Abstract:
A method and apparatus of operating a dual gate TFT electromagnetic radiation imaging device wherein the electrical conditions on each pixel are compared after exposure to radiation and during measurement. The pixel charge electrode is preset to a predetermined voltage level prior to radiation exposure so that the pixel may be operated beyond its linear operating range.

Description:
FIELD OF THE INVENTION 
     This invention relates in general to electromagnetic radiation imaging devices, and more particularly to a method and apparatus for operating an X-ray imaging device beyond the linear range of each pixel sensor. 
     BACKGROUND OF THE INVENTION 
     The use of two-dimensional arrays of thin film transistors for radiation detection is known in the art. One prior art X-ray imaging detector has been developed at the University of Michigan, as described in L. E. Antonuk, J. Boudry, W. Huang, D. L. McShan, E. J. Morton, J. Yorkston, M. J. Longo, and R. A. Street, Multi-Element Amorphous Silicon Detector Array (MASDA), MED PHYS 19, 1455 (1992). In this prior art detector, a scintillating material (e.g. phosphor screen or CsI) converts X-rays directly into light. The light then impinges on an array of a-Si:H photodiodes, which produce charge in proportion to the light intensity. The generated charge is stored on a capacitor and is read out through a thin film transistor (TFT) as each line is addressed. 
     Another prior art detector has been developed by researchers at the University of Toronto in which X-rays are converted directly to charge. This system is described in W. Zhao and J. S. Rowlands, Selenium Active Matrix Universal Read-out Array Imager (SAMURAI), Medical Imaging VII: Physics of Medical Imaging SPIE (1993). Both the prior art MASDA and SAMURAI devices require measurement of charge (or integrated current), which is proportional to X-ray intensity, for each addressed row of the array. 
     Instead of directly measuring the charge generated by the radiation, it is known in the art to allow the charge to accumulate on the gate of a field effect transistor and to modulate the current through the channel. This approach takes advantage of the intrinsic amplification function of a field effect transistor and also allows the signal to be measured without necessarily destroying the charge. This prior art approach to radiation detection has been disclosed in U.S. Pat. Nos. 5,182,624 and 5,235,195 (Tran et al). 
     A modified version of this approach, for video camera use, has also been proposed (see Z-S. Huang and T. Ando, IEEE Transactions on Electronic Devices, ED-37 1432 (1990) and F. Andoh, K. Taketoshi, J. Yamasaki, M. Sugawara, Y. Fujita, K. Mitani, Y. Matuzawa, K. Miyata and S. Araki, Proceedings of IEEE International Solid State Circuits Conference, page 212 (1990)). In this modified version, a three transistor circuit is used at each pixel location. One of the transistors is used for row selection, another is used for modulating the current in proportion to the radiation-induced charge, and third transistor is used to clear the radiation-induced charge when the next row is addressed. 
     One disadvantage of such prior art systems is that the pixel arrays suffer from sensor non-linearity, thereby requiring extremely accurate photolithography in the fabrication process to ensure inter-pixel matching and reduction of parasitic capacitances. Furthermore, such prior art devices are limited to operating in the linear range of each pixel, thereby limiting the range of detectable radiation-generated charges. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a new method and apparatus is provided for driving electromagnetic radiation imaging devices using dual gate thin film transistors. The principal advantage provided by the present invention over the prior art, is the elimination of sensor non-linearity at the pixel level. This advantage is achieved by selecting a range of operation of said pixel which preferably approaches the biasing voltage range, and by duplicating and comparing the pixel during measurement with the electrical conditions of the pixel resulting from exposure to radiation. The pixel charge electrode is preset to a predetermined voltage level prior to radiation exposure so that the pixel may be caused to operate beyond its linear operating range. The imaging device according to the present invention is capable of operating over a wider sensing range since the sensors are not restricted to operation in the linear range. Furthermore, practical implementation of the imager according to the present invention is simplified as a result of more relaxed fabrication design rules over prior art systems, since inter-pixel matching is not required. 
     The method and apparatus for driving electromagnetic radiation imaging devices according to the present invention, may advantageously be used in the electromagnetic radiation imaging device using dual gate thin film transistors as described and claimed in applicant&#39;s international patent application number PCT/CA94/00077, filed Feb. 11, 1994, the contents of which are incorporated herein by reference. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A detailed description of the preferred embodiment is provided herein below with reference to the sole drawing in which: 
     FIG. 1 comprising parts- 1 A and  1 B together, is a block schematic diagram of a parasitic independent, wide dynamic range driver for a dual gate TFT electromagnetic radiation imaging device in accordance with the preferred embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 depicts a 4×3 pixel imaging array in accordance with the present invention. Each pixel comprises a radiation-to-charge transducer, X, having one electrode connected to an independent top contact, T 0 , and an opposite electrode connected to one gate of a dual gate TFT T 1  and to the drain of a further TFT T 2 . The transducer X can be fabricated from amorphous selenium, or any other suitable material for directly converting electromagnetic radiation into electron-hole pairs. The top contact, T 0 , for each transducer X can be connected to a common terminal for all transducers, or can be separately connected, depending on the application. 
     Each pixel is connected to two input source lines. The first source line, S 0 , receives a DC voltage from DC supply  1 , for application to the source of each TFT T 1 . The DC voltage level is set by a control signal, VC 1 , applied to the supply  1 . The other input source line, LN, receives a controlled variable voltage from digital-to-analog (D/A) converter  3 , for application to the source of each TFT T 2 . The DC voltage output from D/A converter  3  is set by a digital input value VC 2 . 
     A single output sense line, SE, is provided for each column of pixels. The drain of each TFT T 1  is connected to an associated output sense line SE, which is further connected to an associated comparator C 0 . 
     Thus, as will be seen from FIG. 1, the two input source lines (S 0  and LN) are shared between two consecutive row pixels while the sense line (SE) is independent. 
     Each pixel is controlled by two gate lines (G 1  and G 2 ), for independent switching of TFTs T 1  and T 2 , as well as simultaneous connections of the TFTs T 1  and T 2  to the different voltage sources (i.e DC supply  1  and D/A converter  3 ). 
     Each sense line SE is connected to one of a pair of identical capacitors C 1  and C 2 , depending on the position of switches S 1  and S 2 . The capacitors C 1  and C 2  can also be connected to ground and to respective inputs of the associated comparator C 0 , via the switches S 1  and S 2 . Thus, the switches S 1  and S 2  are operated to selectively ground capacitors C 1  and C 2 , to connect sense line SE to one of capacitors C 1  and C 2 , and to connect the capacitors C 1  and C 2  to the respective inputs of the comparator C 0 . 
     In operation, the top gate electrode of TFT T 1  is first set to a predetermined voltage V 1 , where V 1  can be zero volts or any other predetermined value. In order to preset this top electrode voltage, D/A converter  3  generates the required voltage V 1 , an enable voltage is applied to the gate lines G 2  for enabling TFTs T 2 , and the top contact T 0  of each charge transducer X is grounded. 
     According to this operation, a predetermined sensor range is selected for operating the TFTs T 1  (which, as discussed above, the extended operating range can be outside of the normal linear pixel operating range which is generally significantly smaller than the TFT biasing voltage range (typically ½ or less). The extended operating range provided by the driving scheme of the present invention preferably approaches the biasing voltage range. 
     Next, the TFT array is exposed to radiation (e.g. X-rays), so that electron-hole pairs are generated in the charge transducers X. During radiation exposure, the top contact TO is normally connected to a high voltage source, but can be connected to an alternate suitable voltage source in the event that a non-zero voltage V 1  has been applied to the top gate TFTs T 1 . 
     Finally, the charge acquired by the top electrodes of the T 1  TFTs is measured on a row-by-row basis, as follows. 
     Firstly, the C 1  and C 2  capacitors are discharged to ground via respective switches S 1  and S 2 . Next, the DC supply  1  is caused to generate a predetermined voltage V 2 . The voltage V 2  depends on the TFT technology employed. The gate lines GI of the selected row are enabled, and the C 1  capacitors are switched to the corresponding sense lines SE for a-predetermined duration so that the charge on the top electrodes of the T 1  TFTs (i.e. charge proportional to that on the pixels), is stored on the respective capacitors C 1 . Next, the second gate line G 2  for the selected row is enabled. 
     Finally, the output voltage from D/A converter  3  is successively incremented from a minimum level, and, for each successive output voltage from D/A converter  3 , a series of operations are performed for each source line, as follows: 
     A) Connect the capacitors C 2  to the sense lines SE for a predetermined duration similar to that discussed above with reference to capacitors C 1 . 
     B) Compare the charges on the respective capacitors C 1  and C 2 , via the associated comparators C 0 . 
     C) If the voltage on a capacitor C 2  is more than that at the respective capacitor C 1 , go to step G for that particular column. 
     D) Discharge capacitors C 2  (except those for which process control has branched to step G). 
     E) Increment the output of D/A converter  3  (i.e. by incrementing the digital input thereto). 
     F) Go to step A. 
     G) Read the digital input value to D/A converter  3 , which indicates the amount charge present on a particular pixel. 
     H) Continue the steps A-H until charges on all pixel rows have been detected. 
     In summary, the method and apparatus of the present invention effectively eliminates the effects of charge leakage, parasitic capacitances and sensor non-linearity at the pixel level, resulting in simplified fabrication of radiation imaging devices which utilize dual gate TFT arrays, and a wider pixel sensing range. 
     A person understanding the present invention may conceive of other embodiments or variations therein, without departing from the sphere and scope as provided by the claims appended hereto.