Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Reexamination Certificate
2001-01-31
2003-11-25
Porta, David (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
C250S370010
Reexamination Certificate
active
06653636
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a sensor with a plurality of sensor elements, each of which includes a radiation-sensitive conversion element which generates an electric signal in dependence on the incident radiation, and also with means for amplifying the electric signal in each sensor element and a read-out switching element in each sensor element which is connected to a read-out line in order to read out the electric signal. The invention also relates to a method of operating such a sensor as well as to an X-ray examination apparatus which includes an X-ray source for emitting an X-ray beam for irradiating an object so as to form an X-ray image, as well as a detector for generating an electric image signal from said X-ray image.
BACKGROUND OF THE INVENTION
Large-surface X-ray detectors are customarily used for X-ray examination applications, notably in the medical field; such detectors consist of a plurality of sensor elements. The sensor elements (pixels) as a rule are arranged in rows and columns in a sensor matrix. Preferably, use is made of the so-called flat dynamic X-ray detectors (FDXD). Such detectors are seen as universal detector components that can be used in a wide variety of X-ray apparatus.
In contemporary FDXD embodiments, the individual sensor elements (matrix cells) comprise a radiation-sensitive conversion element, having an intrinsic storage capacity, and a switching element for reading out the signal present on the conversion element or the storage capacitance after the irradiation. The FDXD preferably utilize conversion elements in the form of photodiodes of amorphous silicon and scintillator elements connected thereto, or alternatively photoconductors, for the direct conversion of the X-rays into electric charges. In other types of sensors for other radiation, of course, other conversion elements can also be used.
Diode switches or transistors, notably TFTs (thin film transistors) of amorphous silicon are preferably used as read-out switching elements. In order to read out the signal, present as a collected charge on the conversion element or the intrinsic storage capacitance thereof, the read-out switching elements are activated and the collected charge is conducted to the relevant read-out line. From there it flows to a charge-sensitive amplifier (CSA). Subsequently, corresponding electronic information is applied to a multiplexer which conducts this information to a data acquisition unit for display on a display device in the form of a monitor.
When such detectors are used, notably in the medical analysis practice, it is desirable to reduce the radiation dose so as to limit the dose whereto the patient is exposed; consequently, only a very small amount of radiation is incident on the individual sensor elements. As a result, the electric signal in the individual sensor elements is also very small. Therefore, the aim is to realize sensors or X-ray detectors having an as high as possible signal-to-noise ratio.
A particularly high signal-to-noise ratio and detection of small doses, of course, is also desirable for other radiation-sensitive sensors. In order to improve the signal-to-noise ratio, the signal can in principle be amplified already in the individual matrix cell of the detector.
U.S. Pat. No. 5,825,033 discloses a semiconductor detector for gamma rays in which the charge generated in each pixel in the detector material is stored in an integration capacitor of a capacitive feedback amplifier. This integration takes place for all pixels simultaneously. In a so-called Correlated Double Sample-and-Hold circuit (CDSH) the noise induced by the resetting of the integration capacitor is eliminated. Subsequent to the CDSH, the individual pixels are connected to a respective unity gain buffer which is connected to a read-out line common to each column. The read-out lines are then combined by appropriate multiplexers. The sensor in this case consists of a matrix with 48×48 individual pixels.
For amplifier circuits for enhancing the signal-to-noise ratio, the signal amplification and the noise are customarily the essential characteristics considered for evaluation. For practical operation there is a further criterion in the form of the stability of the transfer function. For example, when the signal amplification or an offset value of the amplifier fluctuates in time, offset and gain artefacts occur in the imaging detector system; such artefacts can only be corrected partly and with great effort only. Such fluctuations may be caused by changes of the temperature or other operating conditions as well as be due to aging, radiation damage and/or trapping effects in semiconductors.
The threshold voltage and the transconductance are liable to change significantly in time, notably in the frequently used thin film transistors (TFTs) of amorphous silicon, which can also be used notably for the manufacture of integrated amplifier circuits in a matrix cell; this may degrade the stability of the transfer function.
Therefore, it is an object of the present invention to provide a sensor and a method of operating the sensor wherein a high stability of the transfer function and an attractive signal-to-noise ratio are ensured by a comparatively simple and economical construction.
SUMMARY OF THE INVENTION
This object is achieved by means of a sensor which is characterized in that the means for amplifying include a respective source follower transistor whose gate is connected to the conversion element, whose source is connected an active load and to one side of a sampling capacitor, the other side of the sampling capacitor being connected to the read-out line via the read-out switching element, and that a respective reset element is connected to the conversion element in order to reset the conversion element to an initial state.
The active load ideally constitutes a current source which impresses a constant channel current on the source follower transistor. The threshold voltage of the source follower transistor is thus stabilized; this threshold voltage is strongly dependent on the channel current, notably in the case of TFTs of amorphous silicon. As a result of the stable threshold voltage, the condition for correct operation of the source follower transistor with adequate stability of the transfer function is satisfied. Therefore, the source follower transistor has a stable voltage amplification of 1. It is converted into a charge amplification G
Q
=C
S
/C
P
by the sampling capacitor, wherein C
P
is the capacitance on the conversion element and C
S
is the capacitance of the sampling capacitor. The capacitance on the conversion element may again be an intrinsic storage capacitance of the conversion element or an additional capacitance.
Preferably, the active load, the read-out switching element and the reset element are also formed by transistors. All components required for the invention can then be integrated directly in the sensor elements while using the thin film technology which is used any way to form the sensor elements; in the context of this technology the transistors can be made of amorphous silicon or polycrystalline silicon. Because of the stable amplification circuit constructed in conformity with the invention, the use of the TFT transistors of amorphous silicon that can be economically manufactured is not a drawback.
A process with vertical integration can now be advantageously used in such a manner that the surface area of the conversion element, or the storage capacitance within a sensor element, is not reduced.
In one embodiment a discharge switching element, preferably in the form of a transistor, for example a TFT of amorphous or polycrystalline silicon, is connected parallel to the sampling capacitor. This discharge switching element can be used for the simultaneous, accelerated discharging of the sampling capacitor during a reset of the conversion element by means of the reset element, so that the sampling capacitor is also reset to an initial state.
The reset element and the discharge switching element may then hav
Busse Falko
Harkin Gerard Francis
Overdick Michael
Rütten Walter
Porta David
Sung Christine
Vodopia John
LandOfFree
Sensor and method of operating the sensor does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Sensor and method of operating the sensor, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Sensor and method of operating the sensor will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3136223