阅读说明：本技术 放射线检测器 (Radiation detector ) 是由 盐田昌弘 田口滋也 进藤刚宏 饭塚邦彦 芦田伸之 于 2019-06-18 设计创作，主要内容包括：实现一种高灵敏度的放射线检测器。放大器晶体管(3)被构成为光电二极管(1)在放大器晶体管(3)被预先导通的状态下接收光。(A radiation detector with high sensitivity is realized. The amplifier transistor (3) is configured such that the photodiode (1) receives light in a state where the amplifier transistor (3) is turned on in advance.)

1. A radiation detector, characterized by comprising:

a light receiving element that receives light obtained from radiation and converts the light into an electrical signal; and

an amplifier transistor that amplifies the electric signal,

the amplifier transistor is configured such that the light receiving element receives the light in a state where the amplifier transistor is turned on in advance.

2. The radiation detector according to claim 1,

the light receiving element is a photodiode.

3. The radiation detector according to claim 1,

the amplifier transistor has a channel layer,

the channel layer includes an oxide semiconductor having a crystal structure that is not amorphous.

4. The radiation detector according to claim 1,

the amplifier transistor has a channel layer,

the constituent elements of the channel layer include at least indium and zinc.

5. The radiation detector according to any one of claims 1 to 4,

the light receiving element is disposed above the amplifier transistor.

Technical Field

The present invention relates to a radiation detector.

Background

In recent years, as a radiation detector for detecting radiation such as X-rays, development of a solid-state device using an imaging element or the like has been advanced instead of using a conventional sensitizing paper-X-ray film. In particular, radiation detectors (radiation cameras) using TFT (Thin Film Transistor) panels have been actively developed because they have the advantage of not requiring lenses and of being suitable for imaging large screens, as compared with radiation detectors using imaging elements such as CCDs (charge coupled devices) or CMOSs (Complementary Metal-Oxide semiconductor).

Patent document 1 discloses a radiation detector using an APS (active pixel sensor) type radiation detector, which improves the signal-to-noise ratio.

Disclosure of Invention

Technical problem to be solved by the invention

In the APS method, the generated charges are amplified by the TFT to detect the amount of radiation, and therefore, a smaller amount of charges can be detected than in a PPS (passive pixel sensor) type radiation detector.

Patent document 1 discloses how to read out the detection current amplified by the TFT, but it has not been sufficiently studied which part of the current characteristics should be used for the amplification of the TFT. Therefore, the technique disclosed in patent document 1 has a problem that it is difficult to configure the radiation detector with sufficiently high sensitivity.

An object of one embodiment of the present invention is to realize a radiation detector with high sensitivity.

Means for solving the problems

(1) A radiation detector according to an embodiment of the present invention includes: a light receiving element that receives light obtained from radiation and converts the light into an electrical signal; and an amplifier transistor that amplifies the electric signal, wherein the amplifier transistor is configured such that the light receiving element receives the light in a state where the amplifier transistor is turned on in advance.

(2) In the radiation detector according to one embodiment of the present invention, in addition to the configuration of (1) above, the light receiving element is a photodiode.

(3) In addition, in the radiation detector according to an embodiment of the present invention, in addition to the configuration of the above (1) or (2), the amplifier transistor includes a channel layer including an oxide semiconductor having a crystal structure which is not an amorphous state.

(4) In addition, in the radiation detector according to an embodiment of the present invention, in addition to any one of the configurations (1) to (3), the amplifier transistor includes a channel layer, and constituent elements of the channel layer include at least indium and zinc.

(5) In the radiation detector according to an embodiment of the present invention, in addition to any one of the configurations (1) to (4), the light receiving element is disposed above the amplifier transistor.

Effects of the invention

According to one embodiment of the present invention, a radiation detector with high sensitivity can be realized.

Drawings

Fig. 1 is a circuit diagram showing a configuration of a radiation detector according to a first embodiment of the present invention, and shows a first step in an operation mechanism.

Fig. 2 is a circuit diagram showing a configuration of a radiation detector according to a first embodiment of the present invention, and shows a second step in an operation mechanism.

Fig. 3 is a circuit diagram showing a configuration of a radiation detector according to the first embodiment of the present invention, and shows a third step in an operation mechanism.

Fig. 4 is a circuit diagram showing a configuration of a radiation detector according to the first embodiment of the present invention, and shows a fourth step in an operation mechanism.

Fig. 5 is a circuit diagram showing a configuration of a radiation detector according to the first embodiment of the present invention, and shows a fifth step in an operation mechanism.

Fig. 6 is a circuit diagram showing a configuration of a radiation detector according to the first embodiment of the present invention, and shows a sixth step in an operation mechanism.

Fig. 7 is a circuit diagram showing a configuration of a radiation detector according to the first embodiment of the present invention, and shows a modification of the fourth step in the operation mechanism.

Fig. 8 is a schematic diagram of a connection relationship between a photodiode and a gate of an amplifier transistor in the radiation detector shown in fig. 1.

Fig. 9 is a circuit diagram showing a configuration of a radiation detector according to a comparative example, and shows a first step in an operation mechanism.

Fig. 10 is a circuit diagram showing a configuration of a radiation detector according to a comparative example, and shows a second step in an operation mechanism.

Fig. 11 is a schematic diagram of a connection relationship between a photodiode and a gate of an amplifier transistor in the radiation detector shown in fig. 9.

Fig. 12 is a graph illustrating characteristic variations of each of the amplifier transistors of the plurality of radiation detectors shown in fig. 1.

Fig. 13 is a graph illustrating the comparative example of fig. 12.

Fig. 14 is a view showing a cross-sectional structure of a radiation detector according to a first embodiment of the present invention.

Detailed Description

[ first embodiment ]

Fig. 1 to 6 are circuit diagrams each showing a configuration of a radiation detector 100 according to a first embodiment of the present invention, and respectively show first to sixth steps in an operation mechanism. Fig. 7 is a circuit diagram showing a configuration of the radiation detector 100 according to the first embodiment of the present invention, and shows a modification of the fourth step in the operation mechanism. Fig. 1 to 7 also show graphs showing characteristics of a current Id (vertical axis: unit a) flowing through the amplifier transistor 3 with respect to a gate voltage Vg (horizontal axis: unit V) in the amplifier transistor 3. Hereinafter, this modified example of the fourth step is referred to as a fourth' step.

The radiation detector 100 is an APS including a photodiode 1, a reading transistor 2, an amplifier transistor 3, and a reset transistor 4. Fig. 1 to 7 also show a measurement circuit 111 in addition to the radiation detector 100.

Fig. 8 is a schematic diagram of a connection relationship between the photodiode 1 and the gate of the amplifier transistor 3 in the radiation detector 100 shown in fig. 1.

The photodiode 1 generates electric charges using received light. The reading transistor 2 is a switch that performs connection of the amplifier transistor 3 and the measurement circuit 111. The amplifier transistor 3 flows a current Id proportional to the charge generated by the photodiode 1. The reset transistor 4 is used to reset the electric charge generated by the photodiode 1.

In other words, as described below. The photodiode 1 is a light receiving element that receives light (light obtained from radiation) from a scintillator (not shown) and converts the light into an electrical signal, for example. The amplifier transistor 3 amplifies the electric signal. The reading transistor 2 is a switch connected between the amplifier transistor 3 and the measurement circuit 111, and controls the timing of supplying the electric signal amplified by the amplifier transistor 3 to the measurement circuit 111.

The measurement circuit 111 obtains the amount of light emitted by the photodiode 1 based on the level of the electric signal amplified by the amplifier transistor 3. This allows the radiation amount to be detected in units of pixels, not shown, corresponding to the radiation detector 100. The measurement circuit 111 is, for example, a comparator that supplies the electric signal amplified by the amplifier transistor 3 to one input terminal and supplies a voltage serving as a reference to the other input terminal. However, the configuration of the measurement circuit 111 is not limited to this.

The cathode of the photodiode 1 is connected to a node of the amplifier transistor 3 and the reset transistor 4. As shown in fig. 8, the photodiode 1 is configured such that an n layer 6 as an n-type semiconductor layer and a p layer 7 as a p-type semiconductor layer are connected via an i layer 5 containing no impurity. When a bias voltage having a lower potential on the p-layer 7 side than the n-layer 6 is applied and the photodiode 1 receives light, if light is incident on the p-layer 7, respectively, the hall (holes) are concentrated on the p-layer 7 side and the electrons are concentrated on the n-layer 6 side. In the radiation detector 100, the n layer 6 is connected to the gate of the amplifier transistor 3. Thus, in the radiation detector 100, the voltage Vg decreases by the electrons generated by the light reception of the photodiode 1, and the current Id corresponding to the decrease in the voltage Vg can be made to flow.

In the radiation detector 100, the amplifier transistor 3 is configured such that the photodiode 1 receives light in a state where the amplifier transistor 3 is turned on in advance. Hereinafter, an operation mechanism of the radiation detector 100 relating to this configuration will be described with reference to fig. 1 to 6.

First, in the first step, by turning on the reset transistor 4, the voltage Vg is set to the voltage Vreset supplied from the outside via the reset transistor 4 in the on state (refer to the point 8). In the present embodiment, the voltage Vreset is 3V. At this time, since the read transistor 2 is not turned on, the current Id does not flow.

Next, in the second step, the reset transistor 4 is turned off, and the reset of the radiation detector 100 is completed. Since the second step is performed immediately after the reset, the voltage Vg remains as the voltage Vreset. In the second step, the read transistor 2 is turned on, and thus a current Id flows. Then, the charge from the amplifier transistor 3 (proportional to the current Id × the duration of the on state of the read transistor 2) is supplied to the measurement circuit 111.

Next, in the third process, the reading transistor 2 is set to be non-conductive, and the supply of the electric charge from the amplifier transistor 3 to the measurement circuit 111 is ended. In the third step, the current Id does not flow.

Next, in the fourth step, the photodiode 1 receives light, and electric charge is stored on the amplifier transistor 3 side of the photodiode 1, and here, stored on the cathode side of the photodiode 1, and the voltage Vg drops (see point 9). Here, the decrease amount of the voltage Vg with respect to the voltage Vreset in the fourth step is defined as a voltage Δ Vg. In addition, since the read transistor 2 is not turned on, the current Id does not flow.

Next, in the fifth step, the light reception by the photodiode 1 is completed, but at this time, the voltage Vg is (voltage Vreset — voltage Δ Vg). In other words, the voltage Vg in the fifth step is the same as the voltage Vg in the fourth step.

Next, in the sixth step, the read transistor 2 is turned on, and thereby the current Id flows. Then, as in the second step, the charge from the amplifier transistor 3 is supplied to the measurement circuit 111. At this time, as the voltage Vg decreases in the fourth step, the current Id also decreases. The radiation detector 100 repeatedly performs the first to sixth steps each time the radiation dose is detected.

The difference between the amount of charge from the amplifier transistor 3 in the second process and the amount of charge from the amplifier transistor 3 in the sixth process is correlated with the amount of light emission of the light received by the photodiode 1. In this manner, by supplying the electric charge generated by the photodiode 1 to the gate of the amplifier transistor 3, it is possible to take out the differential electric charge amplified more than the electric charge generated by the photodiode 1. That is, the amplifier transistor 3 can be used as an amplifier of an electric signal.

In the fourth' process, the photodiode 1 receives light. The amount of light received by the photodiode 1 in the fourth' step is much larger than the amount of light received by the photodiode 1 in the fourth step. Light is received by the photodiode 1, electric charge is stored in the cathode side of the photodiode 1, and the voltage Vg drops (refer to a point 10). Here, the decrease amount of the voltage Vg with respect to the voltage Vreset in the fourth 'step is referred to as a voltage Δ Vg'. The voltage Δ Vg' is greater than the voltage Δ Vg. In addition, since the read transistor 2 is not turned on, the current Id does not flow.

From the start time point of the first process, the voltage Vg is set to the voltage Vreset, and the gate of the amplifier transistor 3 to which the voltage Vreset is applied is in the on state. From this, it can be seen that the amplifier transistor 3 is configured such that the photodiode 1 receives light in a state where the amplifier transistor 3 is turned on in advance.

According to the radiation detector 100, when the change in the voltage Vg associated with the light reception of the photodiode 1 is small, the current Id is likely to change greatly. This is because the larger the voltage Vg is, the larger the amount of change in the current Id corresponding to the amount of change in the voltage Vg becomes, and it is easy to set the voltage Vreset for determining the voltage Vg before light reception of the photodiode 1 to be large. Therefore, according to the above configuration, the radiation detector 100 with high sensitivity can be realized.

Fig. 9 and 10 are circuit diagrams each showing a configuration of the radiation detector 100a according to the comparative example, and respectively show a first step and a second step in an operation mechanism. Fig. 9 and 10 also show graphs showing characteristics corresponding to the graphs shown in fig. 1 and the like, respectively. Fig. 11 is a schematic diagram of a connection relationship between the photodiode 1 and the gate of the amplifier transistor 3 in the radiation detector 100a shown in fig. 9.

The differences between the radiation detector 100a and the radiation detector 100 are as follows. That is, in the radiation detector 100a, the anode of the photodiode 1 is connected to the node of the amplifier transistor 3 and the reset transistor 4. In the radiation detector 100a, the p layer 7 is connected to the gate of the amplifier transistor 3 (see fig. 11). Thus, in the radiation detector 100a, the voltage Vg rises due to the hall effect caused by the light reception of the photodiode 1, and the current Id corresponding to the rise of the voltage Vg can be made to flow.

In the radiation detector 100a, the amplifier transistor 3 is configured such that the photodiode 1 receives light in a state where the amplifier transistor 3 is set to be non-conductive in advance.

First, in the first step, the reset transistor 4 is turned on, whereby the voltage Vg is substantially 0V (see point 11). Next, in the second step, the photodiode 1 receives light, and electric charge is accumulated on the amplifier transistor 3 side of the photodiode 1, and here, accumulated on the anode side of the photodiode 1, and the voltage Vg rises (see point 12). Here, the rise amount of the voltage Vg and the rise amount of the current Id with respect to the above-described substantially 0V are set to the voltage Δ Vg and the current Δ Id, respectively. When the current Id flows thereafter, the current Id increases as the voltage Vg increases in the second step.

According to the radiation detector 100a, when the change in the voltage Vg accompanying the light reception of the photodiode 1 is small, the change in the current Id is small. Therefore, it is difficult to improve the sensitivity of the radiation detector 100a as compared with the radiation detector 100.

Fig. 12 is a graph illustrating a characteristic variation of each of the amplifier transistors 3 of the plurality of radiation detectors 100 shown in fig. 1.

The horizontal and vertical axes of the graph shown in fig. 12 are defined the same as the horizontal and vertical axes of the graph shown in fig. 1, respectively.

When the amount of light received by the photodiode 1 is large, the voltage Δ Vg becomes large. This can be said to be the same as the meaning of the increase in the level of the electrical signal generated by the light reception of the photodiode 1.

In the radiation detector 100, the amount of charge flowing into the measurement circuit 111 is counted. The charge amount is given by the following equation. In addition, the time for which the current Id flows into the measurement circuit 111 is equal to the duration of the on state of the read transistor 2.

The amount of charge is the time when the current Id × the current Id flows into the measurement circuit 111

Here, a plurality of radiation detectors 100 will be considered. As for the voltage Vreset, almost no deviation is generated for each radiation detector 100. On the other hand, the characteristic of the current Id with respect to the voltage Vg may generate a relatively large deviation for each radiation detector 100 (in other words, for each amplifier transistor 3). This deviation is shown by curves 13 to 15 in fig. 12. That is, when the voltage Vreset is applied to each of the plurality of amplifier transistors 3, a deviation of the current Id (refer to the point 16 to the point 18) according to the deviation of the characteristics is generated.

Hereinafter, a case where the measurement circuit 111 is configured by an integrator will be also considered. In this case, the measurement circuit 111 cannot count charges exceeding a certain amount. Therefore, it is necessary to determine the current Id and the time when the current Id flows into the measurement circuit 111 so that the electric charge supplied to the measurement circuit 111 does not exceed a certain amount. Although the time at which the current Id flows into the measurement circuit 111 is controllable, it is not realistic to make each radiation detector 100 different. Therefore, it is necessary to determine the time when the current Id flows into the measurement circuit 111 so that one of the plurality of measurement circuits 111 corresponding to the plurality of radiation detectors 100 is also not saturated.

That is, it is preferable to determine the time when the current Id flows into the measurement circuit 111 from the characteristic that the current Id is maximum with respect to the specific voltage Vg, i.e., the curve 15 in fig. 12. Specifically, it is preferable that the time is the maximum value of the time in which saturation of the measurement circuit 111 in which the charge is supplied from the amplifier transistor 3 having the characteristic of the curve 15 can be avoided. In addition, not only the current Id but also the voltage Vreset is preferably a maximum value at which saturation of the measurement circuit 111 in which the charge is supplied from the amplifier transistor 3 having the characteristic of the curve 15 can be avoided with respect to a specific time. Thereby, the radiation detector 100 that reliably avoids saturation of the measurement circuit 111 and has the highest sensitivity can be realized.

On the other hand, regarding each of the plurality of radiation detectors 100, the following is described. The light reception by the photodiode 1 generates electric charges, and the voltage Vg decreases by the amount of the voltage Δ Vg. After the light reception, the charge amount based on the reduced current Id is measured by the measurement circuit 111, and the voltage Δ Vg is known from the difference between the charge amounts before and after the light reception, and if the voltage Δ Vg is known, the light emission amount is known. The radiation dose per pixel can thus be monitored.

Further, in the radiation detector 100a, the larger the amount of light emitted by the light received by the photodiode 1, the larger the current Id ideally increases infinitely. Therefore, when the amount of light received by the photodiode 1 is very large, it is difficult to reliably avoid saturation of the measurement circuit 111 regardless of the time when the current Id flows into the measurement circuit 111 and the specific value of the voltage Vreset (see fig. 13). The horizontal and vertical axes of the graph shown in fig. 13 are defined the same as the horizontal and vertical axes of the graph shown in fig. 1, respectively.

Fig. 14 is a diagram showing a cross-sectional structure of the radiation detector 100 according to the first embodiment of the present invention. The radiation detector 100 includes: a glass substrate 51, a gate electrode 52, a channel layer 53, a drain electrode 54, a source electrode 55, insulating films 56 to 60, a photodiode lower electrode layer 61, a photodiode body layer 62, a photodiode upper electrode layer 63, an upper wiring electrode 64, and a base 65 of the photodiode lower electrode layer. The photodiode lower electrode layer 61, the photodiode main body layer 62, the photodiode upper electrode layer 63, the upper wiring electrode 64, and the base 65 of the photodiode lower electrode layer correspond to the photodiode 1. The gate electrode 52, the channel layer 53, the drain electrode 54, and the source electrode 55 correspond to the amplifier transistor 3.

The gate electrode 52 is connected to the base 65 of the photodiode lower electrode layer through a wire, and the base 65 of the photodiode lower electrode layer is connected to the photodiode lower electrode layer 61. Further, the photodiode 1 is disposed above the amplifier transistor 3, and thus the photodiode lower electrode layer 61 is disposed above the amplifier transistor 3. Thereby, the photodiode lower electrode layer 61 can be made to function as the back gate of the amplifier transistor 3. As a result, the threshold voltage of the amplifier transistor 3 is stabilized, and thus, the variation in the radiation amount detection can be suppressed.

[ second embodiment ]

The channel layer 53 preferably contains an oxide semiconductor having a crystal structure which is not amorphous. This makes it possible to further reduce variations in the threshold voltage of the amplifier transistor 3, and thus further suppress variations in radiation amount detection. For example, it is conceivable to adopt, as the channel layer 53, a crystal structure in which the C axis (direction perpendicular to the film surface) is strongly oriented in the channel film growth direction, instead of the amorphous structure.

The constituent elements of the channel layer 53 preferably include at least In (indium) and Zn (zinc). This makes it possible to form the amplifier transistor 3 small. Thus, even if the photodiodes 1 are fabricated at high density, the light receiving area can be secured, and therefore, an apparatus capable of obtaining a high-resolution image from a minute radiation can be developed. For example, it is conceivable to constitute the amplifier transistor 3 by a transistor using an In-Ga (gallium) -Zn oxide semiconductor.

[ conclusion ]

The radiation detector according to aspect 1 of the present invention includes: a light receiving element that receives light obtained from radiation and converts the light into an electrical signal; and an amplifier transistor that amplifies the electric signal, wherein the amplifier transistor is configured such that the light receiving element receives the light in a state where the amplifier transistor is turned on in advance.

According to the above configuration, when the change in the gate voltage of the amplifier transistor accompanying the light reception of the light receiving element is small, the current flowing through the amplifier transistor is likely to be changed greatly. Therefore, according to the above configuration, a radiation detector with high sensitivity can be realized.

In the radiation detector according to aspect 2 of the present invention, in aspect 1 above, the light receiving element is preferably a photodiode.

The radiation detector relating to aspect 3 of the present invention, in the above aspect 1 or 2, the amplifier transistor has a channel layer that includes an oxide semiconductor having a crystal structure that is not amorphous.

According to the above configuration, since the variation in the threshold voltage of the amplifier transistor can be further reduced, the variation in the radiation amount detection can be further suppressed.

In the radiation detector according to aspect 4 of the present invention, in any one of aspects 1 to 3, the amplifier transistor includes a channel layer, and constituent elements of the channel layer include at least indium and zinc.

According to the above configuration, the amplifier transistor can be formed small. Thus, even if the light receiving elements are fabricated with high density, the light receiving area can be secured, and therefore, an apparatus capable of obtaining a high-resolution image from minute radiation can be developed.

In the radiation detector according to aspect 5 of the present invention, in any one of aspects 1 to 4, the light receiving element is disposed above the amplifier transistor.

According to the above configuration, the electrode of the light receiving element having the same potential as the gate of the amplifier transistor can function as the back gate of the amplifier transistor. As a result, the threshold voltage of the amplifier transistor is stabilized, and thus, the variation in the radiation amount detection can be suppressed.

The present invention is not limited to the above embodiments, and various modifications can be made within the scope shown in the claims, and embodiments obtained by appropriately combining technical means disclosed in different embodiments are also included in the technical scope of the present invention. Further, new technical features can be formed by combining the technical methods disclosed in the respective embodiments.

Description of the reference numerals

1 photodiode (light receiving element)

2 read transistor

3 amplifier transistor

4 reset transistor

5 i layer

6 n layers

7 p layer

51 glass substrate

52 gate electrode

53 channel layer

54 drain electrode

55 source electrode

56-60 insulating film

61 photodiode lower electrode layer

62 photodiode body layer

63 photodiode upper electrode layer

64 upper wiring electrode

65 photodiode lower electrode layer substrate

100 radiation detector

111 measuring circuit