  and comparing it with the circuit diagram of  FIG. 3  it can be seen that photosensitive region R 1 , formed by D R1  and C R1 , is permanently connected to the sense transistor M 1  and the other photosensitive regions R 2 , R 3  and R 4  are switchably connected between a lower (e.g., ground) potential and the sense transistor M 1 . 
   Referring to  FIG. 10 , photosensitive region R 1  is disposed adjacent to photosensitive regions R 2 , R 3  and R 4  (albeit diagonally). Hence, a photogenerated electron has a maximum distance of half of the photosensitive region (R 1 , R 2 , R 3  or R 4 ) length/width to diffuse before it would encounter a collection node. 
   The operation of the multiple photosensitive region embodiment of the variable response photosensor  40  is illustrated in TABLE 1. In particular, if the variable response photosensor  40  is operating under bright lighting conditions (high light level) all of the photosensitive regions are connected to the sense transistor M 1 . As the light level falls (medium high light level) only three quarters of the photosensitive regions (e.g., R 1 , R 3  and R 4 ) are connected to the sense transistor M 1 . In the present example, it is irrelevant if photosensitive region R 2  or R 3  is used. However, photosensitive region R 4  is preferred as it minimizes or reduces the diffusion distance for photogenerated electrons. 
   As the light level falls further (medium to low light level) then only half of the photosensitive regions (R 1  and R 4 ) are connected to the sense transistor M 1  (photosensitive region R 4  is preferred for the above-stated reasons). At the lowest light level, (low light level) only a quarter of the photosensitive regions (photosensitive region R 1 ) are connected to the sense transistor M 1 . The rest of the photosensitive regions are grounded. In this case, photogenerated carriers diffuse to photosensitive region R 1  and are collected and detected therein. 
   The above-described variably responsive photosensor is applicable to a broad range of devices including digital cameras, optical mice and X-ray sensors. It will be appreciated that those skilled in the art may use standard techniques to implement the invention in these and other ways. Alterations and modifications may be made to the above without departing from the scope of the invention.