Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Patent
1996-10-04
1998-07-14
Glick, Edward J.
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
25037008, 2502081, H01L 27146, G01T 124
Patent
active
057808589
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates in general to opto-electronic devices, and more particularly to an electromagnetic radiation imaging device incorporating an array of dual gate thin film transistors.
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.
SUMMARY OF THE INVENTION
According to the present invention an electromagnetic radiation imaging detector is provided comprising an energy absorbing layer overlying an array of thin film transistors. Electron-hole pairs (EHPs) are generated in the energy absorbing layer in proportion to the intensity of electromagnetic radiation to which the layer is exposed. A voltage is applied across the absorbing layer for separating the electron-hole pairs. According to the present invention, each of the thin film transistors incorporates a dual gate. The lower gate is used for row selection in the array, and the upper gate is used to collect charge generated by the energy absorbing layer. This charge effectively modulates the current conducted by each thin film transistor so that upon addressing individual rows of pixels, the drain currents from the associated thin film transistors may be measured to provide an accurate reading of the electromagnetic radiation in the vicinity of the pixel.
The composition of the energy absorbing layer is chosen for sensitivity in the desired region of the electromagnetic spectrum. The energy absorbing layer may comprise a single layer material, or a multi-layer stack, as in a PIN photodiode. A list of suitable materials for various spectral regions is disclosed in U.S. Pat. No. 5,235,195 (Tran et al). The energy
REFERENCES:
patent: 4592029 (1986-05-01), Altmann et al.
patent: 4679068 (1987-07-01), Lillquist et al.
Singh Surendra
Waechter David
Glick Edward J.
Litton Systems (Canada) Limited
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