Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
1999-06-22
2004-03-30
Smith, Zandra V. (Department: 2877)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S214100
Reexamination Certificate
active
06713748
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an image detection device for converting a light signal into an electrical signal and, more particularly, to an image detection device suitable for a medical X-ray diagnostic apparatus.
In recent years, an X-ray diagnostic apparatus using an image detection device including an a-Si TFT (amorphous silicon Thin Film Transistor) has been proposed in, e.g., U.S. Pat. No. 4,689,487.
FIG. 13
is a block diagram showing the whole arrangement of this image detection device.
X-rays are emitted from an X-ray source
101
to be incident on an a-Si TFT image detection device
103
through an object
102
to be examined. The image detection device
103
generates and outputs an analog electrical signal corresponding to the quantity of X-rays having passed through it. The analog electrical signal is input to an A/D converter
109
in time-series and converted into a digital signal, and the digital signal is stored in an image memory
106
. The image memory
106
stores image data of one or several images, and sequentially stores image data supplied to specific addresses on the basis of a control signal from a controller
105
. The image data stored in the image memory
106
is extracted and processed by an arithmetic processor
110
, and the result is re-stored in the image memory
106
. The arithmetic result stored in the image memory
106
is converted into an analog signal by a D/A converter
107
and displayed as an X-ray image on an image monitor
108
.
The a-Si TFT image detection device
103
has an arrangement as shown in FIG.
14
. The TFT array is made up of a 2,000×2,000 matrix of pixels (e
1
,
1
) to (e
2000
,
2000
). Individual pixels (ej, j) (j is an integer of 1 to 2,000) are parallel-connected at their two ends. One end of each pixel has a photoelectric conversion film
140
and pixel capacitor
142
to which a bias voltage from a power supply
148
is applied, and the other end has an a-Si TFT
144
having an input terminal connected to the other end of each of the photoelectric conversion film
140
and pixel capacitor
142
, an output terminal connected to a signal line S
1
, and a gate connected to a scanning line G
1
.
When light is incident on the pixel, a current flows through the photoelectric conversion film
140
to accumulate charges in the capacitor
142
. A scanning line driving circuit
152
drives the scanning lines G
1
to turn on columns of the TFTs
144
whose gates are connected to themselves. Charges accumulated in the capacitor
142
having one end connected to the input terminal of each TFT
144
are transferred to an amplifier
154
via a signal line S
1
connected to the output terminal of the TFT
144
. The charge amount corresponds to a light quantity incident on the pixel, and the amplitude of an output signal from the amplifier
154
changes in accordance with the charge amount.
The output signal from the amplifier
154
is converted into a digital signal using an A/D converter (not shown) to display a digital image on a computer display. The pixel region shown in
FIG. 14
has the same arrangement as a TFT liquid crystal display used in a compact information device such as a personal computer, and can be easily manufactured for a low-profile large display.
The arrangement shown in
FIG. 14
has one TFT
144
per pixel. However, an actual device may have a plurality of TFTs
144
per pixel. For example, when charges accumulated in a given capacitor
142
are read out to a corresponding signal line S
1
using a detective circuit having an arrangement as shown in
FIG. 15
, TFTs T
1
and T
2
are used. The TFT T
1
is connected to the scanning line G
1
at its gate and ON/OFF-controlled to output charges in the capacitor
142
to the signal line S
1
. The TFT T
2
is connected between one end of the capacitor
142
and a ground terminal to operate as a protective diode.
Alternatively, a detective circuit shown in
FIG. 16
is of an AMI (Amplified MOS Imager) type for converting charges into a voltage. The detective circuit includes TFTs T
11
to T
14
, a constant current source made up of the TFTs T
11
to T
13
is connected between the capacitor
142
and the signal line S
1
, and the TFT T
14
is connected as a reset transistor between one end of the capacitor
142
and a ground terminal. The gate of the TFT
14
receives a reset signal R
1
. The detective circuit shown in
FIG. 15
directly reads out charges accumulated in the capacitor
142
to the signal line S
1
via the TFT T
1
. To the contrary, the circuit shown in
FIG. 16
converts charges in the capacitor
142
into a voltage to output the voltage.
High S/N ratios and wide dynamic ranges are required of X-ray image detection devices. For this reason, when a plurality of TFTs are formed in one pixel, their TFT characteristics must be made uniform. However, the TFT characteristics vary due to process variations. In particular, variations in OFF resistance and threshold voltage Vth degrade the image quality. Further, variations in OFF resistance increase the leakage current, resulting in large noise, a low S/N ratio, and a narrow dynamic range.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide an image detection device capable of realizing high image quality by suppressing the leakage current and increasing the S/N ratio.
An image detection device of the present invention comprises a signal line and scanning line which run perpendicularly to each other on a substrate, a pixel portion arranged at an intersection of the signal line and scanning line and including a photoelectric conversion film for converting incident light into a signal charge to accumulate the signal charge and a pixel electrode, a signal detective circuit including thin film transistors controlled in operation by the scanning line to read out a potential of the pixel electrode, and a scanning line driving circuit for driving the scanning line, wherein a thin film transistor having a source or drain connected to the pixel electrode out of the thin film transistors included in the signal detective circuit has an LDD structure on a high-potential side solely.
Even when pluralities of signal lines and scanning lines run on the substrate, pixel portions are arranged in a matrix at intersections of the signal lines and scanning lines, and the signal detective circuit includes thin film transistors controlled in operation by the respective scanning lines to read out potentials of corresponding pixel electrodes, the present invention can be applied. In this case, at least one thin film transistor having a source or drain connected to the pixel electrode out of the thin film transistors included in the signal detective circuit has an LDD structure on a high-potential side.
In the image detection device of the present invention, at least one transistor having a source or drain connected to the pixel electrode out of the thin film transistors included in the signal detective circuit has a multi-gate structure.
Alternatively, in the image detection device of the present invention, at least one transistor having a source or drain connected to the pixel electrode out of the thin film transistors included in the signal detective circuit has LDD structures on high- and low-potential sides in which an LDD length is larger on the high-potential side than the low-potential side.
When the signal detective circuit comprises a signal read transistor which has a drain or source connected to the pixel electrode, a source or drain connected to the signal line, and a gate connected to the scanning line, and is controlled in operation by the scanning line to output the potential of the pixel electrode to the signal line, and a protective transistor which has a drain or source and a gate connected to the pixel electrode, and a source or drain connected to a predetermined potential line, and connects the pixel electrode to the predetermined potential line when the potential of the pixel electrode reaches not less than a predetermined potential
Ikeda Mitsushi
Kinno Akira
Tsutsumi Junsei
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