Field effect transistor

Abstract

To provide a field effect transistor that has high radiation resistance and secure circuit characteristics.SOLUTION: A field effect transistor 1 has a laminate structure in which a semi-insulating diamond substrate layer 10, a hydrogen-terminated diamond layer 11, p+ diamond layers 12 and 13, a source electrode 14, a drain electrode 15, a gate electrode 16, and an insulating layer 18 are laminated. The insulating layer 18 is made of aluminum oxide and is located on the hydrogen-terminated region of the hydrogen-terminated diamond layer 11. The field effect transistor 1 has a mutual conductance of 0.5 mS / mm or more under a room temperature condition after being irradiated with 5 MGy X-rays.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to a field-effect transistor. [Background technology]

[0002] Electronic components used in high-radiation environments such as nuclear reactors require high radiation resistance. Diamond is a candidate semiconductor material for achieving such high radiation resistance. Non-Patent Literature 1 describes a MESFET (Metal-Semiconductor Field Effect Transistor), which is an example of a field-effect transistor using a diamond semiconductor. The MESFET in Non-Patent Literature 1 comprises a nitrogen-doped semi-insulating diamond substrate, a p-drift layer made of diamond formed on the substrate, a p+ contact layer made of diamond formed on the p-drift layer, source electrodes and drain electrodes formed thereon, and a gate electrode formed on the p-drift layer. Ruthenium is used for the gate electrode. When such a MESFET is irradiated with 5 MGy and 10 MGy X-rays, it has been shown that the maximum drain current and transconductance of the MESFET remain almost constant with respect to irradiation. [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] Hitoshi Umezawa, Shinya Omachi, and Yoshiaki Mokuno, "Characterization of X-ray Radiation Resistance of Diamond Schottky Barrier Diodes and Metal-Semiconductor Field-Effect Transistors," Proceedings of the 29th International Symposium on Power Semiconductor Devices and ICs, IEEE, July 2017, pp. 379-382. [Overview of the project] [Problems that the invention aims to solve]

[0004] As described above, the MESFET in Non-Patent Document 1 exhibits high resistance to X-ray irradiation. However, the field-effect transistor in Non-Patent Document 1 has a low transconductance of 0.01 mS / mm, which may mean it does not meet the circuit characteristics required for an electronic component.

[0005] The object of the present invention is to provide a field-effect transistor that has high radiation resistance while also ensuring good circuit characteristics. [Means for solving the problem]

[0006] The field-effect transistor of the present invention comprises a non-doped diamond layer with a hydrogen-terminated surface, first and second p+ diamond layers formed on the non-doped diamond layer with hydrogen-terminated regions sandwiched between them, a metallic source electrode formed on the first p+ diamond layer, a metallic drain electrode formed on the second p+ diamond layer, an insulating layer formed on the hydrogen-terminated region of the non-doped diamond layer, and a gate electrode formed on the insulating layer, wherein the transconductance is 0.5 mS / mm or more at room temperature after irradiation with 5 MGy of X-rays.

[0007] The field-effect transistor of the present invention employs a structure in which an insulating layer and a gate electrode are provided on a hydrogen-terminated, undoped diamond layer. This ensures circuit characteristics in which the transconductance is 0.5 mS / mm or more at room temperature after exposure to 5 MGy of X-rays. 5 MGy of X-rays is a typical example of an electronic circuit applying the present invention being used in a nuclear reactor. According to the present invention, circuit characteristics with a transconductance of 0.5 mS / mm or more can be ensured under such conditions. This makes it possible to ensure the necessary circuit characteristics for electronic circuits even under harsh, high-radiation environments such as inside a nuclear reactor during normal operation, severe accidents, or decommissioning. It should be noted that the ability to ensure circuit characteristics under such conditions does not mean that the application of the present invention is limited to nuclear reactors. The present invention can be applied to various environments such as accelerators, radiation therapy, fusion reactors, space environments, and aerospace environments.

[0008] Furthermore, in the present invention, it is preferable that the insulating layer contains aluminum oxide. This improves the radiation resistance properties of the insulating layer.

[0009] Furthermore, in the present invention, it is preferable that the source electrode, drain electrode, and gate electrode are all made of at least one of ruthenium, iridium, platinum, and molybdenum. This improves the radiation resistance of the electrodes. In this case, electrodes made by combining at least one of ruthenium, iridium, platinum, and molybdenum with other metals such as gold may be used. For example, a layer made of other metals such as gold may be laminated on a layer made of at least one of ruthenium, iridium, platinum, and molybdenum.

[0010] Furthermore, in the present invention, it is preferable to further include a recovery electrode, which is an independent electrode different from the source electrode, the drain electrode, and the gate electrode, for restoring circuit characteristics by at least one of defect recovery by thermal recovery and charge extraction. This makes it possible to restore circuit characteristics that have deteriorated due to radiation irradiation. In addition, in the present invention, after being irradiated with 5 MGy of X-rays, the leakage current of the gate electrode is 10 times the operating drain current. -6 It is preferable that it is less than or equal to twice the original value. According to this, the present invention realizes a field-effect transistor in which leakage current is suppressed. [Brief explanation of the drawing]

[0011] [Figure 1] This is a conceptual diagram showing the structure of a field-effect transistor according to one embodiment of the present invention. [Figure 2] Figure 1 is a graph showing the drain voltage-drain current characteristics of one embodiment of the field-effect transistor. Figure 2(a) shows the characteristics before X-ray irradiation, Figure 2(b) shows the characteristics after 10 kGy of X-ray irradiation, Figure 2(c) shows the characteristics after 100 kGy of X-ray irradiation, and Figure 2(d) shows the characteristics after 1 MGy of X-ray irradiation. [Figure 3] Figure 1 is a graph showing the gate bias-drain current characteristics of one embodiment of a field-effect transistor. [Figure 4] Figure 4(a) is a graph showing the maximum drain current as a function of the integrated X-ray dose. Figure 4(b) is a graph showing the transconductance as a function of the integrated X-ray dose. Figure 4(c) is a graph showing the change in threshold voltage as a function of the integrated X-ray dose. [Figure 5] This is a graph showing the sheet resistance according to the embodiment shown in Figure 3. [Figure 6] This is a conceptual diagram to explain the mechanism by which the characteristics shown in the graphs of Figure 4(c) and Figure 5 manifest. [Figure 7] This is a graph showing the leakage current as a function of the integrated X-ray dose in another embodiment of the field-effect transistor shown in Figure 1. [Figure 8]A graph showing the leakage current with respect to the gate voltage when iridium is used as the electrode of the field-effect transistor in FIG. 1. [Figure 9] A graph showing the leakage current with respect to the gate voltage when platinum is used as the electrode of the field-effect transistor in FIG. 1. [Figure 10] A graph showing the leakage current with respect to the gate voltage when molybdenum is used as the electrode of the field-effect transistor in FIG. 1. [Figure 11] A graph showing the leakage current with respect to the gate voltage when a multilayer electrode composed of molybdenum and gold is used as the electrode of the field-effect transistor in FIG. 1.

Embodiments for Carrying Out the Invention

[0012] The field-effect transistor 1 according to an embodiment of the present invention will be described with reference to FIG. 1. As shown in FIG. 1, the field-effect transistor 1 has a stacked structure in which a semi-insulating diamond substrate layer 10, a hydrogen-terminated diamond layer 11 (corresponding to the non-doped diamond layer in the present invention), a p+ diamond layer 12, a p+ diamond layer 13, a source electrode 14, a drain electrode 15, a gate electrode 16, a back gate electrode 17, and an insulating layer 18 are stacked.

[0013] For the semi-insulating diamond substrate layer 10, diamond doped with nitrogen is used. Its thickness is 50 to 500 μm, and the nitrogen concentration is 10 17 ~10 20 atoms / cm 3 . By providing the semi-insulating diamond substrate layer 10 doped with nitrogen, the short-channel effect, the increase in drain conductance in the saturation region, and the decrease in output impedance are suppressed. In order to achieve a mutual conductance of 0.5 mS / mm or more as described later, it is desirable that the defect density of the substrate is 10 6 per cm 2 or less.

[0014] The hydrogen-terminated diamond layer 11 is formed on the semi-insulating diamond substrate layer 10. The hydrogen-terminated diamond layer 11 uses non-doped diamond that is not doped with impurities. The thickness of the hydrogen-terminated diamond layer 11 is preferably 3 μm or less. This makes it possible to prevent a decrease in the output impedance. In the region H where the insulating layer 18 is laminated on the laminated surface 11a which is one surface of the hydrogen-terminated diamond layer 11, a hydrogen-terminated conductive layer is formed. The hydrogen-terminated diamond layer 11 is formed, for example, by using the CVD (chemical vapor deposition) method. The hydrogen-terminated conductive layer is formed by exposing the surface of diamond to hydrogen plasma during the formation of the layer by the CVD method.

[0015] The p+ diamond layers 12 and 13 are formed on the laminated surface 11a of the hydrogen-terminated diamond layer 11. They are arranged to be separated from each other on the hydrogen-terminated diamond layer 11. The p+ diamond layers 12 and 13 use diamond doped with boron atoms as impurities. The concentration of boron in the p+ diamond layers 12 and 13 is 10 19 ~10 22 atoms / cm 3 ³. The p+ diamond layers 12 and 13 are formed, for example, by using the CVD method.

[0016] The insulating layer 18 is formed in the region sandwiched between the p+ diamond layer 12 and the p+ diamond layer 13 on the laminated surface 11a of the hydrogen-terminated diamond layer 11. Aluminum oxide (Al₂O₃) is used for the insulating layer 18. The insulating layer 18 is formed, for example, by using the high-temperature ALD (Atomic Layer Deposition) method.

[0017] The source electrode 14 is formed on the p+ diamond layer 12, the drain electrode 15 on the p+ diamond layer 13, and the gate electrode 16 on the insulating layer 18. The source electrode 14 is in contact only with the p+ diamond layer 12, the drain electrode 15 is in contact only with the p+ diamond layer 13, and the gate electrode 16 is in contact only with the insulating layer 18. The back gate electrode 17 is formed on the surface of the semi-insulating diamond substrate layer 10 opposite to the hydrogen-terminated diamond layer 11. The back gate electrode 17 is in contact only with the semi-insulating diamond substrate layer 10. Ruthenium is used for the source electrode 14, drain electrode 15, gate electrode 16, and back gate electrode 17. These electrodes are formed, for example, using the RF sputtering method.

[0018] The recovery electrode 21 is formed on the laminated surface 11a of the hydrogen-terminated diamond layer 11 in a region adjacent to the region where the p+ diamond layers 12 and 13 and the insulating layer 18 are laminated. The recovery electrode 21 is in contact only with the hydrogen-terminated diamond layer 11. The recovery electrode 21 is made entirely of ruthenium, or the surface of a metal electrode body is protected with ruthenium. Any metal capable of supplying 10W or more of power may be used for the electrode body. A power supply circuit 31 is connected to the recovery electrode 21. The power supply circuit 31 supplies 10W or more of power to the recovery electrode 21 by passing current through it. The heat generated from the recovery electrode 21 due to this power supply raises the temperature of the field-effect transistor 1. The recovery electrode 21 is formed, for example, using the RF sputtering method.

[0019] The recovery electrode 22 has a portion 22a extending along the lamination surface 11a of the hydrogen-terminated diamond layer 11 and a portion 22b extending along the side end surface 11b of the hydrogen-terminated diamond layer 11. Portion 22a is formed in the region adjacent to the region where the p+ diamond layers 12 and 13 and the insulating layer 18 are laminated on the lamination surface 11a. Portion 22b extends from the end of portion 22a along the side end surface 11b to the semi-insulating diamond substrate layer 10. As a result, the recovery electrode 22 is in contact with both the hydrogen-terminated diamond layer 11 and the semi-insulating diamond substrate layer 10. The recovery electrode 22 is either entirely made of ruthenium or has a metal electrode body with a ruthenium-protected surface. Any metal may be used for the electrode body. A pulse supply circuit 32 is connected to the recovery electrode 22. The pulse supply circuit 32 applies a rectangular wave pulse voltage signal to the recovery electrode 22. The pulse voltage signal is, for example, a signal in which a voltage of +100V lasts for a predetermined time and a voltage of -100V lasts for a predetermined time alternate at predetermined time intervals. The recovery electrode 22 is formed, for example, using the sputtering method.

[0020] The field-effect transistor 1 according to this embodiment ensures circuit characteristics such as a transconductance of 0.5 mS / mm or more at room temperature, even after exposure to X-ray irradiation with a high cumulative dose of 5 MGy. This assurance of circuit characteristics in a high-radiation environment was achieved through diligent research by the inventors of the present invention, by adopting the structure of this embodiment, in which p+ diamond layers 12 and 13 are provided at the source and drain, and an insulating layer 18 is provided on the hydrogen-terminated diamond layer 11. As a result of adopting this configuration, as shown in the embodiments described later, circuit characteristics such as a transconductance of 0.5 mS / mm or more at room temperature were ensured even after exposure to X-ray irradiation with a high cumulative dose of 5 MGy. Furthermore, similar circuit characteristics can be ensured even in high-temperature environments (e.g., 450°C).

[0021] The value of 5 MGy corresponds to the cumulative dose over a period of about one week in the high-radiation environment inside a nuclear reactor during a severe accident, and was calculated through simulation. Furthermore, in order to realize a low-noise circuit, an amplification factor of 2 mS or more is required at least at room temperature. Considering its application to electronic circuits used inside a nuclear reactor, an amplification factor of 2 mS or more is preferable in the temperature range from room temperature to 450°C. On the other hand, in order to increase radiation resistance, it is preferable to make the gate width 4 mm or less. As a result, an amplification factor of 0.5 mS / mm or more is required. In addition, by achieving a transconductance of 0.5 mS / mm, noise is reduced and it becomes possible to amplify signals up to high frequency bands. Furthermore, in order to amplify pulse signals without degradation, it is preferable that it also operates for high-frequency signals of 100 MHz or higher.

[0022] Furthermore, the field-effect transistor 1 according to this embodiment is provided with recovery electrodes 21 and 22. The recovery electrodes 21 and 22 restore elements that have deteriorated due to radiation irradiation. There are two types of deterioration due to radiation irradiation. The first type of deterioration is caused by the accumulation of charge in at least one of the hydrogen-terminated diamond layer 11, p+ diamond layers 12 and 13, semi-insulating substrate 10, and insulating layer 18, resulting in fluctuations in the threshold voltage, drain current, etc. This type of deterioration is restored by supplying a pulse voltage signal from the pulse supply circuit 32 to the recovery electrode 22, thereby extracting the charge from the layers where the charge has accumulated. The second type of deterioration is caused by the generation of defects in the insulating layer 18, resulting in fluctuations in the threshold voltage, a decrease in transconductance, a decrease in drain current, etc. This type of deterioration is restored by supplying power from the power supply circuit 31 to the recovery electrode 21, thereby raising the temperature of the field-effect transistor 1 to 600°C or higher, and repairing the point defects.

[0023] Furthermore, ruthenium is used for the source electrode 14, drain electrode 15, and gate electrode 16 in this embodiment. Both the recovery electrodes 21 and 22 are either entirely made of ruthenium, or have a metal electrode body with a ruthenium-protected surface. This improves the radiation resistance of each electrode.

[0024] Furthermore, in the field-effect transistor 1 of this embodiment, it is preferable that the change in circuit characteristics (e.g., drain voltage-drain current characteristics) due to radiation irradiation of a predetermined integrated dose (e.g., 10 kGy) or more saturates. It is preferable that the drain conductance (output impedance) in the saturation region of the drain voltage-drain current characteristics when irradiated with 10 kGy to 1 MGy of radiation is 5 MΩ mm or more. It is preferable that the drain conductance in the linear region of the drain voltage-drain current characteristics when irradiated with 10 kGy to 1 MGy of radiation is 2 MΩ mm or less. The gate voltage V when irradiated with 10 kGy to 1 MGy of radiation GS <-3V leakage current is 10 -7 It is preferable that the A / mm is less than or equal to the subthreshold swing during radiation irradiation of 10 kGy to 1 MGy is less than or equal to 300 mV / decade. It is preferable that at least one or both of the absolute values ​​of the maximum drain current and transconductance do not decrease significantly from the time of no radiation irradiation to the time of 5 MGy irradiation. It is preferable that the threshold voltage fluctuation range is 3 V or less from the time of no radiation irradiation to the time of 5 MGy irradiation.

[0025] [Example 1]

[0026] The following describes an embodiment of the field-effect transistor according to the present invention. The field-effect transistor according to this embodiment corresponds to the field-effect transistor according to the above embodiment, excluding the recovery electrodes 21 and 22. The field-effect transistor was fabricated by the following method. First, a 1 μm thick hydrogen-terminated diamond layer 11 was formed on a diamond Ib(001) substrate (corresponding to the semi-insulating diamond substrate layer 10) at a methane concentration of 0.5% using a plasma CVD apparatus (Seki Technotron, AX5010-INT). Next, a mask was formed using the plasma CVD apparatus to selectively form p+ diamond layers 12 and 13 in predetermined areas on the hydrogen-terminated diamond layer 11. Next, p+ diamond layers 12 and 13 with a thickness of 0.4 μm were formed on the diamond layer 11 in areas where the mask was not formed using the HF-CVD method. Next, the mask was removed, and an 83 nm thick aluminum oxide layer was formed at 350°C by the high-temperature ALD method using an ALD plasma processing apparatus, and then an insulating layer 18 was formed by etching using a reactive ion etching apparatus. Next, a source electrode 14, a drain electrode 15, and a gate electrode 16 made of ruthenium were formed using RF sputtering with a sputtering apparatus.

[0027] The operational performance of the field-effect transistor 1, fabricated as described above, under high-radiation conditions was evaluated as follows. First, at room temperature, the relative positions of the field-effect transistor 1 and the X-ray irradiation device were kept constant, and X-rays were continuously irradiated from the X-ray irradiation device to the field-effect transistor 1. This was done in stages until the irradiated radiation reached values ​​of 1 kGy, 3 kGy, 10 kGy, 30 kGy, 100 kGy, 300 kGy, and 1000 kGy. The performance of the field-effect transistor was evaluated using a semiconductor device parameter analyzer at the stage before X-ray irradiation and at each stage immediately after irradiating with X-rays up to the above values. The radiation levels to the field-effect transistor 1 were determined based on the evaluation of the dose per unit time using a cellulose triacetate film dosimeter and the irradiation time. As a result of the dose evaluation, the dose per unit time was 1.1 kGy / min.

[0028] The results of the above performance evaluation are shown in Figures 2 to 4. The graphs in Figures 2(a) to 2(d) show the drain voltage-drain current characteristics. Figure 2(a) shows the characteristics before X-ray irradiation, Figure 2(b) shows the characteristics after 10 kGy of X-ray irradiation, Figure 2(c) shows the characteristics after 100 kGy of X-ray irradiation, and Figure 2(d) shows the characteristics after 1 MGy of X-ray irradiation. Initial fluctuations occurred in the characteristics up to 10 kGy of X-ray irradiation, but since no significant changes are observed between the graphs in Figures 2(b) to 2(d), it is clear that the changes in the device characteristics due to radiation irradiation are minimal. Initial fluctuations are a phenomenon that can be observed even in high-temperature environments, and stabilization can be achieved by processing in an environment of about 400°C for about 1 hour. According to Figures 2(b) to 2(d), the drain conductance (output impedance) in the saturation region is 5 MΩ mm or more. The drain conductance in the linear region is 2 MΩ mm or less.

[0029] The graph in Figure 3 shows the gate voltage-drain current characteristics when the drain voltage is -10V. In the graph, □ represents the characteristics when irradiated with 100kGy of X-rays, × represents the characteristics when irradiated with 100kGy of X-rays, and ○ represents the characteristics when irradiated with 1MGy of X-rays. According to Figure 3, V GS <-3V leakage current is 10 -7 The values ​​are less than A / mm. The subthreshold swings are 238, 215, and 264 mV / decade for X-ray irradiation at 100 kGy, 300 kGy, and 1 MGy, respectively. In other words, all subthreshold swings are less than 300 mV / decade. This suggests that the interface level does not increase even when the irradiated X-ray dose is high. Furthermore, the threshold voltage shift after the initial fluctuation is less than 1 V.

[0030] The graph in Figure 4(a) shows the maximum drain current for the irradiated X-rays. The graph in Figure 4(b) shows the transconductance for the irradiated X-rays. The graph in Figure 4(c) shows the change in threshold voltage for the irradiated X-rays. In all of Figures 4(a) to 4(c), the drain voltage was set to -10V. According to Figure 4(a), the maximum drain current J DSThe absolute value of increases from the unirradiated state to the 10 kGy irradiation period, showing initial fluctuations, and then remains almost constant beyond that irradiation level. In other words, the absolute value of the maximum drain current does not decrease significantly from the initial fluctuation until the 1 MGy irradiation period. Also, according to Figure 4(b), the transconductance g m The absolute value of increases during the initial fluctuations from the unirradiated state to the 10 kGy irradiation, and then saturates and becomes almost constant at higher irradiation levels. In other words, the transconductance hardly decreases from the initial fluctuations up to the 1 MGy irradiation. Also, according to Figure 4(c), the threshold voltage V T The absolute value of the threshold voltage increases from the unirradiated state to 10 Gy irradiation, showing initial fluctuations, and then saturates to a nearly constant value at higher irradiation levels. Furthermore, the fluctuation range of the threshold voltage from the initial fluctuation to 1 MGy irradiation is less than 1 V. In another embodiment, when a similar performance evaluation was performed with 1 to 5 MGy of X-ray irradiation, the maximum drain current, transconductance, and threshold voltage all saturated to a constant value. In other words, combining the results of this other embodiment with the results shown in Figure 4, the absolute values ​​of the maximum drain current and transconductance do not decrease significantly from the initial fluctuation to 5 MGy irradiation, and the fluctuation range of the threshold voltage is less than 1 V.

[0031] As a result of this embodiment, the sheet resistance of the field-effect transistor 1 decreased as the cumulative dose increased to about 100 kGy, as shown in Figure 5, and remained constant as the cumulative dose increased beyond 100 kGy. The reason why such a characteristic in the sheet resistance in response to the X-ray irradiation dose appears is thought to be due to the following mechanism (see Figure 6). When X-rays are irradiated, electron-hole pairs are generated in the Al2O3 that forms the insulating layer 18. Since defects exist in Al2O3, the generated electrons are captured by the defects, inducing holes on the hydrogen-terminated diamond layer 11 side. The induction of holes leads to an increase in carrier density, which improves conductivity and decreases the sheet resistance. Here, the defects are thought to be oxygen vacancies, impurities, disorder in the crystal structure, etc., which are defects that capture charge.

[0032] Because the above-mentioned defects are limited, after a certain amount of irradiation, almost all defects capture electrons, and it becomes impossible to capture any more electrons. Therefore, once a certain amount of irradiation (approximately 100 kGy in this result) is performed, further irradiation beyond that point hardly results in the capture of new electrons or the induction of holes. Thus, the sheet resistance stabilizes even with irradiation exceeding a certain amount.

[0033] Furthermore, as shown in Figure 4(c), the threshold voltage rises until the cumulative dose reaches approximately 100 kGy, and then remains constant above 100 kGy. This is thought to be due to the following mechanism (see Figure 6). In a P-channel, normally-on FET (field-effect transistor), the FET can be turned off by applying a positive voltage to the gate to remove holes from the channel. In this case, the threshold voltage can be considered as the positive voltage that must be applied to turn off the FET.

[0034] If X-ray irradiation causes electrons to accumulate in the Al2O3 forming the insulating layer 18, and holes are induced on the hydrogen-terminated diamond layer 11 side, the density of holes on the surface of the hydrogen-terminated diamond layer 11 will increase. Therefore, a larger positive voltage will need to be applied to eliminate the holes and turn off the device.

[0035] Up to a certain amount of irradiation (approximately 100 kGy in this result), electrons generated by the irradiation accumulate in Al2O3, increasing the density of holes on the surface of the hydrogen-terminated diamond layer 11 and thus raising the threshold voltage. However, for irradiation exceeding this amount, the defects are filled, and almost no electron accumulation occurs. Therefore, the threshold voltage remains stable for irradiation exceeding a certain amount.

[0036] [Example 2]

[0037] The leakage current of the gate electrode 16 was measured for various X-ray irradiation doses up to 3 MGy to the field-effect transistor 1. The field-effect transistor 1 was the same as in Example 1, except that each electrode consisted of a gold layer stacked on top of a ruthenium layer (a structure in which the gold layer protects the ruthenium layer from X-ray irradiation by forming a gold layer on the surface of the ruthenium layer). Voltages of 0V, 0V, and 1V were applied to the source electrode 14, drain electrode 15, and gate electrode 16, respectively. Figure 7 shows the results, illustrating the leakage current I as a function of X-ray irradiation dose. gs This is a graph. The solid line shows the measured leakage current, and the dashed line represents the prediction using linear approximation of the measured value. According to this, the leakage current is expected to be 5.2nA for an irradiation dose of 5MGy. This value corresponds to 10 for an operating drain current of 17mA. -6 It is less than double. In this embodiment, no change in the appearance of the electrode before and after X-ray irradiation, nor any trace of interdiffusion between the ruthenium layer and the gold layer after X-ray irradiation was observed. In contrast, previous experiments by the inventors have shown that when radiation is irradiated onto an electrode having a nickel layer instead of a ruthenium layer, interdiffusion occurs between the nickel layer and the gold layer, indicating that electrodes using nickel and gold cannot be used in high-radiation environments.

[0038] [Example 3]

[0039] After irradiating the field-effect transistor 1 with 10 MGy of X-rays, the leakage current of the gate electrode 16 was measured while varying the gate voltage. The field-effect transistor 1 used consisted of four electrodes: one made of iridium (hereinafter referred to as Example 3-Ir), one made of platinum (hereinafter referred to as Example 3-Pt), one made of molybdenum (hereinafter referred to as Example 3-Mo), and one made of molybdenum and gold (hereinafter referred to as Example 3-Mo / Au). In Example 3-Mo / Au, each electrode has a structure in which a layer of gold is stacked on top of a layer of molybdenum (a structure in which the gold layer protects the molybdenum layer from X-ray irradiation by forming a gold layer on the surface of the molybdenum layer). Graphs in Figures 8 to 11 show the results for Examples 3-Ir, 3-Pt, 3-Mo, and 3-Mo / Au, respectively.

[0040] As shown in Figures 8 to 11, respectively, after irradiation with 10 MGy of X-rays, 10 -11 An increase in leakage current was observed up to approximately A. However, the leakage current was 10 compared to the operating current of field-effect transistor 1, which is 17mA. -8 It is less than twice the size and is perfectly usable as a radiation-resistant electrode.

[0041] <Variation>

[0042] The above describes preferred embodiments of the present invention, but the present invention is not limited to the embodiments described above, and various modifications are possible within the scope of the means for solving the problem.

[0043] For example, in the above-described embodiment, ruthenium is used for the source electrode 14, drain electrode 15, gate electrode 16, and back gate electrode 17, thereby ensuring high radiation resistance for the electrodes. However, electrodes may also be used in which the surface of the electrode body, made of another metal (e.g., molybdenum), is coated with gold to protect it from radiation.

[0044] Furthermore, in the above-described embodiment, aluminum oxide is used as the insulating layer 18. However, other materials may be used for the insulating layer 18. For example, silicon dioxide or calcium fluoride may be used. Also, the insulating layer 18 may be composed of multiple materials from among these materials and other materials. For example, the insulating layer 18 may be composed of a material that combines aluminum oxide and other materials.

[0045] Furthermore, in the above-described embodiment, the degradation of the insulating layer 18 is restored by raising the temperature of the field-effect transistor 1 to 600°C or higher by applying a pulse voltage signal to the recovery electrode 21. However, the degradation of the insulating layer 18 may also be restored by raising the temperature of the field-effect transistor 1 to 600°C or higher by applying a pulse voltage signal to the gate electrode 16 or the back gate electrode 17. In this case, the recovery electrode 22 does not need to be provided on the field-effect transistor 1. [Explanation of Symbols]

[0046] 1. Field-effect transistor 11 Hydrogen-terminated diamond layer 12, 13 p+ diamond layer 14 Source electrodes 15 Drain electrode 16 Shuttle gates 18 Insulating layer 21, 22 Recovery electrodes

Claims

1. a non-doped diamond layer having a hydrogen-terminated surface; first and second p+ diamond layers formed on the non-doped diamond layer with a hydrogen terminated region between them; a metallic source electrode formed on the first p+ diamond layer; a metallic drain electrode formed on the second p+ diamond layer; an insulating layer formed on the hydrogen-terminated region of the undoped diamond layer; and a gate electrode formed on the insulating layer.

2. A field effect transistor as described in claim 1, characterized in that after being exposed to X-rays of 1 kGy or more, the mutual conductance is 0.5 mS / mm or more under room temperature conditions.

3. The field effect transistor according to claim 2, characterized in that after being irradiated with 5 MGy of X-rays, the mutual conductance is 0.5 mS / mm or more under room temperature conditions.

4. A field effect transistor according to claim 1, characterized in that after being irradiated with X-rays of 1 kGy or more, the magnitude of fluctuation in threshold voltage is 3 V or less under room temperature conditions.

5. 5. The field effect transistor according to claim 1, wherein the insulating layer contains aluminum oxide.

6. 6. The field effect transistor according to claim 1, wherein the source electrode, the drain electrode and the gate electrode are made of at least one of ruthenium, iridium, platinum and molybdenum.

7. The field effect transistor according to any one of claims 1 to 6, further comprising a recovery electrode which is an independent electrode different from any of the source electrode, the drain electrode and the gate electrode, and which recovers circuit characteristics by at least one of recovering defects by thermal recovery and extracting electric charges.

8. After being irradiated with 5 MGy of X-rays, the leakage current of the gate electrode was 10 -6 8. The field effect transistor according to claim 1, wherein the resistance of the field effect transistor is equal to or less than 1000 Ω / s.

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