Cmos active pixel with very high functional dynamics

Abstract

The invention relates to a structure of an active pixel of the CMOS type ( 1 ) that comprises: at least one photodiode ( 10 ), characterised in that it comprises means for reading any bias voltage in the evolution phase of the photodiode ( 10 ) upon exposure.

1 . A structure of an active pixel of the CMOS type ( 1 ) comprising:
 at least one photodiode ( 10 ),   means for reading a voltage with any polarity in the time-dependent variation phase of the photodiode ( 10 ) during exposure, including a buffer amplifier ( 30 ) including a read-out transistor (T 2 ) and a charging transistor (T 3 ),   characterized in that it further includes a capacitive coupling capacitor ( 50 ) mounted in series between the photodiode ( 10 ) and the buffer amplifier ( 30 ).   
     
     
         2 . The structure of an active pixel of the CMOS type ( 1 ) according to  claim 1 , characterized in that the read-out transistor (T 2 ) has negative threshold voltage. 
     
     
         3 . The structure of an active pixel of the CMOS type ( 1 ) according to  claim 1 , characterized in that the charging transistor (T 3 ) is located inside the structure of the pixel ( 1 ). 
     
     
         4 . The structure of an active pixel of the CMOS type ( 1 ) according to  claim 1 , characterized in that the charging transistor (T 3 ) is located outside the structure of the pixel ( 1 ). 
     
     
         5 . The structure of an active pixel of the CMOS type ( 1 ) according to any of  claims 1  to  2 , characterized in that a threshold voltage of the read-out transistor (T 2 ) is less than a voltage, the most negative voltage, capable of passing through the photodiode ( 10 ). 
     
     
         6 . The structure of an active pixel of the CMOS type ( 1 ) according to  claim 1 , characterized in that it includes a means with which an output signal from the buffer amplifier ( 30 ) may be sent onto a communications bus ( 40 ). 
     
     
         7 . The structure of an active pixel of the CMOS type ( 1 ) according to  claim 6 , characterized in that the means for sending the output signal of the buffer amplifier ( 30 ) onto a communications bus ( 40 ) is a transistor (T 4 ) connected to a selection signal (SEL). 
     
     
         8 . The structure of an active pixel of the CMOS type ( 1 ) according to any of  claims 1  to  7 , characterized in that it includes means for setting up a voltage on the coupling capacitor ( 50 ) within the pixel ( 1 ). 
     
     
         9 . The structure of an active pixel of the CMOS type ( 1 ) according to  claim 8 , characterized in that the setting-up means are resistors ( 60 ) mounted between the buffer amplifier ( 30 ) and a positive voltage source ( 70 ). 
     
     
         10 . The structure of an active pixel of the CMOS type ( 1 ) according to  claim 9 , characterized in that the resistor ( 60 ) consists of an NMOS transistor (TR) connected as a diode. 
     
     
         11 . The structure of an active pixel of the CMOS type ( 1 ) according to  claim 9 , characterized in that the drain and the gate of the transistor (TR) are connected to two different power supply sources ( 70 ,  72 ). 
     
     
         12 . The structure of an active pixel of the CMOS type ( 1 ) according to any of  claims 8  to  11 , characterized in that the setting-up means are an injection of a positive charge under ultraviolet radiation into the plate ( 51 ) of the coupling capacitor ( 50 ) connected to a buffer amplifier ( 30 ). 
     
     
         13 . The structure of an active pixel of the CMOS type ( 1 ) according to any of  claims 8  to  12 , characterized in that the setting-up means are an injection of a positive charge by the tunnel effect into the plate ( 51 ) of the coupling capacitor ( 50 ) connected to the buffer amplifier ( 30 ). 
     
     
         14 . The structure of an active pixel of the CMOS type ( 1 ) according to  claim 13 , characterized in that the photodiode ( 10 ) is used like one of the plates ( 51 ,  53 ) of the coupling capacitor ( 50 ). 
     
     
         15 . The structure of an active pixel of the CMOS type ( 1 ) according to any of  claims 1  to  14 , characterized in that it includes a reset transistor (T 1 ) capable of being disabled by application of a negative voltage. 
     
     
         16 . The structure of an active pixel of the CMOS type ( 1 ) according to any of  claims 1  to  15 , characterized in that the transistors (T 1 , T 2 , T 3 , T 4 , TR) are of the same type, either NMOS or PMOS, and in that the read-out transistor (T 2 ) is a depleted transistor. 
     
     
         17 . A method for operating a pixel ( 1 ) according to any of  claims 1  to  16 , characterized by the fact that it comprises the steps of:
 1) resetting a photodiode ( 10 ), before taking shots, to an initial voltage V, with a signal RAZ enabling the reset transistor (T 1 ) 
 2) disabling the reset transistor (T 1 ) and letting the photodiode ( 10 ) vary under illumination during the exposure; 
 3) performing a first read-out at the end of the exposure with a selection signal SEL, and 
 4) performing a second read-out during or suddenly after re-enabling the reset transistor (T 1 ).