Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Reexamination Certificate
2000-12-29
2003-02-11
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
Reexamination Certificate
active
06518576
ABSTRACT:
This application claims the benefit of Korean Patent Application No. 1999-68050, filed on Dec. 31, 1999, which is hereby incorporated by reference for all purposes as if fully set forth herein.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates an X-ray imaging device. More particularly it relates to an X-ray imaging device in which residual charges within a photo-sensing layer are rapidly removed.
2. Discussion of the Related Art
Imaging systems that employ X-rays have been successfully used in medical, scientific and industrial applications. One type of X-ray imaging device uses a photosensitive array panel comprised of a plurality of photo-sensing cells arranged in a matrix. Those cells sense irradiating X-rays that have passed through an object being imaged by generating electric charges in proportion to the intensity of the irradiating X-rays. The electric charges from the photo-sensing cells are sent to a signal converter that converts those charges into electrical signals, that are in turn sent to an image output device. The image output device processes the electrical signals to produce a screen display of the intensity patterns of the X-rays that irradiate the photosensitive array panel.
FIG. 1A
schematically illustrates a sectional view of a photo-sensing cell, while
FIG. 1B
schematically illustrates a planar view of part of a photosensitive array panel. Referring now to
FIG. 1A
, the photo-sensing cell includes a gate line
22
, a thin film transistor (TFT)
24
, and a charging capacitor (Cst) that are formed on a glass substrate
20
. A pixel electrode
32
electrically connects to a drain electrode
26
of the TFT and to the charging capacitor Cst. A photo-sensing layer
34
is formed on the pixel electrode
32
. A dielectric insulating layer
36
is formed on the photo-sensing layer
38
, and a conductive upper electrode
38
is formed on the insulating layer
36
.
The photo-sensing layer
34
is photoconductive and is used to convert X-rays into electric charges. It is beneficially formed from amorphous selenium having a thickness of hundreds of micrometers.
As shown in FIG.
1
A and
FIG. 1B
, the TFT
24
includes a gate electrode
30
that electrically connects to the gate line
22
. Thus, control signals can be applied to the TFT via the gate line
22
. The TFT
24
also includes a source electrode
28
that electrically connects to a data line
40
(see
FIG. 1B
) that is formed on the photo sensitive cell array panel in a direction perpendicular to the gate line
22
. The drain electrode
26
electrically connects to the pixel electrode
32
.
The pixel electrode
32
is formed within the photo-sensing cell to have an area as large as possible. This enables efficient collection of the electric charges generated in the photo-sensing layer
34
so that they can be stored in the charging capacitor Cst. A high voltage generator
42
connects to the upper electrode
38
. That generator supplies a high voltage such that a strong electric field is produced in the photo-sensing layer
34
.
In operation, X-rays pass through an object and then irradiate the photo-sensing layer
34
. The irradiating X-rays (photons) produce electron-hole pairs within the photo-sensing layer
34
. The high voltage (several tens of kV) from the high voltage generator
42
is applied to the upper electrode
38
. That high voltage produces an electric field in the photo-sensing layer
34
that causes the electron-hole pairs to separate. The holes are collected by the pixel electrode
32
and then stored in the charging capacitor Cst. The TFT
24
serves as a switch that controls the outflow of stored electric charges in the charging capacitor Cst. When a gate voltage is applied, via the gate line
22
, to the gate electrode
30
of the TFT
24
, a channel is defined between the source electrode
28
and the drain electrode
26
. Electrons stored in the charging capacitor Cst can then pass through the drain electrode
26
and through the source electrode
28
to the data line
40
.
Referring now to
FIG. 2
, an X-ray imaging system includes a driving apparatus that converts the electric charges from the charging capacitors Cst into electrical signals that are applied to an output. The driving apparatus includes a photosensitive array panel
60
having photo-sensing cells
62
that are arranged in a matrix. A gate driver
64
connects to gate lines GL
1
through GLm, and a data reader
66
connects to data lines DL
1
through DLn. An output
68
displays electrical signals from the data reader
66
as an image.
In the photo sensitive array panel
60
, the photo-sensing cells
62
are positioned near intersections of the m gate lines GL
1
through GLm and the n data lines DL
1
through DLn. Still referring to
FIG. 2
, each of the photo-sensing cells
62
includes a photo sensor
70
, a charging capacitor Cst, and a TFT
72
. In each photo-sensing cell
62
, a gate electrode
74
of the TFT
72
electrically connects to one of the gate lines GL
1
through GLm, and thus to the gate driver
64
. Additionally, a source electrode
76
thereof electrically connects to one of the data lines DL
1
through DLn, and thus to the data reader
66
. Furthermore, a drain electrode of each TFT
72
is connected to a charging capacitor Cst.
When a gate control signal from the gate driver
64
is applied to one of the gate lines GL
1
through GLm, the gate electrodes
74
connected to that gate line turn on their associated TFTs
72
. Then, a conductive channel is defined between the drain electrode
78
and the source electrode
76
of each of the ON TFTs
72
. Thus, the electric charge stored in the charging capacitor Cst of each ON TFT is transferred via one of the data lines DL
1
through DLn to the data reader
66
. In practice, the gate driver
64
applies a pulse-shaped gate control signal sequentially to each of the m gate lines GL
1
through GLm. Thus, the stored electric charges are all applied to the data reader
66
in scan lines.
The data reader
66
generates electrical data signals that correspond to the electric charges from the photo sensitive array panel
60
. The data reader
66
sequentially applies groups of n data signals that correspond to the intensity of the X-rays irradiated onto the photo sensitive cell array panel
60
, plus a reference signal, to the output
68
. The output
68
includes a differential amplifier and an analog-to-digital converter (not shown). The data signals applied to the output
68
are analog signals having noise. The output
68
differentially amplifies the data signals with the reference signal to remove that noise, and then converts the noise-removed analog signal into a digital signal that is suitable for producing an image output on a screen.
As previously mentioned X-rays that irradiate the photo sensitive array panel
60
produce electron-hole pairs in the photo-sensing layer
34
. The high voltage from the high voltage generator produces an intense electric field that separates the electron-hole pairs. Thus, as shown in
FIG. 1A
, holes are collected by the pixel electrode
32
and stored in the charging capacitor Cst. However, the separated electrons accumulate in a region near the boundary between the insulating layer
36
and the photo-sensing layer
34
. Such electrons are referred to as “residual charges.” The residual charges do not simply disappear, they remain even after the electric charges stored in the charging capacitor Cst have been applied to the data reader
66
.
The residual charges have an impact on charges stored in subsequent X-ray irradiations, and thus on subsequent electrical signals. A prior art approach to dealing with residual charges is to irradiate visible light onto the photo sensitive array panel after X-ray irradiation. For example, U.S. Pat. No. 5,563,421 discloses turning off the high voltage applied to the upper electrode
38
after X-ray irradiation and then irradiating the panel with visible light. The visible light produces new electron-hole pairs in the photo-sensing layer
34
Hannaher Constantine
LG.Philips LCD Co. , Ltd.
McKenna Long & Aldridge LLP
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