JPS6236606B2 - - 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 provided on one side and a ZnO sintered body or a ZnO sintered body containing an appropriate additive. Due to surface barriers. However, the surface barrier type ZnO varistor obtained in this way is still weak against surges and has a small voltage nonlinearity index a [voltage nonlinearity index a is I=(V/C) ^a :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 However, some impurities tend to diffuse into the ZnO sintered body, and since the silver is applied in the form of a paste, its thickness becomes uneven, so the amount of impurities diffusing into the ZnO sintered body varies depending on the location. It depends on the situation. On the other hand, a technique described in Japanese Patent Application Laid-Open No. 52-66982 has been devised to improve such a varistor. This conventional method uses bismuth (Bi) or its compound and manganese (Mn) in a ZnO sintered body.
Or by applying a substance containing that compound, Zn
The properties can be easily controlled by diffusing Bi and Mn components into the sintered body. However, since this conventional method utilizes the non-ohmic nature inside the sintered body, the voltage-current characteristics are symmetrical, and in order to obtain a low rise voltage, the thickness of the element must be made thinner. However, it was difficult to obtain one with low pressure due to its mechanical strength. 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, voltage nonlinear resistors that have asymmetric voltage-current characteristics and are strong against surges are desired for use in DC circuits. 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 (or a ZnO layer with appropriate additives),
2 is a layer consisting of at least one of cobalt oxide (Co ₂ O ₃ ) or manganese oxide (MnO ₂ ); 3;
4 is an ohmic electrode. With such a configuration, along the interface between the ZnO layer 1 and the layer 2 made of at least one of Co ₂ O ₃ and MnO ₂
Since the shot key barrier 5 is formed on the ZnO layer 1 side, current easily flows when a voltage is applied using the electrode 3 as the anode, and on the other hand, when a voltage is applied using the electrode 4 as the anode, a Since the Yottoky barrier 5 is reverse biased, an element exhibiting asymmetric voltage nonlinearity is obtained in which current does not flow up to a certain voltage, and current suddenly flows above that voltage. 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. After that, after thoroughly cleaning with an organic solvent, a high-frequency sputtering device is used to sputter onto the main surface of a substrate made of a mirror-polished ZnO sintered body.
A sputtered film of CO ₂ O ₃ or MnO ₂ was provided. 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 a and surge resistance of the obtained device. Considering that a surge current of several tens of amperes at most flows in a low-voltage circuit, the surge withstand capacity is 50 A with a shock current waveform of 8 x 20 microseconds.
Voltage V across the element at 1 mA after applying
Expressed as a rate of change of _{1 nA} .

[Table] Example 2 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, the same method as described in Example 1 was applied.
A Co ₂ O ₃ or MnO ₂ sputtered film was provided, and an Al evaporated electrode was further attached thereon, and the electrical properties of each film were measured. The structure of the obtained device is shown in FIG. In the figure, 6 is a Co ₂ O ₃ or MnO ₂ film, 7 is a ZnO film, 8,
9 is an electrode, and 10 is a glass substrate. Such a device also exhibited asymmetrical voltage-current characteristics as shown in FIG. 2, remarkable non-ohmic properties, and excellent stability against surges. The electrical characteristics of the obtained device are shown in Table 2. Note that the area of the electrode 8 in this case is set to be almost the same as in the first embodiment.

[Table] Example 3 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. Thereafter, Co ₂ O 3 having the composition shown in Table 3 was applied to one surface of the mirror-polished ZnO single crystal, which was thoroughly washed with an organic solvent and polished using a high-frequency sputtering device, using the same method as shown in Examples 1 and _2. Alternatively, a sputtered MnO ₂ film was provided, and then an Al evaporation electrode was attached. The sample prepared in this way also has an asymmetrical voltage as shown in Figure 2.
The current characteristics are shown. The electrical characteristics of the obtained device are shown in Table 3. 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] In this example described above, Co ₂ O ₃ and MnO ₂
Although we have not discussed the case where both are added at the same time, it is clear that devices with almost similar characteristics can be obtained using these composite films. Further, the sputtering target of Co ₂ O ₃ or MnO ₂ component described in Example 2 can be made by a conventional ceramic method. The same applies to the ZnO sputtering target described in Example 2.
That is, after forming Co ₂ O ₃ or MnO ₂ or ZnO into a predetermined shape, it may be fired in air. ₁₀₀₀ ℃~1200 for _Co2O3 or _MnO2
°C and 1000 °C to 1400 °C for ZnO sintered bodies
calcination is preferred. A suitable time for firing is 1 to 5 hours. 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 and other properties of the sputtered film can be controlled by changing the amount of oxygen gas replaced. For the Co ₂ O ₃ or MnO ₂ layer in the example,
Although it was formed to a thickness of 500 to 1000 Å, similar characteristics were obtained even when it was thinner than 500 Å, and similar characteristics were obtained when it was thicker than 1000 Å. 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 a sintered body is used 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 current, for example, one of 50 Å or more can be easily produced. It is also extremely easy to dope additives to improve the properties of ZnO. It is obtained by adding an appropriate amount of an organic binder to the ZnO sintered body and ZnO powder used in Example 1, forming it into a disk, and then firing it in air at 1000°C to 1400°C. A suitable 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 2,
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 2, 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. Furthermore, the present invention utilizes a shot key barrier formed at the ZnO side interface, and the properties of the shot key barrier are 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 rising voltage of 6V and a barrier type ZnO Varitas 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
Although it is inferior to Zener diodes in terms of a, it is significantly superior in terms of 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, it is superior in both a and surge resistance. 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 configuration diagram of one embodiment of a voltage nonlinear resistor according to the present invention, Fig. 2 is a diagram showing typical voltage-current characteristics, and Fig. 3 is a structural diagram of another embodiment. . 1... ZnO layer, 2, 6... A layer consisting of at least one of cobalt oxide or manganese oxide,
3, 4, 8, 9...Ohmic electrode, 5...Shot key barrier, 7...ZnO film, 10...Glass substrate.

Claims (1)

[Claims] 1. A first layer containing ZnO as a main component and a second layer consisting of at least one of cobalt oxide or manganese oxide, wherein the first layer is in contact with the second layer. A voltage nonlinear resistor, further comprising an electrode provided on the other surface of each of the first and second layers. 2. The voltage nonlinear resistor according to claim 1, wherein the first layer is a polycrystalline material. 3. The voltage nonlinear resistor according to claim 1, wherein the first layer is a sputtered film. 4 Claim 1 in which the first layer is a single crystal
Voltage non-linear resistor as described in Section. 5. A film made of at least one of cobalt oxide or manganese oxide is formed on one surface of a substrate containing ZnO as a main component 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 film and on the other side of the substrate. 6. Voltage non-linearity according to claim 5, which uses a sintered body obtained by granulating and molding a powder mainly composed of ZnO and firing it in air at 1000°C to 1400°C as a substrate. Method of manufacturing resistors. 7. The voltage according to claim 5, 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.