JPS63305573A - Superconductive three-terminal element - Google Patents

Abstract

PURPOSE:To enable the enlarging of voltage gains, by making first to third superconductors have superconductive transition temperatures equal to or larger that a liquid helium temperature and making superconductive gap energy of the third superconductor larger than that of the first and second superconductors. CONSTITUTION:This element is composed of a superconductor 10, a tunnel barrier 11, a superconductor 12, a tunnel barrier 13, and a superconductor 14. The superconductors 10, 12, 14 have respective superconductive transition temperatures equal to or larger than a liquid helium temperature. Superconductive gap energy of the superconductor 1 4 is made larger than that of the superconductors 10, 11. Since this element can be hence operated by application of a base.collector voltage larger then an emitter.base voltage required for implanting quasi-particles into a base electrode, large voltage and power gains can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a superconducting device, and is particularly directed to a superconducting three-terminal device consisting of two tunnel junctions.

(Prior art) A superconducting three-terminal device consisting of a superconducting electrode and two tunnel junctions is described in, for example, Uploaded Physics Letters 3.
2, No. 6, pp. 392-395. Figure 3 is a diagram for explaining a conventional superconducting three-terminal device, in which three superconductors 1 and 1 have the same superconducting gap energy Δ.
．． 2.3 and tunnel barriers 4 and 5 sandwiched between them. The superconductors 1.2.3 form emitter, base and collector electrodes, respectively. FIG. 4 is an energy diagram for explaining the operation of the superconducting three-terminal element. (a) shows the operation when the element is in the off state, with a voltage VBC smaller than 2Δ/e (e is the electron charge) between the base and collector, and zero voltage between the emitter and base. is applied. The dotted line in the figure indicates the Fermi energy EF, and the energy states indicated by diagonal lines are each occupied by an electron. In this state, the electrons in superconductor 2 cannot tunnel because they cannot find an energy state that is not occupied by electrons in superconductor 3, and no tunnel current flows between superconductors 2 and 3. Figure 4(b) shows the operation when the element is in the on state, with a base-collector voltage VBC smaller than 2Δ/e between the base and collector, and a voltage VBC smaller than 2Δ/e between the emitter and base. A large voltage VBE is also applied. In this state, the electrons in the superconductor 1 can find an energy state in the superconductor 2 that is not occupied by electrons, and tunnel from the superconductor 1 to the superconductor 2 as shown by the arrow in the figure. The tunneled electrons become quasiparticles in the superconductor 2, and diffuse into the tunnel barrier 5.
reach. The quasiparticles can store energy in the superconductor 3 and find an energy state not occupied by electrons, and tunnel from the superconductor 2 to the superconductor 3 as shown by the arrow in the figure. This tunnel current becomes a collector current IC, flows through the collector electrode, and becomes an output current of the element. As can be seen from the above description, this element functions as a switching element. Electrons injected from the emitter electrode 1 exist as quasiparticles in the base, and recombine into a pair of Coopers (superconducting electrons) with a finite survival time 1 on average. As a result of recombination, the resulting pair of coopers cannot reach the collector electrode 3, but instead flows from the base electrode 2 to the base terminal, resulting in a loss current IB. Therefore, in order for most of the electrons injected from the emitter electrode 1 to the base electrode 2 as the emitter current IE to reach the collector electrode 3, the rate at which quasiparticles form a Cooper pair due to recombination r'R (=t-1)
It is necessary that the ratio of tunneling through the tunnel junction 5 is sufficiently smaller than that of FT. This can be done by selecting the operating temperature or by making the thickness of the tunnel junction 5 sufficiently thin to increase the proportion of tunneling. At the same time, in the operation of the device, the emitter-base voltage is
Quasiparticles cannot be injected into the base electrode unless it is set to Δ/e or more, and the base-collector voltage is 2Δ
If it is not within /e, leakage current will occur between the base and collector in the off state. Therefore, when the element is operated as an active element, the ratio VBE/VBC of VBE, which is the input voltage, and VBC, which is the output voltage, is 1 or more, and the element uses a normal conductor with an electrical resistance of voltage gain. Some improvement is possible. FIG. 5 is an energy diagram showing the operating state of a superconducting three-terminal device having an emitter electrode of a normal conductor. The emitter-base voltage VBE required to inject quasi-particles from the emitter electrode 1 to the base electrode 2 needs to be Δ/e or more, so the ratio of input terminal to output voltage VBEA'BC in this case is close to 1/2. The device can have a voltage gain close to 2. However, it is difficult for a three-terminal element having such a small voltage gain or power gain to operate as an active element in a high frequency band such as microwave or millimeter wave in actual applications.

(Problems to be Solved by the Invention) In conventional superconducting three-terminal devices, materials having the same superconducting gap are used for the superconductor 2 serving as the base electrode and the superconductor 3 serving as the collector electrode. Because of this, voltage gain could not be obtained. An object of the present invention is to eliminate the drawbacks of the conventional superconducting three-terminal elements described above and to provide a superconducting three-terminal element with a large voltage gain.

(Means for Solving the Problems) According to the present invention, a first superconductor or normal conductor, a second superconductor, the first superconductor or normal conductor, and the second superconductor The first tunnel barrier sandwiched between
In a superconducting three-terminal device consisting of a third superconductor and a second tunnel barrier sandwiched between the second and third superconductors, each superconductor has superconductivity at a temperature higher than liquid helium temperature. The third superconductor has a transition temperature, and the superconducting gap energy of the third superconductor is equal to that of the first superconductor. A superconducting three-terminal element characterized by having a superconducting gap energy larger than that of the second superconductor is obtained.

(Function) In order to put a superconducting three-terminal element into practical use, the higher its operating temperature is, the more advantageous it is, but in terms of temperature stability and temperature controllability, it is preferable to use the element by immersing it in liquid helium. The most practical. That is, as described in the section of Problems to be Solved by the Prior Art Invention, superconducting gap energy plays an important role in the operation of the device, but superconducting gap energy changes depending on the operating temperature. Therefore, fluctuations in operating temperature cause significant deterioration of the operating margin of the element or changes in characteristics such as voltage gain. In an element immersed in liquid helium, the operating temperature is not only fixed at 4.2°K, but even if the element generates heat due to operation, this amount of heat is converted by the heat of vaporization when the liquid helium evaporates. and the operating temperature of the device hardly changes. Therefore, in the following operation of the device, it is assumed that the device operates while being immersed in liquid helium. Further, from this, it is assumed that the superconductor transition temperature of the superconductor constituting the element is 4.2° or higher. Superconductors with a superconducting transition temperature higher than the temperature of liquid helium can be roughly divided into metal superconductors such as niobium and lead, and mixtures of yttrium, barium, copper, and oxygen (YBCO).
There are oxide superconductors represented by (abbreviated as ).

The superconducting transition temperature of the metal superconductor is as low as 9° for niobium.
It is about a degree. On the other hand, the transition temperature of the oxide superconductor is about 90° at high<YBCO. The transition temperature is proportional to the superconducting gap, so if a metal superconductor is used for the base electrode and an oxide superconductor is used for the collector electrode, the emitter-base voltage VBE required to inject quasiparticles into the base electrode will be reduced. It is possible to operate the superconducting three-terminal element by applying a base-collector voltage VBC that is nearly 10 times larger, and a large voltage gain and power gain can be obtained.

(Examples) Hereinafter, the present invention will be described based on Examples. FIG. 1 is a block diagram for explaining the present invention in detail. This embodiment consists of a first normal conductor 10 made of aluminum, a first tunnel barrier 11 made of niobium oxide, a second superconductor 12 made of niobium, a second tunnel barrier 13 made of germanium, and YBCO. It consists of a third superconductor 14. The superconductors 10, 12 and 14 form an emitter electrode, a base electrode and a collector electrode, respectively. Second
The figure is an energy diagram for explaining the operation of the conductive three-terminal element, with (a) showing the off state and (b) showing the on state. The operating temperature is liquid helium temperature (4.2°
Assume k). The superconducting transition temperatures of niobium and YBCO are approximately 10 times different, and the ratio Δ21Δ1 between the superconducting gap Δ1 of niobium and the superconducting gap Δ2 of YBCO can be estimated to be approximately 10. In the state of (a), a voltage approximately close to (Δ2+Δ1)/e is applied between the base and collector. Electrons in the base electrode store energy in the collector electrode, and no energy state not occupied by electrons can be found, and no collector current; tIc flows to the collector electrode. In the state of (b), a voltage VBE of Δ1/e or more is applied between the emitter and the base, and electrons are injected from the emitter electrode 10 into the base electrode 12. The injected electrons become quasiparticles, diffuse in the base electrode 12, tunnel through the tunnel barrier 13, and reach the collector electrode 1.
Appears during the 4th. Assuming that the recombination in the base electrode 12 is sufficiently smaller than the rate at which it tunnels through the tunnel barrier 13, the emitter current IE is approximately equal to the collector current IC.

The ratio VBC/VBE of the voltage applied between the emitter and the base and between the base and the collector is approximately equal to Δdam 1 (~10). Therefore, in the superconducting three-terminal element, the voltage gain and power gain are approximately 10. It can be seen that the following can be obtained.

The superconducting three-terminal device shown in the example of FIG. 1 is shown in FIG. 3(a).
It can be produced according to the manufacturing steps shown in ~(d). on a substrate 15 of magnesium oxide (MgO) with a thickness of 5
A YBCO thin film 14 with a thickness of 100 A is formed by sputtering, and then a germanium thin film 13 with a thickness of 100 A and a niobium thin film 12 with a thickness of 100 A are deposited (see Fig. 3).
a)).

Next, a part of the niobium thin film 12 is thermally oxidized in an oxygen atmosphere to form a niobium oxide film 11, and then a niobium oxide film 11 with a thickness of 2000 mm is formed.
Depositing a thin aluminum film 10 (FIG. 3(b))
.

The resist film applied by photolithography is patterned, and the aluminum thin film 10 is etched using chlorine-based gas CCl4, and the niobium oxide film 11 is etched using argon gas. Subsequently, after removing the resist film, the newly applied resist film is buttered, and a niobium thin film 12 and a germanium thin film 13 are formed using a fluorine-based gas CF4.
(Fig. 3(C)).

Next, a silicon oxide film 5i0216 is formed by sputtering, an opening is formed by etching, and wiring for an emitter electrode, a base electrode, and a collector electrode are formed using patterned niobium thin films 17, 18, and 19, respectively (Fig. 3). (d)). Through the above steps, the superconducting three-terminal element can be manufactured.

In the embodiments described above, YBCO was used as the third superconductor, but it is not limited to this, and composites of other rare earth metals and barium/copper/oxides, such as lanthanum, barium, copper, etc. - Oxide or lanthanum, strotium, copper, or oxide in which barium is replaced with strotium may be used.

(Effects of the Invention) As explained in detail above, according to the present invention, a superconducting three-terminal element with large voltage gain and power gain can be provided, and can be applied as an active element in ultra-high frequency bands such as microwaves and millimeter waves. It is possible.

Fig. 1 is a diagram showing an embodiment of the superconducting three-terminal element of the present invention, and Fig. 2 is an energy diagram for explaining the operation of the element. (b) shows the on state. 3(a) to 3(d) are manufacturing process diagrams of the superconducting three-terminal device, FIG. 4 is a diagram showing a conventional superconducting three-terminal device, and FIG. 5 is a diagram showing a conventional superconducting three-terminal device. In the energy diagram for explaining the operation of the terminal element, (a) shows the off state and (b) shows the on state.

FIG. 6 is an energy diagram for explaining the operation of the conventional superconducting three-terminal device in which the superconductor constituting the emitter electrode is replaced with a normal moving body.

1.10...Emitter electrode, 2.12...Base electrode, 3.14...Collector electrode, 4.11...First tunnel barrier, 5.13...Second tunnel barrier , 15...MgO
Substrate, 16... Silicon oxide film, 17... Emitter electrode, 18... Base electrode, 19...
...Collector electrode 2] -
Fig. 2 (b) Fig. 3 (C) ,),) L Q - Pulse Pulse Pus Fig. 5 (a) (b) Fig. 6

Claims (1)

[Claims] a first superconductor or normal conductor, a second superconductor, a first tunnel barrier sandwiched between the first superconductor or normal conductor and the second superconductor; In a superconducting three-terminal device consisting of a third superconductor and a second tunnel barrier sandwiched between the second and third superconductors, each of the superconductors has A superconducting three-terminal device having a conduction transition temperature, wherein the superconducting gap energy of the third superconductor is larger than the superconducting gap energy of the first and second superconductors.