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] The present 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 this high radiation resistance. Non-Patent Document 1 describes a metal-semiconductor field-effect transistor (MESFET), an example of a field-effect transistor using a diamond semiconductor. The MESFET in Non-Patent Document 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 and drain electrodes formed thereon, and a gate electrode formed on the p-drift layer. Ruthenium is used for the gate electrode. It has been shown that when such a MESFET was irradiated with 5 MGy and 10 MGy of X-rays, the maximum drain current and transconductance of the MESFET remained nearly constant with the irradiation. [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] J. Umezawa, S. Omagari, and Y. Mokuno, "Characterization of X-ray Radiation Hardness 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. Summary of the Invention [Problem to be solved by the invention]

[0004] As described above, the MESFET of Non-Patent Document 1 exhibits high resistance to X-ray irradiation. However, the field-effect transistor of Non-Patent Document 1 has a low mutual conductance of 0.01 mS / mm, and may not satisfy the circuit characteristics required for an electronic component.

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

[0006] The field effect transistor of the present invention comprises a non-doped diamond layer whose surface is hydrogen-terminated and formed on the undoped diamond layer with a hydrogen-terminated region sandwiched therebetween. and a first p+ diamond layer formed on the first p+ diamond layer. a metallic source electrode and a metallic drain electrode formed on the second p+ diamond layer; an electrode and an insulating layer formed on the hydrogen-terminated region of the undoped diamond layer; and a gate electrode formed on the insulating layer. At this time , 1 kGy or more or After 5MGy of X-ray irradiation, the mutual conductance is 0.5mS / mm or more at room temperature. It is also preferable that the magnitude of the change in threshold voltage is 3 V or less under room temperature conditions after exposure to X-rays of 1 kGy or more.

[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. For example, 1 kGy or more After 5MGy X-ray irradiation, the circuit characteristics showed that the mutual conductance was 0.5mS / mm or more under room temperature conditions. Alternatively, after exposure to X-rays of 1 kGy or more, the circuit characteristics can be such that the magnitude of the threshold voltage fluctuation is 3 V or less under room temperature conditions. For example, 5MGy of X-ray irradiation is sufficient for the electronic circuit to which this invention is applied to be used in a nuclear reactor. etc.It is assumed that the present invention will be used in such environments. According to the present invention, circuit characteristics such as a transconductance of 0.5 mS / mm or more can be ensured in such environments. This makes it possible to ensure the circuit characteristics required for electronic circuits even in harsh, high-radiation environments such as inside a nuclear reactor during normal operation, a severe accident, or decommissioning. Note that the fact that circuit characteristics can be ensured in such environments does not mean that the application of the present invention is limited to inside a nuclear reactor. The present invention can also be applied in various environments, such as accelerators, radiation therapy, nuclear fusion reactors, space environments, and aerospace environments.

[0008] In the present invention, the insulating layer preferably contains aluminum oxide, which improves the radiation resistance of the insulating layer.

[0009] In the present invention, it is also preferable that the source electrode, drain electrode, and gate electrode each contain at least one of ruthenium, iridium, platinum, and molybdenum. This improves the radiation resistance of the electrodes. In this case, an electrode may be used that combines at least one of ruthenium, iridium, platinum, and molybdenum with another metal such as gold. For example, a layer of another metal such as gold may be stacked on a layer of at least one of ruthenium, iridium, platinum, and molybdenum.

[0010] In addition, in the present invention, it is preferable that a recovery electrode, which is an independent electrode different from any of the source electrode, the drain electrode, and the gate electrode, is further provided for recovering circuit characteristics by at least one of recovering defects by thermal recovery and extracting charges. This makes it possible to recover circuit characteristics deteriorated by irradiation with radiation. 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 lower than the operating drain current. -6 It is preferable that the value is equal to or less than 1.0 times. As a result, the present invention realizes a field effect transistor in which leakage current is suppressed. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a conceptual diagram showing the structure of a field-effect transistor according to one embodiment of the present invention. [Figure 2] 2 is a graph showing the drain voltage-drain current characteristics of one embodiment of the field effect transistor of Fig. 1. Fig. 2(a) relates to the characteristics before X-ray irradiation, Fig. 2(b) relates to the characteristics when irradiated with 10 kGy of X-rays, Fig. 2(c) relates to the characteristics when irradiated with 100 kGy of X-rays, and Fig. 2(d) relates to the characteristics when irradiated with 1 MGy of X-rays. [Figure 3] 2 is a graph showing gate bias-drain current characteristics of the field effect transistor of FIG. 1 according to an embodiment. [Figure 4] Figure 4(a) is a graph showing the maximum drain current versus the cumulative dose of X-rays, Figure 4(b) is a graph showing the transconductance versus the cumulative dose of X-rays, and Figure 4(c) is a graph showing the change in threshold voltage versus the cumulative dose of X-rays. [Figure 5] 4 is a graph showing the sheet resistance according to the example of FIG. 3. [Figure 6] FIG. 6 is a conceptual diagram for explaining the mechanism by which the characteristics shown in the graphs of FIG. 4(c) and FIG. 5 appear. [Figure 7] 10 is a graph showing leakage current versus cumulative dose of X-rays in another embodiment of the field effect transistor of FIG. [Figure 8] 2 is a graph showing leakage current versus gate voltage when iridium is used as an electrode for the field effect transistor of FIG. 1. [Figure 9] 2 is a graph showing leakage current versus gate voltage when platinum is used as an electrode of the field effect transistor of FIG. 1. [Figure 10] 2 is a graph showing leakage current versus gate voltage when molybdenum is used as an electrode for the field effect transistor of FIG. 1. [Figure 11] 2 is a graph showing leakage current versus gate voltage when a multi-layer electrode made of molybdenum and gold is used as the electrode of the field effect transistor of FIG. 1. DETAILED DESCRIPTION OF THE INVENTION

[0012] A field effect transistor 1 according to one 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 layered 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 layered.

[0013] Nitrogen-doped diamond is used for the semi-insulating diamond substrate layer 10. Its thickness is 50 to 500 μm, and the nitrogen concentration is 10 17 ~10 20 atoms / cm 3 The provision of the nitrogen-doped semi-insulating diamond substrate layer 10 suppresses the short channel effect, the increase in drain conductance in the saturated region, and the decrease in output impedance. In order to achieve a mutual conductance of 0.5 mS / mm or more as described below, the defect density of the substrate must be 10 6 pieces / cm 2 It is desirable that the following:

[0014] The hydrogen-terminated diamond layer 11 is formed on a semi-insulating diamond substrate layer 10. The hydrogen-terminated diamond layer 11 uses undoped 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 output impedance. A hydrogen-termination conductive layer is formed in a region H where the insulating layer 18 is laminated on the lamination surface 11a, which is one surface of the hydrogen-terminated diamond layer 11. The hydrogen-terminated diamond layer 11 is formed, for example, by using a CVD (chemical vapor deposition) method. The hydrogen-termination conductive layer is formed by exposing the surface of the diamond to hydrogen plasma when forming the layer by the CVD method.

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

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

[0017] The source electrode 14 is formed on the p+ diamond layer 12, the drain electrode 15 is formed on the p+ diamond layer 13, and the gate electrode 16 is formed 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, the drain electrode 15, the gate electrode 16, and the back gate electrode 17. These electrodes are formed using, for example, an RF sputtering method.

[0018] The recovery electrode 21 is formed on the stacking 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 stacked. 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 a metal electrode body whose surface is protected with ruthenium. Any metal can be used for the electrode body as long as it can supply 10 W or more of power. A power supply circuit 31 is connected to the recovery electrode 21. The power supply circuit 31 supplies 10 W or more of power by passing a current through the recovery electrode 21. The heat generated by this power supply from the recovery electrode 21 increases the temperature of the field-effect transistor 1. The recovery electrode 21 is formed, for example, using an RF sputtering method.

[0019] The recovery electrode 22 has a portion 22a extending along the deposition 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. The portion 22a is formed in a region of the deposition surface 11a adjacent to the region where the p+ diamond layers 12 and 13 and the insulating layer 18 are deposited. The portion 22b extends from the end of the portion 22a along the side end surface 11b to the semi-insulating diamond substrate layer 10. This allows the recovery electrode 22 to be in contact with both the hydrogen-terminated diamond layer 11 and the semi-insulating diamond substrate layer 10. The recovery electrode 22 is made entirely of ruthenium, or a metal electrode body whose surface is protected with ruthenium. 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 pulse voltage signal to the recovery electrode 22. The pulse voltage signal is, for example, a signal in which a state in which a voltage of +100 V continues for a predetermined time and a state in which a voltage of −100 V continues for a predetermined time are alternately switched at predetermined time intervals. The recovery electrode 22 is formed by, for example, sputtering.

[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 being exposed to a cumulative X-ray dose of 5 MGy. The inventors of the present invention have conducted extensive research to ensure circuit characteristics in such high radiation environments by adopting the structure of this embodiment, in which p+ diamond layers 12 and 13 are provided on 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 examples below, circuit characteristics, such as a transconductance of 0.5 mS / mm or more at room temperature, can be ensured even after being exposed to a cumulative X-ray dose of 5 MGy. Furthermore, similar circuit characteristics can be ensured even in high-temperature (e.g., 450°C) environments.

[0021] The 5MGy value corresponds to the cumulative radiation dose over a period of approximately one week in a high-radiation environment inside a nuclear reactor during a severe accident, and was calculated through simulation. Furthermore, a gain of 2 mS or greater at room temperature is required to achieve low-noise circuits. Considering application to electronic circuits used inside nuclear reactors, a gain of 2 mS or greater is preferable in the temperature range from room temperature to 450°C. On the other hand, to enhance radiation resistance, a gate width of 4 mm or less is preferable. Therefore, a gain of 0.5 mS / mm or greater is required. Furthermore, achieving a transconductance of 0.5 mS / mm reduces noise and enables signal amplification up to a high frequency band. Furthermore, to amplify pulse signals without degradation, operation with high-frequency signals of 100 MHz or greater is preferable.

[0022] The field-effect transistor 1 according to this embodiment also includes recovery electrodes 21 and 22. The recovery electrodes 21 and 22 recover elements degraded by radiation exposure. There are two types of degradation due to radiation exposure. The first type is degradation caused by charge accumulation in at least one of the hydrogen-terminated diamond layer 11, the p+ diamond layers 12 and 13, the semi-insulating substrate 10, and the insulating layer 18, resulting in threshold fluctuations, drain current fluctuations, and the like. This type of degradation is recovered by supplying a pulse voltage signal from the pulse supply circuit 32 to the recovery electrode 22, thereby extracting charge from the accumulated layer, as described above. The second type of degradation is degradation caused by defects generated in the insulating layer 18, resulting in threshold fluctuations, a decrease in transconductance, a decrease in drain current, and the like. This type of degradation is recovered by repairing point defects by supplying power from the power supply circuit 31 to the recovery electrode 21 and raising the temperature of the field-effect transistor 1 to 600°C or higher, as described above.

[0023] In this embodiment, ruthenium is used for the source electrode 14, drain electrode 15, and gate electrode 16. The recovery electrodes 21 and 22 are either entirely made of ruthenium or have a metal electrode body whose surface is protected with ruthenium, thereby improving 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 (for example, drain voltage-drain current characteristics) due to radiation irradiation of a predetermined cumulative dose (for example, 10 kGy) or more is saturated. It is preferable that the drain conductance (output impedance) in the saturated region of the drain voltage-drain current characteristics when irradiated with radiation of 10 kGy to 1 MGy is 5 MΩmm or more. It is preferable that the drain conductance in the linear region of the drain voltage-drain current characteristics when irradiated with radiation of 10 kGy to 1 MGy is 2 MΩmm or less. It is preferable that the gate voltage V GS <10 leakage current at -3V -7A / mm or less is preferable. The subthreshold swing when irradiated with 10 kGy to 1 MGy of radiation is preferably 300 mV / decade or less. From the time when no radiation is applied to the time when irradiated with 5 MGy of radiation, it is preferable that at least one or both of the absolute values ​​of the maximum drain current and the mutual conductance does not decrease substantially. From the time when no radiation is applied to the time when irradiated with 5 MGy of radiation, it is preferable that the fluctuation range of the threshold voltage is 3 V or less.

[0025] [Example 1]

[0026] An example of a field-effect transistor according to the present invention will be described below. The field-effect transistor according to this example corresponds to the field-effect transistor according to the above-described embodiment, except that the recovery electrodes 21 and 22 are omitted. 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 a semi-insulating diamond substrate layer 10) using a plasma CVD apparatus (Seki Technotron, AX5010-INT) with a methane concentration of 0.5%. Next, a mask was formed using the plasma CVD apparatus to selectively form p+ diamond layers 12 and 13 in predetermined regions on the hydrogen-terminated diamond layer 11. Next, 0.4 μm-thick p+ diamond layers 12 and 13 were formed in the unmasked regions of the diamond layer 11 using the HF-CVD method. Next, the mask was removed, and an 83 nm-thick aluminum oxide layer was formed at 350°C using an ALD plasma processing apparatus, followed by etching using a reactive ion etching apparatus to form an insulating layer 18. Next, a source electrode 14, a drain electrode 15 and a gate electrode 16 made of ruthenium were formed by RF sputtering using a sputtering device.

[0027] The performance of the field-effect transistor 1 fabricated as described above in a high-radiation environment was evaluated as follows. First, in a room-temperature environment, the field-effect transistor 1 was continuously irradiated with X-rays from the X-ray irradiator while maintaining a constant positional relationship between the field-effect transistor 1 and the X-ray irradiator. This was repeated in stages until the irradiated radiation reached 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 before and immediately after X-ray irradiation. The radiation dose to the field-effect transistor 1 reaching each of the above values ​​was determined based on the dose per unit time and the exposure time using a cellulose triacetate film dosimeter. The dose was determined to be 1.1 kGy / min.

[0028] The results of the performance evaluation are shown in Figures 2 through 4. Graphs 2(a) through 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 X-ray irradiation, Figure 2(c) shows the characteristics after 100 kGy X-ray irradiation, and Figure 2(d) shows the characteristics after 1 MGy X-ray irradiation. Although there was some initial fluctuation in the characteristics before the 10 kGy X-ray irradiation, the lack of significant change between the graphs in Figures 2(b) through 2(d) clearly indicates that the device characteristics change little due to radiation exposure. This initial fluctuation is also observed in high-temperature environments, and stabilization occurs after approximately one hour of treatment at approximately 400°C. According to Figures 2(b) through 2(d), the drain conductance (output impedance) in the saturated region is greater than 5 MΩmm. The drain conductance in the linear region is less than 2 MΩmm.

[0029] The graph in Figure 3 shows the gate voltage-drain current characteristics when the drain voltage is -10V. In the graph, squares indicate characteristics when irradiated with 100kGy of X-rays, crosses indicate characteristics when irradiated with 100kGy of X-rays, and circles indicate characteristics when irradiated with 1MGy of X-rays. GS <10 leakage current at -3V-7 The subthreshold swing is 238, 215, and 264 mV / decade when irradiated with 100 kGy, 300 kGy, and 1 MGy X-rays, respectively. In other words, all subthreshold swings are 300 mV / decade or less. This suggests that the interface state density does not increase even when the irradiated X-rays reach a high cumulative dose. Furthermore, the shift in threshold voltage after the initial fluctuation is 1 V or less.

[0030] The graph in Figure 4(a) shows the maximum drain current versus the X-ray irradiation. The graph in Figure 4(b) shows the transconductance versus the X-ray irradiation. The graph in Figure 4(c) shows the change in threshold voltage versus the X-ray irradiation. In all of Figures 4(a) to 4(c), the drain voltage was set to -10 V. According to Figure 4(a), the maximum drain current J DS The absolute value of g increases from unirradiated to 10 kGy irradiation, and fluctuates initially, but remains almost constant at higher irradiation levels. In other words, the absolute value of the maximum drain current does not decrease from the initial fluctuation to 1 MGy irradiation. Also, according to Figure 4(b), the transconductance g m The absolute value of increases in the initial fluctuation from unirradiated to 10 kGy irradiation, and saturates and becomes almost constant at higher irradiation. In other words, the transconductance does not decrease at all from the initial fluctuation to 1 MGy irradiation. Also, according to Figure 4(c), the threshold voltage V T The absolute value of increases from unirradiated to 10 Gy irradiation, showing an initial fluctuation, but saturates and assumes a substantially constant value at higher irradiation levels. The threshold voltage fluctuation range from the initial fluctuation to 1 MGy irradiation is 1 V or less. In another example, similar performance evaluation was performed using X-ray irradiation of 1 to 5 MGy, and the maximum drain current, transconductance, and threshold all saturated and assumed constant values. In other words, combining the results of this example with the results shown in FIG. 4, the absolute values ​​of the maximum drain current and transconductance do not decrease substantially from the initial fluctuation to 5 MGy irradiation, and the threshold fluctuation range is 1 V or less.

[0031] As a result of this example, as shown in Figure 5, the sheet resistance of the field-effect transistor 1 decreased as the cumulative dose increased up to about 100 kGy, and remained constant relative to the cumulative dose once it exceeded 100 kGy. The reason why the sheet resistance exhibits such characteristics depending on the X-ray dose is thought to be due to the following mechanism (see Figure 6). X-ray irradiation generates electron-hole pairs in the Al2O3 that forms the insulating layer 18. Because Al2O3 contains defects, the generated electrons are captured by the defects, inducing holes in the hydrogen-terminated diamond layer 11. The induction of holes leads to an increase in carrier density, improving conductivity and reducing sheet resistance. The defects here are thought to be defects that capture charge due to oxygen vacancies, impurities, or crystal structure disturbances.

[0032] Because there is a limit to the number of defects mentioned above, after a certain level of irradiation, almost all defects capture electrons and are no longer able to capture electrons. Therefore, once a certain level of irradiation (approximately 100 kGy in this study) is reached, new electron capture and hole induction hardly occur with irradiation beyond that level. Therefore, the sheet resistance remains stable with irradiation beyond a certain level.

[0033] Furthermore, as shown in Figure 4(c), the threshold voltage increases until the cumulative dose reaches approximately 100 kGy, and then remains constant once the dose exceeds 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 and removing holes from the channel. In this case, the threshold voltage can be thought of as the positive voltage that must be applied to turn the FET off.

[0034] If X-ray irradiation causes electrons to accumulate in the Al2O3 that forms the insulating layer 18 and induces holes on the hydrogen-terminated diamond layer 11 side, the density of holes on the surface of the hydrogen-terminated diamond layer 11 will increase, and therefore a correspondingly large positive voltage will need to be applied to remove the holes and turn off the element.

[0035] When irradiated up to a certain level (approximately 100 kGy in this study), electrons generated by the irradiation accumulate in Al2O3, increasing the density of holes on the surface of the hydrogen-terminated diamond layer 11, thereby raising the threshold voltage. However, when irradiated beyond this level, the defects are filled and almost no electron accumulation occurs. Therefore, the threshold remains stable for irradiations above a certain level.

[0036] [Example 2]

[0037] The leakage current of the gate electrode 16 was measured when the X-ray irradiation dose to the field-effect transistor 1 was varied up to 3 MGy. The field-effect transistor 1 was the same as in Example 1, except that each electrode had a structure in which a layer of gold was stacked on a layer of ruthenium (a structure in which the gold layer was formed on the surface of the ruthenium layer, thereby protecting the ruthenium layer from X-ray irradiation). Voltages of 0 V, 0 V, and 1 V were applied to the source electrode 14, the drain electrode 15, and the gate electrode 16, respectively. Figure 7 shows the results, showing the leakage current I versus the X-ray irradiation dose. gs The solid line shows the actual measured value of leakage current, and the dashed line shows the prediction using linear approximation to the actual measured value. According to this, the leakage current is predicted to be 5.2nA for an irradiation dose of 5MGy. This value is 10 times lower than the operating drain current of 17mA. -6 The electrode thickness was less than 100 μm. In this example, no change in the appearance of the electrode before and after X-ray irradiation, and no traces of interdiffusion between the ruthenium layer and the gold layer after X-ray irradiation were observed. In contrast, previous experiments by the present inventors have shown that when radiation is irradiated onto an element having an electrode using a nickel layer instead of a ruthenium layer, interdiffusion occurs between the nickel layer and the gold layer, and therefore 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. Field-effect transistors 1 used included one in which each electrode was made of iridium (hereinafter referred to as Example 3-Ir), one in which each electrode was made of platinum (hereinafter referred to as Example 3-Pt), one in which each electrode was made of molybdenum (hereinafter referred to as Example 3-Mo), and one in which each electrode was made of molybdenum and gold (hereinafter referred to as Example 3-Mo / Au). Example 3-Mo / Au had a structure in which each electrode had a layer of gold laminated on a layer of molybdenum (a structure in which the gold layer was formed on the surface of the molybdenum layer, protecting the molybdenum layer from X-ray irradiation). The 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, after 10 MGy of X-ray irradiation, -11 The leakage current increased to about 10 A. However, the leakage current was 10 mA compared to the operating current of 17 mA for the field-effect transistor 1. -8 The size is less than twice that of the conventional electrode, and it can be used as a radiation-resistant electrode.

[0041] <Modification>

[0042] The above is a description of a preferred embodiment of the present invention, but the present invention is not limited to the above-described embodiment, and various modifications are possible within the scope of the means for solving the problems.

[0043] For example, in the above-described embodiment, ruthenium is used for the source electrode 14, the drain electrode 15, the gate electrode 16, and the back gate electrode 17, thereby ensuring high radiation resistance for the electrodes. However, these electrodes may be made of other metals (e.g., molybdenum) whose surfaces are coated with gold to protect them from radiation.

[0044] 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. The insulating layer 18 may also be made of a combination of these materials or other materials. For example, the insulating layer 18 may be made of a material that combines aluminum oxide with another material.

[0045] In the above-described embodiment, the temperature of the field-effect transistor 1 is increased to 600° C. or higher by applying a pulse voltage signal to the recovery electrode 21, thereby recovering the deterioration of the insulating layer 18. However, the temperature of the field-effect transistor 1 may also be increased to 600° C. or higher by applying a pulse voltage signal to the gate electrode 16 or the back gate electrode 17, thereby recovering the deterioration of the insulating layer 18. In this case, the recovery electrode 22 may not be provided in 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 electrode 15 Drain electrode 16 gate electrode 18 Insulating layer 21, 22 Recovery electrode

Claims

1. an undoped diamond layer whose surface is hydrogen-terminated; first and second p+ diamond layers formed on the undoped diamond layer with a hydrogen-terminated region 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 undoped diamond layer; a gate electrode formed on the insulating layer, A field effect transistor characterized in that after being irradiated with X-rays of 1 kGy or more, the mutual conductance is 0.5 mS / mm or more under room temperature conditions.

2. A field effect transistor as described in claim 1, characterized in that after being exposed to 5 MGy of X-rays, the mutual conductance is 0.5 mS / mm or more under room temperature conditions.

3. A non-doped diamond layer having a hydrogen-terminated surface; first and second p+ diamond layers formed on the undoped diamond layer with a hydrogen-terminated region 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 undoped diamond layer; a gate electrode formed on the insulating layer, A field effect transistor characterized in that after being irradiated with X-rays of 1 kGy or more, the magnitude of the change in threshold voltage is 3 V or less under room temperature conditions.

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

5. 5. 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.

6. A non-doped diamond layer having a hydrogen-terminated surface; first and second p+ diamond layers formed on the undoped diamond layer with a hydrogen-terminated region 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 undoped diamond layer; a gate electrode formed on the insulating layer, A field effect transistor further comprising a recovery electrode, which is an independent electrode different from any of the source electrode, the drain electrode, and the gate electrode, for recovering circuit characteristics by at least one of recovering defects by thermal recovery and extracting charges.

7. A non-doped diamond layer having a hydrogen-terminated surface; first and second p+ diamond layers formed on the undoped diamond layer with a hydrogen-terminated region 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 undoped diamond layer; a gate electrode formed on the insulating layer, After receiving 5 MGy of X-ray irradiation, the leakage current of the gate electrode was 10 times lower than the operating drain current. -6 1. A field effect transistor characterized in that:

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