Radiation detector

Details Radiation detector Radiation detector

2000-04-18

2004-02-24

Hannaher, Constantine (Department: 2878)

Radiant energy

Invisible radiant energy responsive electric signalling

Semiconductor system

C378S097000, C378S098800

Reexamination Certificate

active

06696687

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-160213, filed Jun. 7, 1999; and No. 2000-108099, filed Apr. 10, 2000, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to a radiation detector using a thin-film transistor (TFT) as a read switching element.
A radiation detector that uses a TFT as a switching element for each pixel is formed by repeating processes for forming a thin film on one surface of a glass substrate, patterning the thin film by etching, forming another thin film to overlap the etched pattern, and patterning the latter thin film.
FIG. 1
shows the arrangement of a conventional radiation detector. The radiation detector has a plurality of pixels arrayed in a matrix. Each pixel
203
is constructed by a photoelectric conversion element
102
which can directly convert incoming radiation (e.g., X-rays) into a charge and is formed of selenium, a capacitor
103
for storing the generated charge, and a signal read TFT
101
. A charge stored in the capacitor
103
of each pixel is read to an integrating circuit
112
including an amplifier
107
, capacitor
110
and switching device
111
, and multiplexer
109
, via the TFT
101
and a signal line
105
.
As shown in
FIG. 2
, a gate driver
108
turns on/off each TFT
101
by changing the potential on a vertical select line
106
. Upon switching the TFT
101
between ON and OFF, an offset voltage &Dgr;Vout is generated by the TFT
101
. The offset voltage &Dgr;Vout is given by:
&Dgr;
V
out=(
C
gs/
C
out)×(
V
on−
V
off)
where Cgs is the effective capacitance
115
between the gate of the TFT
101
and the signal line
105
, Von is the ON voltage of the TFT
101
, Voff is the OFF voltage of the TFT
101
, and Cout is the capacitance of a capacitor
110
of an integrating circuit
112
, as shown in FIG.
3
.
More specifically, when Cout is small, &Dgr;Vout increases. Especially, when the radiation detector is used in X-ray fluoroscopy, the integrating circuit
112
uses a capacitor
110
having a small capacitance Cout due to a weak detection signal, and the signal voltage must therefore be amplified. The offset voltage &Dgr;Vout is also amplified considerably together with the signal voltage. The amplified offset voltage &Dgr;Vout practically narrows down the dynamic range of the integrating circuit
112
. Also, the amplified offset voltage &Dgr;Vout saturates the integrating circuit
112
. These factors cause an S/N ratio drop.
BRIEF SUMMARY OF THE INVENTION
It is an object of the present invention to improve the S/N ratio of a radiation detector.
The radiation detector comprises a plurality of charge conversion elements which are laid out in a matrix and convert incoming radiation into charges, a plurality of capacitors for storing the charges generated by the charge conversion elements, and charge read elements for reading the charges stored in the capacitors. Signals other than signals originated to charges stored in the capacitors, which are produced upon, e.g., turning on/off the charge read elements are canceled by adjustment means, thus improving the S/N ratio.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.


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