Abstract:
An imaging circuit, an imaging sensor, and a method of imaging. The imaging cell circuit including one or more imaging cell circuits, each imaging cell circuit comprising: a transistor having a floating body for holding charge generated in the floating body in response to exposure of the floating body to electromagnetic radiation; means for biasing the transistor wherein an output of the transistor is responsive to the electromagnetic radiation; and means for selectively connecting the floating body to a reset voltage supply.

Description:
FIELD OF THE INVENTION 
   The present invention relates to the field of solid state imagers; more specifically, it relates to a body potential imaging cell and array. 
   BACKGROUND OF THE INVENTION 
   Imaging sensors are well known in the art. Imaging sensors are used in cameras and other detectors to provide an image of a scene or object for display or output purposes. Imaging sensors are operative over various sections of the electromagnetic spectrum and at various intensity levels. Imaging sensors operate by converting electromagnetic energy to charge. The charge is collected and is sensed by an output device. Typically, the charge is collected on a capacitor and the voltage of the capacitor is sensed. Devices using these imaging arrays have relatively slow shutter speeds, low contrast and high power consumption. 
   Therefore, there is a need for an imaging cell with improved sensitivity and performance. 
   SUMMARY OF THE INVENTION 
   The present invention describes an imaging cell which uses a floating body of a transistor for charge collection. Upon exposure to incident electromagnetic radiation the body of the collection transistor charges up, the threshold voltage of the collection transistor decreases. The collection transistor is then used in a circuit whose output is dependent upon the threshold voltage of the collection transistor. A reset transistor is provided to discharge the floating body between exposures. 
   A first aspect of the present invention is an imaging circuit, comprising one or more imaging cell circuits, each imaging cell circuit comprising: a transistor having a floating body for holding charge generated in the floating body in response to exposure of the floating body to electromagnetic radiation; means for biasing the transistor wherein an output of the transistor is responsive to the electromagnetic radiation; and means for selectively connecting the floating body to a reset voltage supply. 
   A second aspect of the present invention is an imaging sensor, comprising: a first transistor having a source region, a drain region, a channel region and a gate; a second transistor having a floating body for holding charge generated in response to exposure of the floating body to electromagnetic radiation, a source region, a drain region and a gate; and the source, first channel and first drain regions of the first transistor connected in series between a reset voltage supply and the floating body of the second transistor. 
   A third aspect of the present invention is a method of imaging, comprising: forming a transistor having a floating body for holding charge generated in response to exposure of the floating body to electromagnetic radiation; biasing the transistor wherein an output of the transistor is responsive to the electromagnetic radiation; and selectively connecting the floating body to a reset voltage supply to store or discharge charge stored on the floating body. 
   A fourth aspect of the present invention is a method of forming an imaging sensor, comprising: forming a first transistor having a source region, a drain region, a channel region and a gate; forming a second transistor having a floating body for holding charge generated in response to exposure of the floating body to electromagnetic radiation, a source region, a drain region and a gate; and connecting the source, first channel and first drain regions of the first transistor in series between a reset voltage supply and the floating body of the second transistor. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     The features of the invention are set forth in the appended claims. The invention itself, however, will be best understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a circuit diagram of a first imaging cell circuit according to the present invention; 
       FIG. 2  is a circuit diagram of a second imaging cell circuit according to the present invention; 
       FIG. 3  is a circuit diagram of a third imaging cell circuit according to the present invention; 
       FIG. 4A  is a plan view and  FIG. 4B  is a cross-sectional view through line  4 B- 4 B of a first embodiment of the detector section of an imaging cell according to the present invention; 
       FIG. 5A  is a plan view and  FIG. 5B  is a cross-sectional view through line  5 B- 5 B of a second embodiment of the detector section of an imaging cell according to the present invention; 
       FIG. 6A  is a plan view and  FIGS. 6B and 6C  are a cross-sectional views through lines  6 B- 6 B and  6 C- 6 C respectively of  FIG. 6A  of a third embodiment of the detector section of an imaging cell according to the present invention; 
       FIG. 7A  is a plan view and  FIGS. 7B and 6C  are a cross-sectional views through lines  7 B- 7 B and  6 C- 6 C respectively of  FIG. 7A  of a third embodiment of the detector section of an imaging cell according to the present invention; and 
       FIG. 8  is an exemplary imaging array utilizing imaging cells of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  is a circuit diagram of a first imaging cell circuit according to the present invention. In  FIG. 1 , an imaging cell circuit  100  includes a detector section  105  and a read out section  110 . Detector section  105  includes a reset PFET T 1  and a collection NFET T 2 . Read out section  110  includes NFETs T 3  and T 4 . The drain of PFET T 1  is connected to a VRES pin. The VRES pin is connected to a reset voltage supply. The reset voltage level may be ground or some other starting bias voltage for the well. The gate of PFET T 1  is connected to a RESET pin, and the source of PFET T 1  is connected to the body of NFET T 2 . The drain of NFET T 2  is connected to a V 1  pin, the gate of NFET T 2  is connected to a VGATE pin and the source of NFET T 2  is connected to the gate of NFET T 3 . NFET T 3  is a source follower. The drain of NFET T 3  is connected to a voltage source VDD and the source of NFET T 3  is connected to the drain of NFET T 4 . The gate of NFET T 4  is connected to a ROW SELECT pin and the source of NFET T 4  is connected to a DATA OUT pin. 
   There are three phases of operation of imaging cell circuit  100 . In a first, or reset phase, a voltage level on pin V 1  is set to ground, a voltage on the RESET pin is set to ground, turning on PFET T 1  and pulling the body of NFET T 2  to VRES, a voltage on the VGATE pin is set to VDD and a voltage on the ROW SELECT pin is set to ground, turning off NFET T 4 . Unless otherwise specified, ground indicates a voltage of zero volts. NFET T 3  is off with the voltage applied to the gate of T 3  being about 0 volts. 
   In a second, or incident radiation gathering phase, the voltage on the RESET pin is set to VDD turning off PFET T 1  and isolating the body of NFET T 2 . The voltage on pin V 1  remains at ground, the voltage on the VGATE pin is set to about 0 to about VDD/2 with NFET T 2  lightly off and the voltage on the ROW SELECT pin remains at ground. With no incident radiation impinging on NFET T 2 , the body of NFET T 2  remains at reset voltage supply. With incident radiation impinging on the body of NFET T 2 , charge is collected in the body of NFET T 2  which changes the voltage potential between the body and gate of NFET T 2 . This causes the threshold voltage VT of NFET T 2  to change. An example of incident electromagnetic radiation is light and particularly visible light. The more negative the body voltage on NFET T 2 , the higher the VT of NFET T 2 . After a predetermined period of time (the “shutter” time), the third phase is commenced. 
   In the third, or read phase, the voltage on the RESET pin remains at VDD, the voltage on the V 1  pin is changed to a voltage between VDD and VGATE-Vtmax of transistor T 2  (selected to maximize the effect of body voltage on the V T  of NFET T 2 ), the voltage level on the VGATE pin is set to VDD (or a lower voltage such that VGATE-Vtmax of transistor T 2  is greater than the Vt of transistor T 3 ) and the voltage on the ROW SELECT pin is set to VDD, turning on NFET T 4 . The transfer characteristic between NFET T 2  and NFET T 3  is the ratio of the swing between ground and VDD through NFET T 3  and the change in V T  generated by incident radiation collection in NFET T 2 . The output signal on a DATA OUT pin is a voltage which is dependent upon the amount of radiation which was converted to charge and collected in the body of NFET T 2 . This voltage can be converted to a digital signal by an analog to digital converter. 
   In imaging cell circuit  100 , the physical connection between the drain of PFET T 1  and the body of NFET T 2  is a direct silicon to silicon connection as described infra. In an alternative circuit, PFET T 1  is replaced with an NFET, in which case the polarity of the voltages applied to the RESET pin are reversed and the physical connection between the drain of now NFET T 1  and the body of NFET T 2  is a silicon to wire to silicon connection. 
     FIG. 2  is a circuit diagram of a second imaging cell circuit according to the present invention. In  FIG. 2 , an imaging cell circuit  115  is similar to imaging cell circuit  100  of  FIG. 1  except that a read out stage  120  includes an NFET T 5  acting as a resistor between VDD and source of NFET T 2 , the gate and drain of NFET T 5  connected to VDD and the source of NFET T 5  connected to the source of NFET T 2 . Operation of imaging cell circuit is the same as for imaging cell circuit  100  of  FIG. 1  with the exception that the drain of NFET T 2  is always at ground. 
     FIG. 3  is a circuit diagram of a third imaging cell circuit according to the present invention In addition, the body potential imager can be formulated in a  2  transistor form factor. In  FIG. 3 , and imaging cell  122  includes PFET T 1  and NFET T 2 . The gate of PFET T 1  is connected to the RESET pin, the drain of PFET T 1  is connected to the VRES pin and the source of PFET T 1  is connected the body of NFET T 2 . The gate of NFET T 2  is connected to the ROW SELECT pin, the drain of NFET T 2  is connected to VDD, the source of NFET T 2  is connected to the DATA OUT pin. 
   There are three phases of operation of imaging cell  122 . In a first or reset phase, NFET T 1  is turned on setting by a voltage level on the RESET pin to ground and, a voltage on the ROW SELECT pin is set to ground. Then NFET T 2  is turned off. 
   In a second or incident radiation gathering phase, with incident radiation impinging on NFET T 2  charge accumulates in the body of NFET T 2 . PFET T 1  is turned off and NFET T 2  may be turned off or turned on if it is desirable to poll the value of the charged body of NFET T 2  during illumination. 
   In a third or read phase, NFET T 2  is turned on and the body of NFET T 2  will source a current on the DATA OUT pin which is dependent upon the body bias induced on the gate of NFET T 2  by the quantity of stored carriers in the body of NFET T 2 . This may then be amplified or directly converted to a voltage as application needs dictate. 
     FIG. 4A  is a plan view and  FIG. 4B  is a cross-sectional view through line  4 B- 4 B of a first embodiment of the detector section of an imaging cell according to the present invention. In the first embodiment of the present invention, imaging cell circuit  100  of  FIG. 1 ,  115  of  FIG. 2  or  122  of  FIG. 3  is fabricated in a silicon-on-insulator (SOI) substrate. In  FIGS. 4A and 4B , PFET T 1  includes a P type drain region  125 , a P type source region  130 , an N type channel region  135  between the source and drain regions, and a gate  140  formed over the channel region. Drain region  125 , source region  130  and channel region  135  are formed in an N-well  150 . Spacers  145  are formed on the source/drain sides of gate  140 . A gate dielectric  155  is formed under gate  140 . 
   NFET T 2  includes, a N type drain region  160 , an N type source region  165 , a P type channel region  170  between the source and drain regions, and a gate  175  formed over the channel region. Drain region  160 , source region  165  and channel region  170  are formed in a P-well  180 . P-well  180  is the body of NFET T 2 . Spacers  185  are formed on the source/drain sides of gate  175 . Gate dielectric  155  is formed under gate  175 . 
   Shallow trench isolation (STI)  190  and a buried oxide layer (BOX)  195  which physically contacts the STI form the isolation for the elements of the pixel. Source region  130  (or an extension of source region  130 ) physically contacts P-well  180 . BOX  195  is formed on a silicon substrate  200 . Thus body of NFET T 2  is electrically isolated except for a connection to source region  130  and floats when PFET T 1  is turned off. Incident radiation striking P-well  180  through gate  175  creates electron/hole charge pairs in P-well  180 . Gate  175  needs to be relatively transparent to incident radiation. In one example gate  175  is polysilicon about 500 Å to about 1500 Å thick. 
   It can be readily seen, that if PFET T 1  is replaced with an NFET, P type source region  130  becomes an N type drain region and that the N type drain region and P-well  180  cannot be physically connected as a PN diode would result. Thus a connection using, for example, a metal wire, needs to be made between the N type drain region and P-well  180 . 
     FIG. 5A  is a plan view and  FIG. 5B  is a cross-sectional view through line  5 B- 5 B of a second embodiment of the detector section of an imaging cell according to the present invention. In the second embodiment of the present invention, imaging cell circuit  100  of  FIG. 1 ,  115  of  FIG. 2  or  122  of  FIG. 3  is fabricated in a bulk silicon substrate. The only differences between  FIGS. 5A and 5B  and  FIGS. 4A and 4B , is BOX  195  is not present in  FIGS. 5A and 5B , N-well  150  and P-well  180  extend under STI  190 , substrate  200  is N-type and P-well  180  may be doped to a different concentration than an optional P-type region  205  that may be formed between drain  160  and source  165 . In one example, P-well  180  may be more heavily doped P-type in order to provide robust electrical isolation and P-type region  205  more lightly P-doped than P-well  180 . 
     FIG. 6A  is a plan view and  FIGS. 6B and 6C  are a cross-sectional views through lines  6 B- 6 B and  6 C- 6 C respectively of  FIG. 6A  of a third embodiment of the detector section of an imaging cell according to the present invention. In the third embodiment of the present invention, imaging cell circuit  100  of  FIG. 1 ,  115  of  FIG. 2  or of  122  of  FIG. 3  is fabricated in a silicon-on-insulator (SOI) substrate. In  FIGS. 6A ,  6 B and  6 C, PFET T 1  includes, P type drain region  125 , P type source region  130 , N type channel region  135  between the source and drain regions, and gate  140  formed over the channel region. Drain region  125 , source region  130  and channel region  135  are formed in an N-well (not shown). Spacers  145  are formed on the source/drain sides of gate  140 . A gate dielectric (not shown) is formed under gate  140 . 
   NFET T 2  includes, a N type drain region  210 , an N type source region  215 , a P type channel region  220  between the source and drain regions, and a gate  225  formed over the channel region. Drain region  210 , source region  215  and channel region  220  are formed in a P-well  230 . P-well  230  is the body of NFET T 2 . Spacers  235  are formed on the source/drain sides of gate  225 . A gate dielectric  240  is formed under gate  225 . A P-well extension  245  is contiguous and integral with P-well  230 . While gate  225  is illustrated as overlapping P-well extension  245 , gate  225  may be aligned so as not to overlap the P-well extension. P-well extension  245  acts as an extension of the body of NFET T 2 . 
   P-well  230  and P-well extension  245  are bounded by STI  190  and a BOX  195  which physically contacts the STI. Source region  130  physically contacts P-well extension  245 . Thus the body of NFET T 2  is electrically isolated except for a connection to source region  130  and floats when PFET T 1  is turned off. Incident radiation striking P-well  230  through gate  225  and striking P-well extension  245  creates electron/hole charge pairs in P-well  230  and P-well extension  245 . Most of the charge pairs are generated in P-well extension  245  because the P-well extension is generally many times larger in area than P-well  230 . In one example, the area of P-well extension  245  is about 10 times or greater than the area of P-well  230 . 
   Optionally, source  210  and/or drain  215  may extend into P-well extension  245  in order to reduce the dark current and speedup the collection of electrons and holes. 
   While a physical connection through silicon between P-well  230  and P-well extension  245  is illustrated in  FIGS. 6A ,  6 B and  6 C, the P-well extension may be physically isolated from but electrically connected to the P-well of NFET T 2  by, for example, a metal wire. 
   As described supra, PFET T 1  can be replaced with an NFET. 
     FIG. 7A  is a plan view and  FIGS. 7B and 6C  are a cross-sectional views through lines  7 B- 7 B and  7 C- 7 C respectively of  FIG. 7A  of a third embodiment of the detector section of an imaging cell according to the present invention. In the fourth embodiment of the present invention, imaging cell circuit  100  of  FIG. 1 ,  115  of  FIG. 2  or  122  of  FIG. 3  is fabricated in a bulk silicon substrate. The only differences between  FIGS. 7A ,  7 B and  7 C and  FIGS. 6A and 6B  is BOX  195  is not present in  FIGS. 7A ,  7 B and  7 C, P-well  230  and P-well extension  245  extend under STI  190 , substrate  200  is N-type and P-well extension  245  or both P-well extension  245  and P-well  230  may be doped to a different concentration than an optional P-type region  250  formed between drain  210  and source  230  in P-well  230  and formed in P-well extension  245 . In one example, P-well  230  and P-well extension  245  may be more heavily doped P-type in order to provide robust electrical isolation and doped P-type region  250  more lightly P-doped than the P-well or P-well extension. 
     FIG. 8  is an exemplary imaging array utilizing imaging cells of the present invention. In  FIG. 8 , an imaging array  260  includes an array of pixels  265  arranged in rows and columns. Pixels  265  comprise either imaging cell circuits  100  (see  FIG. 1 ) or  115  (see  FIG. 2 ). Pixels  265  comprise one of the embodiments of the present invention as illustrated in  FIGS. 4A and 4B ,  5 A and  5 B,  6 A,  6 B and  6 C, or  7 A,  7 B and  7 C as described infra. There are N+1 rows 0 through N and M+1 columns 0 through M columns. A RESET X, ROW X SELECT and VGATE X signal bus is coupled to each pixel in row X (where X is a whole positive integer between 0 and N). RESET X, ROW X SELECT and VGATE X signal buses are generally driven from a row decoder circuit (not shown). A VDD, VRES and V 1  or GND power source is coupled to each pixel  265 . A COL Y DATA OUT signal bus is coupled to each pixel in column Y (where Y is a whole positive integer between 0 and M). Generally each COL Y DATA OUT signal bus is connected to a current amplified analog to digital converter. 
   Thus, the present invention provides an imaging cell with improved sensitivity and performance. 
   The description of the embodiments of the present invention is given above for the understanding of the present invention. It will be understood that the invention is not limited to the particular embodiments described herein, but is capable of various modifications, rearrangements and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. For example, the semiconductor material silicon, may be replaced with other semiconductor materials. Therefore, it is intended that the following claims cover all such modifications and changes as fall within the true spirit and scope of the invention.