Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Utility Patent
1998-07-21
2001-01-02
Ham, Seungsook (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
C250S208100
Utility Patent
active
06169286
ABSTRACT:
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's international patent application number PCT/CA94/00077, filed Feb. 11, 1994, the contents of which are incorporated herein by reference.
REFERENCES:
patent: 4689487 (1987-08-01), Nishiki et al.
patent: 5182624 (1993-01-01), Tran et al.
patent: 5235195 (1993-08-01), Tran et al.
patent: 5396072 (1995-03-01), Schiebel et al.
patent: 5436442 (1995-07-01), Michon et al.
patent: 0 574 690 A2 (1993-06-01), None
L.E. Antonuk et al., “Demonstration of megavoltage and diagnostic x-ray imaging with hydrogenated amorphous silicon arrays”, Med. Phys. 19 (6), 1455 (Nov./Dec. 1992).
Z-S. Huang et al., IEEE Transactions on Electronic Devices, ED-37 1432 (1990).
F. Andoh et al., “Proceedings of IEEE International Solid State Circuits Conference”, p. 212 (1990).
W. Zhao, “Digital Radiology Using Self-scanned Readout of Amorphous Selenium”, SPIE vol. 1896 Physics of Medical Imaging (1993).
1294339 Ontario, Inc.
Fay Sharpe Fagan Minnich & McKee LLP
Ham Seungsook
Hanig Richard
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