High Sensitivity and High Dynamic-Range CMOS Image Sensor Pixel Structure with Dynamic C-V Characteristics

Abstract

A new photogate pixel structure for high performance CMOS Image Sensors is proposed. A new photogate structure is incorporated into the photodiode active-pixel structure. The proposed pixel structure exhibits the dynamic integration capacitance characteristics, which can be controlled by varying the control-voltage at the photogate node. Since the sensitivity is inversely proportional to the integration capacitance, the dynamic integration capacitance characteristics can provide the new functionality and controllability for high sensitivity and high dynamic range. At a low voltage level of the photogate, the pixel sensitivity of the new photogate pixel structure is maximized due to the minimum value of the integration capacitance. At a high voltage of the photogate, the dynamic range of the new structure can be maximized due to the increased well capacity. In addition, at an optimum bias voltage of the photogate, both the dynamic-range and the sensitivity can be simultaneously improved. Consequently, the new pixel structure allows performance tunability as well as optimization in both the dynamic range and the sensitivity of the image sensor cell.

1 . A active pixel structure based on the photodiode- 2  ( 206 ) of the photogate ( 314 ) combining the 3-T photodiode- 1  ( 205 ) pixel structure (see  FIG. 2(   a ) and  FIG. 3) ,
 using the dynamic integration capacitance (C int ) Characteristics with a transition voltage level (V PG -V TH ) (see  FIG. 2(   b )),   which can be controlled by varying the photogate bias voltage (V PG ) at the photogate node ( 207 ,  313 ),   for high sensitivity (high conversion-gain) due to the small integration capacitance (C PD ) of the photodiode- 1  ( 205 ) and high dynamic-range (high well-capacity due to low conversion-gain) due to the large integration capacitance (C PD +C PG ) of the combination of the photodiode- 1  ( 206 ) (see  FIG. 2(   b )).   
   
   
       2 . A active pixel structure with the photogate ( 314 ) using the dynamic integration capacitance (C int ) Characteristics with a transition voltage level (V PG -V TH ) (see  FIG. 2(   b )), as claimed in  claim 1 , which has high sensitivity at low light intensity due to the minimum value of C int  at above the transition voltage (V PG -V TH ) (see  FIG. 2(   b )), can be decreased by a low voltage level of the photogate (example: V PG =0V). 
   
   
       3 . A active pixel structure with the photogate ( 314 ) using the dynamic integration capacitance (C int ) Characteristics with a transition voltage level (V PG -V TH ) (see  FIG. 2(   b )), as claimed in  claim 1 , which has high dynamic-range due to the increased well capacity, can be maximized by the maximum value of C int  (=C PD +C PG ) at below the transition voltage (V PG -V TH ) (see  FIG. 2(   b )), can be increased by a high voltage level of the photogate (example: V PG =3.3V). 
   
   
       4 . A active pixel structure with the photogate ( 314 ) using the dynamic integration capacitance (C int ) Characteristics with a transition voltage level (V PG -V TH ) (see  FIG. 2(   b )), as claimed in  claim 1 , at an optimum bias condition of the proposed photo-gate (example: V PG =2.75V), which has both high dynamic range and high sensitivity at low light intensity (example: 10 lux) due to the dynamic integration capacitance (C int ) Characteristics. 
   
   
       5 . A active pixel structure with the photogate ( 314 ) using the dynamic integration capacitance (C int ) Characteristics with a transition voltage level (V PG -V TH ) (see  FIG. 2(   b )), as claimed in  claim 1 , which can improve the sensitivity at low light intensity or the dynamic-range by the dynamic integration capacitance (C int ) Characteristics, as the incident light intensity, which can be controlled by varying the photogate bias voltage (V PG ) electrically, that can be automatically controlled by the negative feedback circuitry. 
   
   
       6 . A active pixel structure with the photogate ( 314 ) using the dynamic integration capacitance (C int ) Characteristics with a transition voltage level (V PG -V TH ) (see  FIG. 2(   b )), as claimed in  claim 1 , which has the linear output voltage waveform (see  FIG. 9)  at low light intensity due to the small integration capacitance (C int ) above the transition voltage level (V PG -V TH ) (see  FIG. 8 ), which can lead to the high sensitivity compared to the conventional pixel. 
   
   
       7 . A active pixel structure with the photogate ( 314 ) using the dynamic integration capacitance (C int ) Characteristics with a transition voltage level (V PG -V TH ) (see  FIG. 2(   b )), as claimed in  claim 1 , which has the non-linear output voltage waveform (see  FIG. 10)  at high light intensity due to the dynamic integration capacitance (C int ) (see  FIG. 8 ),
 which can lead to the high dynamic-range compared to the conventional pixel.