JPS6010792A - Superconducting transistor - Google Patents

Abstract

PURPOSE:To enable switching operation at high speed by forming a second electrode consisting of a superconductor shaped on a first electrode composed of a superconductor through a thin insulating film and a third electrode on the second electrode through a sufficiently thick insulating film and controlling currents flowing between the first and second electrodes by voltage applied to the third electrode. CONSTITUTION:The motion of the whole carriers in superconductors consisting of first and second electrodes 2, 3 is controlled by changing DC voltage applied to a third electrode 6 corresponding to a gate for a field-effect transistor, thus controlling currents flowing between the two superconductors. Consequently, currents flowing between the electrodes 2, 3 can be turned ON-OFF by turning voltage applied to the electrode 6 ON-OFF, and switching operation is enabled. Voltage to be applied to the electrode 6 may be extremely small because g/h (g respresents the correlation of the two superconductors and h a Planck's constant) is extremely large, and super-high speed switching operation is enabled because switching time is approximately zero.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to superconducting elements, and particularly to the structure of superconducting transistors that utilize AC and DC Josephson effects.

[Technical background of the invention and its problems]

As a conventional superconducting element, a Josephson element using a single Josephson junction is known, but this is a two-terminal element and cannot separate input and output, making it difficult to use. Therefore, a circuit with separate input and output as shown in FIG.
has been done.

This is a magnetic field control type, and the current flowing through the Josephson junction (JJ) is varied by the magnetic flux generated by the signal current Ia. However, in this structure, when integrating multiple Josephson elements, it is necessary to space each element at least a certain distance apart to prevent the influence of magnetic fields from adjacent elements, which makes it difficult to achieve high integration. There was a problem. Furthermore, since the switching speed is determined by the inductance, there is a drawback that the switching operation is slow.

Recently as a solution to such problems.

As shown in FIG. 2, a three-terminal transponder in which two Josephson junction elements are connected in series has been proposed.

Both the device structure and principle are similar to the normal semiconductor pie I.
- Similar to a transistor, the principle is that when an injector current It flows, an accessor current Ia changes rapidly. That is, S', -I,
- Flow current Ii through the 8-fold junction; 8. By injecting quasiparticles into S, the superconducting energy gap of S is reduced and the tunneling effect of quasiparticles from S to S is promoted.
A current Ia is passed through the 88-'1-8m junction.

f:r Oh, 5Iss in Figure 2! *8, IT indicates superconducting metal, 1mI2 indicates insulating film.

By the way, this Yuzu three-terminal element is realized by connecting five ordinary bipolar transistors in series with two Josephson junction elements by switching and amplifying operations using quasi-particle implantation as described above. therefore,
Although the switching operation is faster than that of the circuit shown in FIG. 1, it is not as fast as that of a bipolar transistor. Furthermore, it was necessary to realize a double Josephson junction element, which was extremely difficult in terms of manufacturing technology.

[Purpose of the invention]

An object of the present invention is to provide a superconducting transistor that is highly integrated, enables high-speed switching operation, and is easy to manufacture.

[Summary of the invention]

The gist of the present invention is to realize high-speed switching operation by controlling the current flowing between Josephson junction elements by voltage control.

It is known that in a Josephson junction device in which a thin insulating film is sandwiched between superconductors, the current flowing between the conductors changes depending on the potential difference between the conductors. Focusing on this point, the inventors of the present invention have conducted extensive research, and as a result, electrodes were provided on either superconductor of the Josephson element via a sufficiently thick insulating film.

It has been found that the potential difference between the conductors can be varied by changing the voltage applied to the electrodes, and the current flowing between the conductors can be variably controlled.

That is, the present invention provides an electrode *X made of a superconductor formed on an insulating substrate, a second electrode made of a superconductor formed on this electrode with a thin insulating film interposed therebetween, and this second electrode. A superconducting transistor is formed with a third electrode formed on the top via a sufficiently thick insulating film, and the current flowing between the first electrode and the second electrode is controlled by the voltage applied to the third electrode. This is what I did.

〔Effect of the invention〕

According to the present invention, the current flowing between the first electrode and the second electrode is controlled by the voltage applied to the third electrode.
And the current flowing between the second electrodes can be controlled ON/OFF. In this case, since the switching operation is controlled by voltage, it is possible to speed up the switching operation. In addition, it is possible to bring the Josephson junction elements, which are different from those of the magnetic field control type, sufficiently close together, which is extremely effective for high integration.

Further, there is an advantage that there is no need to form double Josephson junction elements, and manufacturing thereof is extremely easy.

[Embodiments of the invention]

FIG. 3 is a perspective view showing a schematic structure of a superconducting transistor according to an embodiment of the present invention. In the figure, a 7Fi insulating substrate is shown, and first and second electrodes 2.3 made of a superconductor such as Nb are formed on this substrate 1 so as to partially overlap with each other. A sufficiently thin insulating film 4 is formed between the first and second electrodes 2 and 3 so that Cooper pairs formed by superconducting electrons can tunnel therethrough. That is, the insulating film 4. A Josephson junction element is constructed from the first and second single poles 2.3. Also.

A third electrode 6 is formed on the overlapping portion of each of the electrodes 2.3 via an insulation @5 that is sufficiently thick that the Josephson effect can be ignored. Then, as shown in Figure 4, the third
The current I flowing between the first and second electrodes 2.3 is controlled by the voltage vK applied to the electrode 6.

In such a structure, a voltage V =
When V (tl) is applied, a current flows between the first and second electrodes 2.3 forming the Josephine junction confection.

Here, J,=2Pρ. Deno represents the correlation between the two superconductors (electrodes 2 and 3), and ρ. is the average density of carriers. Also, θ is the phase difference of the order parameters of each superconductor, q = 2e, e is the elementary charge, fi = b
/2g and 5 is a blank constant.

Now, when no voltage is applied to the third electrode 6, that is, when v=0, due to the DC Josephson effect] J (tJ = Jr, sinθ...
(A superconducting current 21 flows between the first and second electrodes 2.3 by tunneling through the insulating film 4.

Furthermore, when a DC voltage of v=vy is applied to the third electrode 6, J(fl= Jo SI
n'(θ1 + Vf weight) ・Su・・・・・・・(
3) An alternating current flows. here.

ω= q V? /, F, = 4.836 x 10”
The alternating current has a value of VJ'(H2) - (41) and a correspondingly high frequency. This represents the frequency of superconducting electrons moving back and forth between the superconductors (electrodes 2.3). The number of superconducting electrons going back and forth is
If we consider a closed circuit in which the numbers going from electrode 3 to electrode 3 and from electrode 3 to electrode 2 are equal, and electrodes 2 and 3 are connected externally, the current flowing through this closed circuit will be essentially zero. .

As is clear from the above explanation, the 8F! In the structure shown in FIG. 3, carriers (
It is possible to control the motion of the entire Cooper pair) and, as a result, the current flowing between the two superconductors. Therefore, the voltage applied to the third electrode 6 is turned on. FFTl?
Knee IVc Yotsuko, □1□□2゜. The current flowing between 12.3 and 3 can be turned ON and OFF, allowing switching operation. Also,? 8 is very large, the voltage to be applied to the third electrode 6 only needs to be extremely small, and the switching time 1 is approximately zero. Therefore, the third electrode 6
By applying a minute voltage to the oscillator, ultra-high-speed switching operation becomes possible.

Note that the present invention is not limited to the embodiments described above. For example, the material forming the first and second electrodes is Nb.
The material is not limited to , and any superconductor may be used. moreover,
The material for forming the third electrode may or may not be a superconductor. Further, the material, thickness, etc. of the insulating film formed between the first and second electrodes may be determined as appropriate within a range that allows the Cooper pair to tunnel. Furthermore, the second and third
The material, thickness, etc. of the insulating film formed between the electrodes may be appropriately determined within a range where the Josephson effect can be ignored. In addition, various modifications can be made without departing from the gist of the present invention.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram showing the principle configuration of a magnetic field-controlled Josephson junction element, and Figure 2 is a schematic diagram showing the principle configuration of a three-terminal element in which two Josephson junction elements are connected in series.
FIG. 3 is a perspective view showing a schematic structure of a superconducting transistor according to an embodiment of the present invention, and FIG. 4 is a sectional view for explaining the operation of the transistor. J... Insulating substrate, 2... First electrode (superconductor). 3... Second electrode (superconductor), 4... Thin insulating film. 5... Thick insulating film, 6... Third electrode. Applicant's agent Patent attorney Hiko Rin Ushiki 0 Figure 1 Figure 2 Figure 3 Figure 4

Claims (1)

[Claims] A first electrode made of a superconductor is formed on an insulating substrate, and a thin insulating film is formed on this first electrode through which Cu-9 pairs, which are superconducting electrons, can tunnel. A second electrode made of a superconductor, and a third electrode formed on the second electrode with an insulating film so thick that the Josephson effect can be ignored. A superconducting transistor characterized in that a current flowing between the first and second electrodes described above is controlled by an applied voltage.