JPH08129070A - Semiconductor radiation detector - Google Patents

Abstract

PURPOSE: To provide a semiconductor X-ray detector which can increase packaging density and has a good signal detection characteristic and whose output voltage is hard to saturate. CONSTITUTION: Electric charges Q corresponding to a count of X-ray photons entering a sensor element 1 are stored in a capacitor 3 of an integration circuit S, and a voltage V corresponding to the electric charges Q is outputted. At this time, a current I which is the sum of a leak current I1 of the sensor element 1 and a direct current I2 corresponding to an entering rate of the X-ray photons is sent to a MOSFET 41. Tone current operates within a weak inversion area of the MOSFET 41 and therefore a resistance of the MOSFET 41 is changed in inverse proportion to the current I running between a drain D and a source S. As a result, when the electric charges are not stored in the capacitor 3, a value of the resistance is high and the detecting characteristic is turned good. On the other hand, as the electric charges are stored in the capacitor 3, the resistance value is lowered to increase the discharging amount of charges stored in the capacitor 3. An output voltage of an amplifier 2 is accordingly difficult to saturate.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor radiation detector used in, for example, an X-ray imaging apparatus for taking an X-ray image from radiation that has passed through a subject, particularly X-rays that have passed through the subject.

[0002]

2. Description of the Related Art In recent years, as an X-ray image pickup device for a chest, a line sensor in which a large number of semiconductor X-ray detection elements are arranged side by side instead of a method of directly irradiating a film with a transmitted X-ray to obtain an X-ray image A method has been developed in which a two-dimensional sensor converts a transmitted X-ray into an electric signal to obtain an X-ray image.

As shown in FIG. 6, this semiconductor X-ray detector has an integral circuit composed of a sensor element 11 for detecting X-rays, an amplifier 12 including an operational amplifier, a capacitor 13 and a resistor 14 connected in parallel with the sensor element 11. And a circuit S ′. Then, each time the X-ray enters the sensor element 11, a predetermined amount of charge Q is stored in the capacitor 13, and the output voltage of the amplifier 12 rises stepwise as shown in FIG. 7. The output voltage of the amplifier 12 is differentiated by a shaping amplifier or the like (not shown) connected to the next stage and output as a count pulse, and the X-ray incident amount is measured from this count value. On the other hand, the electric charge Q stored in the capacitor 13 is gradually discharged through the resistor 14,
The stored charge of No. 3, that is, the output voltage of the amplifier 12 is prevented from being saturated.

Here, if the resistance value of the resistor 14 is too small, the charge stored in the capacitor 13 is rapidly discharged as shown in FIG. The change in voltage cannot be detected. On the other hand, if the resistance value of the resistor 14 is too large,
The voltage V of the amplifier 12 is saturated immediately and it becomes impossible to detect the incident X-ray accurately. For this reason, it is said that the resistance for connecting to the amplifier 12 is preferably several hundred K parm to several tens M parm.

[0005]

However, a large area is required to form a resistance of several hundreds of K-Ohm to several tens of M-Oum on an integrated circuit. Therefore, when such a resistance is formed, a semiconductor X-ray is formed. It is very difficult to increase the integration density of detectors.

On the other hand, the resistance connected in parallel to the amplifier 12 is
As described above, from the viewpoint of signal detection, one having a large resistance value is preferable, and one having a small resistance value is preferable in order to prevent the voltage of the amplifier 12 from being saturated. In that case, the resistance value must be an intermediate value that satisfies the requirements of both parties. Therefore, in order to solve such a problem, it is an object of the present invention to provide a semiconductor radiation detector capable of increasing the integration density, having good signal detection characteristics, and being less likely to saturate the output voltage.

[0007]

In order to achieve the above object, the present invention provides a semiconductor radiation detector comprising a sensor element for detecting radiation and an integrating circuit for integrating a signal of the sensor element. The circuit is characterized by comprising an amplifier for amplifying the signal of the sensor element, a capacitor connected in parallel with the amplifier, and a transistor connected in parallel with the capacitor and operating as a variable resistor.

[0008]

The operation of the present invention will be described with reference to FIG. 1. The charge Q corresponding to the energy of the photons of the X-rays incident on the sensor element 1 is stored in the capacitor 3 of the integrating circuit S and depends on the charge Q. The voltage is output. At this time, a direct current I, which is a combination of the leak current I1 of the sensor element 1 and the direct current I2 corresponding to the incident rate of the X-ray photons, flows through the MOSFET 41, which is a transistor, as shown in FIG. , The resistance Rm between the drain D and the source S of the MOSFET 41 is Rm = n.UT / IUT = kT / q k: Boltzmann constant, T: absolute temperature, q: When the electric charge n is defined by n = 1 to 2 (constant determined depending on the device), room temperature T = 300 k, leak current I1 of the sensor element 1 = 10 nA, and n = 1.7, X-rays are transmitted to the sensor element 1. In the non-incident state, the resistance Rm of the MOSFET 41 is about 4.4M parm, and the X-ray is the sensor element 1.
Then, when the DC current I2 determined according to the incident rate of X-ray photons becomes 20 nA, the resistance Rm of the MOSFET 41 becomes approximately 1.5 M parm.

For this reason, in the semiconductor X-ray detector including the MOSFET 41, the resistance value of the resistor Rm is very large in a state where no electric charge is stored in the capacitor 3, so that the signal detection characteristic is good, and the incident light is incident. The resistance R increases as the number of photons increases and the accumulated charge of the capacitor 3 increases.
Since the resistance value of m is small and the discharge of the accumulated charge is promoted, the output voltage V of the amplifier 2 is less likely to be saturated. Further, since the MOSFET 41 is used instead of the fixed resistance, the integration density can be greatly increased despite the large resistance.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS.

FIG. 1 is a schematic view showing an embodiment of a semiconductor X-ray detector according to the present invention. In the figure, 1 is a sensor element when detecting an X-ray. Reference numeral S denotes an integrating circuit, which includes an amplifier 2 including an operational amplifier, a capacitor 3 connected in parallel to the amplifier 2, and a MOSFET 41 which is a transistor connected in parallel to the capacitor 3. MO
The gate G of the SFET 41 is short-circuited to the drain D, the drain D is the output of the amplifier 2, and the source S is the amplifier 2.
Are connected to the inverting input of each.

When X-rays are incident on the sensor element 1, a predetermined amount of electric charge Q is stored in the capacitor 3 according to the photon energy of the incident X-rays, and the amplifier 2 is used as in FIG. 7 shown in the conventional example. Output voltage is increased stepwise, this output voltage is differentiated by a shaping amplifier (not shown) connected to the next stage, and the X-ray incident amount is measured from the count pulse number.

Here, a MOSFET which is a transistor
Reference numeral 41 denotes a current I which is a combination of the leak current I1 of the sensor element 1 and the direct current I2 corresponding to the incident rate of X-ray photons.
However, since the current I becomes a minute current due to the characteristics of the sensor element 1, as shown in FIG.
Will operate in the weak inversion region. In the weak inversion region, the drain D and the source S of the MOSFET 41 are
It is known that the resistance Rm between them is defined by the following equation: Rm = n · UT / I. Where UT = kT / q (k: Boltzmann constant, T: absolute temperature,
q: elementary charge (q = 1.6 × 10 ⁻⁹ coulomb) n is a constant determined according to the device and represents a value of 1 to 2.

For example, assuming that the capacitor 3 is completely discharged and the room temperature T = 300 k, the leak current I1 of the sensor element 1 is 10 nA, and n = 1.7, the MOSFET is
The resistance Rm of 41 is about 4.4M parm, which is connected in parallel to the amplifier 2 from the viewpoint that the semiconductor X-ray detector has a good signal detection characteristic and the output voltage V is less likely to be saturated, as described in the conventional example. It becomes a preferable resistance. On the other hand, X-rays are incident on the sensor element 1, charges are stored in the capacitor, the output voltage V of the amplifier 2 rises, and the current I from the amplifier 2 together with the leak current I1 is 30 nA.
Then, the resistance Rm of the MOSFET 41 is about 1.5M parm.

As described above, the MOSFET 41 has a current I flowing between the drain D and the source S in the weak inversion region.
Since the resistance changes in inverse proportion to, the resistance value is high and the detection characteristic is improved in the state where the electric charge is not stored in the capacitor 3. On the other hand, when the electric charge is stored in the capacitor 3, the resistance value is decreased and the amount of electric charge stored in the capacitor 3 is increased, so that the output voltage of the amplifier 2 is less likely to be saturated.
Further, since a transistor is used instead of the resistor, a very high resistance can be formed in a small area, and the integration density of semiconductor detectors can be increased.

As shown in FIG. 3, a bipolar transistor 42 may be used in place of the MOSFET of FIG. 1 as a transistor connected in parallel to the amplifier 2, and a diode 43 may be used as shown in FIG. Good.

FIG. 5 shows an example in which the output terminal 2b of the amplifier 2 to the MOSFET 41 is taken out via the buffer 2a separately from the output terminal 2c to the capacitor 3 in the embodiment of FIG. , It is not always necessary to connect them to the same amplifier output. Such a configuration may be indispensable when the circuit is configured only on one side, not in operation, and corresponds to the fact that the input and output need to have the same potential to some extent in DC feedback. .

Although the most typical X-rays have been described in the above embodiments, the present invention is not limited to this, and radiations such as α-rays and γ-rays may be used.

[0019]

According to the present invention, since the transistor as a discharge resistor used in the integrating circuit operates in the weak inversion region, the resistance changes in inverse proportion to the current I flowing between the drain D and the source S. Therefore, in the state where the electric charge is not stored in the capacitor, the resistance value is increased and the signal detection characteristic is improved, and when the electric charge is stored in the capacitor, the resistance value is decreased and the discharge amount of the electric charge stored in the capacitor is decreased. And the output voltage of the integrating circuit is less likely to be saturated.
Therefore, according to the present invention, it is possible to obtain a semiconductor radiation detector which has a good signal detection characteristic and whose output voltage is less likely to be saturated.

Since a transistor is used instead of the resistor, a very high resistance can be formed in a small area, and the integration density of semiconductor radiation detectors can be greatly increased.

[Brief description of drawings]

FIG. 1 is an embodiment of a semiconductor X-ray detector according to the present invention.

FIG. 2 is a diagram showing a relationship between an input voltage and an output current of a transistor.

FIG. 3 is another embodiment of the semiconductor X-ray detector according to the present invention.

FIG. 4 is another embodiment of the semiconductor X-ray detector according to the present invention.

FIG. 5 is another embodiment of the semiconductor X-ray detector according to the present invention.

FIG. 6 is a diagram showing a conventional semiconductor X-ray detector.

FIG. 7 is a diagram showing the relationship between the output voltage of the integrating circuit and time when X-rays enter the sensor element.

FIG. 8 is a diagram showing the relationship between the output voltage of the integrating circuit and time when X-rays enter the sensor element.

[Explanation of symbols]

 1-Sensor element 2-Amplifier 3-Capacitor 41-MOSFET

Claims (1)

[Claims] 1. A semiconductor radiation detector comprising a sensor element for detecting radiation and an integrating circuit for integrating and outputting a signal of the sensor element, wherein the integrating circuit comprises an amplifier for amplifying the signal of the sensor element. A semiconductor radiation detector comprising: a capacitor connected in parallel with the amplifier; and a transistor connected in parallel with the capacitor and operating as a variable resistance.