JPS6236607B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a voltage nonlinear resistor that has asymmetric voltage-current characteristics and is stable against surges. Traditionally, Zener diodes and ZnO have been used as voltage nonlinear resistors that exhibit asymmetric voltage-current characteristics.
A surface barrier type ZnO varistor is known, in which a non-ohmic electrode is provided on one side of a sintered body and an ohmic electrode is provided on the other side. Zener diodes utilize the tunnel effect or avalanche effect of a silicon single-crystal PN junction when reverse biased. Zener diodes exhibit excellent voltage nonlinearity, but are susceptible to surges (abnormal overvoltage). This is because it is technically difficult to create a uniform, large-area PN junction. Surface barrier type ZnO varistors are made by adding appropriate additives such as Bi ₂ O ₃ or Co ₂ O ₃ to either a ZnO sintered body or a ZnO sintered body containing appropriate additives added to improve properties. Apply a silver paste containing
The electrode is baked at 700 to 800°C to form a non-ohmic electrode, and the other side is provided with an ohmic electrode such as an aluminum sprayed electrode. The thus obtained surface barrier type ZnO varistor exhibits voltage nonlinearity in which current suddenly flows around 4 to 8 V. This is formed at the interface between a silver electrode containing an appropriate additive such as Bi ₂ O ₃ or Co ₂ O ₃ and a ZnO sintered body or a ZnO sintered body containing an appropriate additive, which is provided on one side. Due to surface barrier. However, the surface barrier type ZnO varistor obtained in this way is still weak against surges, and the voltage nonlinearity index α is small (the voltage nonlinearity index α is I=(V/C)〓:I is current, V is voltage, and C is a constant). This is because silver is applied in the form of a paste and then baked at 700 to 800°C, so a considerable amount of silver and additives diffuse into the ZnO sintered body and move from the electrode area into the ZnO sintered body. A slowly decreasing impurity distribution is formed, so a clear surface barrier is not formed, and the diffusion rate at the grain boundary in the ZnO sintered body is much higher than the diffusion rate inside the ZnO particles, so Some impurities tend to diffuse into the sintered body,
Furthermore, since the silver is applied in the form of a paste, its thickness becomes uneven, which causes the amount of impurities to diffuse into the ZnO sintered body to vary depending on the location. In recent years, the use of semiconductors in electrical equipment and electronic equipment has progressed, and in particular, most low-voltage circuits are made of semiconductors. However, as is well known, these semiconductor devices are vulnerable to surges. Therefore, surge countermeasures are required for devices that are particularly susceptible to surges, such as various signal control circuits that are susceptible to lightning surges, printer circuits that generate switching surges, and various control circuits for automobiles. Among these, for those used in linear circuits, voltage nonlinear resistors with asymmetrical resistance-current characteristics and strong resistance to surges are desired. The present invention was made in view of this situation,
The present invention provides a voltage nonlinear resistor that has asymmetric voltage-current characteristics and is strong against surges. FIG. 1 shows the basic structure of a voltage nonlinear resistor according to the present invention. In the figure, 1
is a ZnO layer, 2 is a rare earth oxide layer (or a rare earth oxide layer containing an appropriate additive), and 3 and 4 are ohmic electrodes. With this configuration, a shot key barrier 5 is formed on the ZnO layer 1 side along the interface between the ZnO layer 1 and the rare earth oxide layer 2, so that when a voltage is applied using the electrode 3 as an anode, On the other hand, if a voltage is applied using the electrode 4 as an anode, the shot key barrier 5 will be reverse biased, so the current will not flow until a certain voltage, and will suddenly flow above that voltage. An element exhibiting asymmetric voltage nonlinearity in which current flows is obtained. By the way, as a technology for constructing voltage nonlinear elements using ZnO and rare earth oxides,
There are methods shown in Japanese Patent Publication No. 40039 and Japanese Patent Publication No. 52-40749, but this conventional technology is
It has a structure in which rare earth oxides are dispersed inside the ZnO sintered body, and is thought to take advantage of the non-ohmic nature inside the sintered body, and the voltage-current characteristics are as follows.
Unlike the present invention, it is believed that only symmetrical voltage nonlinearity can be obtained; therefore,
In this conventional device, in order to obtain a low rise voltage, it is sufficient to reduce the thickness of the element, but it is considered difficult to obtain a rise voltage of 10 V or less in terms of mechanical strength. On the other hand, in the present invention, an element exhibiting asymmetric voltage nonlinearity can be obtained, and therefore an element suitable for a low voltage circuit can be easily obtained. Example 1 ZnO powder was molded into a diameter of 12 mm by a normal molding method.
mm, molded to a thickness of 1.5 mm and heated to 1250°C in air.
It was baked for 2 hours. This sintered body was a polycrystalline body. Thereafter, both main surfaces of the sintered body were polished, and one surface in particular was mirror polished using fine alumina powder. Thereafter, after thorough cleaning with an organic solvent, a sputtering film of a rare earth oxide was provided on the main surface of the substrate made of a mirror-polished ZnO sintered body using a high-frequency sputtering device. Furthermore, Al vapor-deposited electrodes were provided on both sides of the obtained device. The electrical characteristics of each were measured. The device thus obtained exhibited asymmetrical voltage nonlinear characteristics as shown in FIG. 2, excellent non-ohmic properties, and stable properties against surges.
Table 1 shows the voltage non-linearity index α and surge resistance of the obtained device. Considering that a surge current of only a few tens of amperes flows at most in a low-voltage circuit, the surge withstand capacity is 50A with a shock current waveform of 8 x 20 microseconds.
Voltage across the element at 1mA after applying
It was expressed as a rate of change in V ₁ mA.

[Table] Example 2 A rare earth oxide having the composition shown in Table 2 was added as a main component on a ZnO sintered body substrate obtained in the same manner as in Example 1. A film was formed. Furthermore, Al is placed on both ends of the obtained device.
A vapor-deposited electrode was installed and its electrical properties were measured. The voltage-current characteristics showed asymmetric characteristics as described in Example 1, but by adding cobalt oxide and manganese oxide to the rare earth oxide film, a product with even better α- and surge resistance was obtained. Obtained.

【table】

[Table] Example 3 Al was vacuum-deposited on a glass substrate, and a high-frequency sputtering device was used to deposit Al on this Al-deposited film.
A sputtered ZnO film was provided. Further, on this ZnO sputtered film, a sputtered film whose main component is a rare earth oxide containing various additives with the composition shown in Table 3 is provided by the same method as described in Example 2, and then Al is further deposited on top of the sputtered film. Electrodes were attached and the electrical properties of each were measured. The structure of the obtained device is shown in FIG. In the figure, 6 is a rare earth oxide main component film containing additives, 7
is a ZnO film, 8 and 9 are electrodes, and 10 is a glass substrate. The device thus obtained also exhibited asymmetric voltage-current characteristics as shown in FIG.
Again, in this case, as in the case of Example 2,
By adding additives to the rare earth oxide, a device with better α and surge resistance was obtained.
The electrical characteristics of the obtained device are shown in Table 3. Note that the area of the electrode 8 in this case is set to be approximately the same as the area of the electrode provided in Examples 1 and 2.

【table】

[Table] Example 4 A ZnO single crystal was cut into a piece having a diameter of 2 mm and a thickness of 0.3 mm, and both sides thereof were polished. In particular, one surface was polished to a mirror surface using fine alumina powder. Thereafter, it was thoroughly washed with an organic solvent, and a rare earth oxide or A sputtered film of rare earth oxide containing additives was provided, and then an Al vapor deposition electrode was attached. The sample prepared in this manner also exhibited asymmetric voltage-current characteristics as shown in FIG. The electrical characteristics of the obtained device are shown in Table 4.
In this case, the surge resistance was measured under the same current density conditions as in Examples 1 and 2 due to the electrode area.

[Table] As can be seen from the examples described above, the element having the basic structure shown in FIG. 1 is asymmetric and exhibits significant nonlinearity. Although voltage nonlinearity can be obtained to a certain degree even without adding any additives to the rare earth oxide layer formed on the ZnO substrate,
Addition of Co ₂ O ₃ and MnO ₂ significantly improves the properties. This is thought to be because the added additive forms traps and surface states in the sputtered rare earth oxide film and at the interface with ZnO. Therefore, regarding the effect of the amount of additives in rare earth oxides, the effects of the second, third, As shown in Table 4, similar effects can be expected. And the second
0.1-40 mol% _Co2O3 , _MnO2 as shown in _the table
When added in the range of 0.1 to 40 mol%, an improvement effect can be obtained. In the examples, Pr ₂ O ₃ ,
Although the explanation has been made using only examples of Nd ₂ O ₃ and Sm ₂ O ₃ , it is clear from these examples that similar effects can be expected even when other rare earths are used. In addition, each rare earth oxide was used alone in the examples, but
It is also clear that devices with similar characteristics can be obtained using these composite films. For the rare earth oxide layer or the rare earth oxide main component layer containing additives in the examples, the thickness is 500 to 1000 Å.
However, similar characteristics were obtained even when the thickness was less than 500 Å. Similar characteristics were also obtained when the thickness was greater than 1000 Å. In any case, the atmosphere during sputtering is preferably an inert gas atmosphere such as argon, or an atmosphere in which about 50% of the atmosphere is replaced with oxygen gas. The resistance value of the sputtered film can be controlled by the amount of oxygen gas replaced. In addition, in the example, the electrode was formed by vacuum evaporation.
Although Al electrodes were used, as can be seen from the above description, the electrodes are not limited to Al as long as they are ohmic electrodes, and may be formed by thermal spraying, baking, or other methods instead of vapor deposition. Next, when using a sintered body for the ZnO substrate part,
Since a large-area sintered body can be easily and inexpensively produced, a large-area element can be produced, and therefore a large surge voltage, for example, 50 A or more, can be easily produced. It is also extremely easy to dope additives to improve the properties of ZnO. The ZnO sintered bodies used in Examples 1 and 2 are obtained by adding an appropriate amount of organic binder to ZnO powder, molding it into a disk, and then sintering it in air at 1000°C to 1400°C. . The appropriate firing time is 1 hour to 5 hours. Note that if the firing temperature is lower than 1000°C, sintering will not be sufficient, and if it is higher than 1400°C, ZnO will evaporate and the substrate will not be dense. On the other hand, when a sputtered film is used as ZnO, since the resistance of the ZnO portion is low, an element with low voltage can be obtained even in a large current range. Regarding the ZnO sputtered film in Example 3,
Although it is formed to a thickness of 5,000 to 10,000 Å, in principle, the thickness may be sufficient to form a shot key barrier (approximately 300 Å) or more. Although glass was used for the substrate in Example 3, it is not necessary to be limited to glass; a stable insulator that can withstand heat generation during sputtering, such as a sintered alumina substrate or a sintered magnesia substrate, may also be used. good. Furthermore, when a single crystal is used as ZnO, there are fewer defects on the bonding surface, so it is possible to obtain an element with long-term stability and stability against repeated surges. Further, the present invention utilizes a shot-key barrier formed at the ZnO side interface, and the properties of the shot-key barrier will be almost similar if the main component is ZnO. Therefore, the present invention is not limited to substrates made only of ZnO, but includes substrates mainly composed of ZnO to which additives are added to improve characteristics, for example, by lowering the specific resistance of ZnO, when applying an impact current. Added Al ₂ O ₃ etc. to reduce the voltage rise of
A substrate containing ZnO as a main component may also be used. However, in either method, it is possible to obtain a voltage nonlinear resistor with asymmetric voltage-current characteristics and an element that is strong against surges. For comparison, we will discuss representative examples of the characteristics of a Zener diode with a rise voltage of 6V and a barrier type ZnO varistor using ZnO.
100 or more, 5, and the former destroys the surge resistance, and the latter -14.8%. Note that the Zener diode is a normal silicon Zener diode. Surface barrier type ZnO varistors are preheated to 1350℃.
On one main surface _of the _ZnO sintered body obtained by firing for ₁ hour at After applying silver paste and firing in air at 700°C for 1 hour, a sprayed Al electrode was provided on the other side. As described above, the voltage nonlinear resistor of the present invention is inferior to the Zener diode in α, but
It has excellent surge resistance. This is because, as mentioned above, it is difficult to create a uniform and large-area PN junction with a Zener diode. On the other hand, compared to surface barrier type ZnO varistors, both α and surge resistance are superior. This is thought to be because, as described above, in a surface barrier type varistor, a surface barrier is created by diffusion, which inevitably causes non-uniformity in the amount and distance of diffusion of impurities.

[Brief explanation of the drawing]

FIG. 1 is a basic structural diagram of one embodiment of the voltage nonlinear resistor according to the present invention, FIG. 2 is a diagram showing typical voltage-current characteristics, and FIG. 3 is a diagram of the configuration of another embodiment. . 1... ZnO layer, 2, 6... Rare earth oxide layer or rare earth oxide layer containing additives, 3, 4, 8, 9
...Ohmic electrode, 5...Shot key barrier,
7...ZnO film, 10...Glass substrate.

Claims (1)

[Scope of Claims] 1. A first layer containing ZnO as a main component and a second layer containing at least a rare earth oxide;
A voltage nonlinear resistor, characterized in that the layer is in contact with the second layer, and further an electrode is provided on the other surface of each of the first and second layers. 2. The voltage nonlinear resistor according to claim 1, using praseodymium oxide, neodymium oxide, or samarium oxide as the rare earth oxide. 3. Claim 1, wherein the second layer contains at least 0.1 to 40 mol% of cobalt in the form of CO ₂ O ₃ and 0.1 to 40 mol % of manganese in the form of MnO ₂ Voltage nonlinear resistor as described. 4. The voltage nonlinear resistor according to claim 1, wherein the first layer is a polycrystalline sintered body. 5. The voltage nonlinear resistor according to claim 1, wherein the first layer is a sputtered film. 6 Claim 1 in which the first layer is a single crystal
Voltage non-linear resistor as described in section. 7 A film mainly composed of rare earth oxides is formed on one side of a substrate mainly composed of ZnO by sputtering in an inert gas or an atmosphere consisting of an inert gas and oxygen gas. A method for manufacturing a voltage nonlinear resistor, comprising providing electrodes on the top and on the other side of the substrate. 8. Voltage nonlinearity according to claim 7, in which the substrate is a sintered body obtained by granulating and molding a powder containing ZnO as a main component and firing it in air at 1000°C to 1400°C. Method of manufacturing resistors. 9. The voltage according to claim 7, in which the substrate is formed by forming an electrode film on a heat-resistant substrate and further forming a ZnO film containing ZnO as a main component by a sputtering method. Method of manufacturing non-linear resistors.