JPH0291983A - superconducting transistor - Google Patents

Abstract

PURPOSE:To obtain a transistor characterized by less dispersion in characteristics and high speed operation by forming a constituent parts with the same kind of material such as an oxide, and using fine particles of oxide superconductors. CONSTITUTION:Source and drain regions 2 and 3 are provided in a tunnel medium substrate 1. The regions 2 and 3 comprise an oxide superconductor whose carrier concentration is lower than metal but higher enough so as to indicate superconduction. A fine particle part 4 is also provided in the substrate 1. The part 4 comprises an oxide superconductor whose carrier concentration is higher enough to indicate the superconduction of the same material as that of the regions 2 and 3 but lower than the carrier concentration of the regions 2 and 3. The medium 1 acts as barriers between the region 2 and the part 4 and between the region 3 and the part 4. On the part 4, barrier layer 5 is formed. The barrier 5 comprises the same material as that of the part 4, but the composition is different. The carrier concentration of the barrier 5 is lower enough so that superconductivity is not indicated. The gate region of an oxide conductor layer 6 having the same composition and material as those of the regions 2 and 3 is formed. Therefore the constituent parts are formed with the same kind of oxide having the different compositions. Dispersion in characteristics is less. A superconductive transistor is operated at a high speed owing to double tunnel effects and the like between the regions 2 and 3 aided by the fine particle part 4.

Description

[Detailed Description of the Invention] [Summary] Regarding a transistor using an oxide superconductor, particularly a superconducting transistor in which conductive fine particles are interposed in a portion corresponding to a channel region between a source and a drain, as the conductive fine particles. The aim is to provide a high-speed superconducting transistor with less variation in characteristics by using an oxide superconductor, which has a lower carrier concentration than metals and maintains a high carrier concentration to the extent that it exhibits superconductivity. A source part and a drain part made of an oxide superconductor, and an oxide which is interposed between the source part and the drain part and is made of the same substance as those and whose carrier concentration is maintained high enough to exhibit superconductivity. A fine particle portion made of a superconductor is interposed between the fine particle portion and the source portion and the drain portion while maintaining a thickness that allows carriers to tunnel, and the fine particle portion, the source portion, and the drain portion have a composition. A tunnel medium part is formed on the fine particle part and is made of the same material with a different composition and has a carrier concentration low enough not to exhibit superconductivity, and a tunnel medium part is made of the same material with a different composition and exhibits superconductivity. A gate insulating film having a low carrier concentration as shown in FIG. and a high gate conductivity layer.

[Industrial application field]

The present invention relates to a transistor using an oxide superconductor, and particularly to a superconducting transistor in which conductive fine particles are interposed in a portion corresponding to a channel region between a source and a drain.

In recent years, oxide high-temperature superconductors whose critical temperature exceeds the boiling point of liquid nitrogen, ie, 77 (K), have been discovered, and their use is expected in the field of electronics.

Josephson junctions, which have been used in the field of superconducting electronics for many years, have the advantages of high speed and low power consumption, but because they are essentially two-terminal devices, they cannot be realized by circuit engineering alone. It has a function.

Therefore, if a three-terminal device that utilizes superconductivity phenomenon, that is, a device that operates as a transistor, could be obtained, it would be advantageous because it would be possible to realize various functions.In particular, if it could be realized using high-temperature superconductors, it would lead to advances in many fields. can bring.

[Conventional technology]

FIG. 3 shows a cutaway side view of essential parts of the superconducting transistor developed by the present inventors.

In the figure, 31 is a silicon substrate, 32 and 33 are tantalum (Ta) F formed by sputtering, for example.
Source and drain electrodes made of an IT film, 34 metal fine particles made of platinum (Pt), 35 a tantalum oxide film, 36 a gate electrode made of Al, for example, S is a source terminal, D is a drain terminal, and G is a gate terminal. are shown respectively.

In this superconducting transistor, electrons move in two consecutive tunnels from the source to the metal particles 34 and from the metal particles 34 to the drain, and the effective carrier transit time is extremely short, resulting in high-speed operation. is possible.

Here, the capacitance of the metal fine particles 34 is small, and
The potential change of a metal particle due to the charge of one electron is the thermal noise voltage kT/e (k = Portzmann constant, T = absolute temperature, e
= elementary charge), this potential change acts as a kind of barrier against the movement of electrons. Therefore, by attaching a gate to the metal fine particles 34 and applying a voltage, the barrier is effectively changed, and the current flowing between the drain and the source can be controlled.

[Problem to be solved by the invention]

In the superconducting transistor as described above, it is necessary to make the metal fine particles 34 extremely small, as well as the interface between the metal fine particles 34 and the tantalum oxide film 35 which is the tunnel medium, and the interface between the tantalum oxide film 35 which is the tunnel medium and the source electrode 32 or It is necessary to precisely control the barrier hide at the interface with the drain electrode 33.
If Hyde control is insufficient, the dark voltage of the transistor will fluctuate. However, since these interfaces are between metals and their oxides, or between semiconductors and metals, it is quite difficult to precisely control the barrier hide that is generated there. Yes, it is possible at the laboratory stage, but the manufacturing yield on a mass production line would be very poor.

Therefore, in order to put a superconducting transistor using fine metal particles into practical use, it is first important to eliminate the variation in characteristics.

Generally, a superconducting transistor using fine metal particles has a structure in which a large number of metal fine particles, for example, 1000 or more, are arranged in parallel between a source and a drain. In other words, superconducting transistors having many single metal particles are arranged in parallel, and if there are variations in the characteristics of each of them, the transistor as a whole will not exhibit any transistor characteristics.

Apart from the above-described superconducting transistor using fine metal particles, the present inventor also wishes to realize a superconducting transistor made of an oxide superconductor, as described in Japanese Patent Application No. 63-212566. ).

FIG. 4 shows a cutaway side view of essential parts of the superconducting transistor.

In the figure, 41 is an oxide superconductor collector layer, 42 is an oxide barrier layer, 43 is an oxide superconductor base layer, and 44 is an oxide superconductor collector layer.
is an oxide barrier layer, 45 is an oxide superconductor emitter layer,
46 is a collector electrode, 47 is a base electrode, 48 is an emitter electrode, and 49 is an insulating substrate.

In the illustrated superconducting transistor, the collector layer 41
, the base layer 43, and the emitter layer 45 exhibit superconducting characteristics at the operating temperature. The barrier layers 42 and 44 are made of the same oxide as the collector layer 41, base layer 43, and emitter layer 45, but have different compositions, so the carrier concentration that carries superconducting current is a small value. Does not exhibit superconductivity.

Since all of the superconducting transistors are composed of essentially the same type of crystal structure material, there is no possibility that an undesired barrier will be formed between the base layer 43 and the collector layer 41, for example. Therefore, it has achieved excellent device characteristics such as good current transfer characteristics and good uniformity and reproducibility of electrical characteristics regarding contact between semiconductors and superconductors. , unlike the superconducting transistor that uses fine metal particles seen in Figure 3, does not utilize two consecutive tunnel effects. For this reason,
In the superconducting transistor shown in FIG. 4, carriers injected into the base layer 43 from the emitter layer 45 must travel within the base layer 43 before reaching the collector layer 41. Normally, the operating speed of this type of transistor is limited by the time that carriers travel through the base layer 43. However, in oxide superconductors, carriers are highly localized, and as a result, carrier mobility is known to be small - more than three orders of magnitude lower than that of semiconductors used in fishing boats. . Therefore, in terms of switching speed, the expected improvement in characteristics cannot be obtained.

Considering the above-mentioned points, it is expected that improvements will be made to the superconducting transistor that utilizes the tunnel effect as explained with reference to FIG.

The present invention aims to provide a high-speed superconducting transistor with less variation in characteristics by using fine particles made of an oxide superconductor.

[Means to solve the problem]

FIG. 1 is a cross-sectional side view of a main part of a superconducting transistor for explaining the present invention in detail.

In the figure, l is the oxide tunneling medium substrate (or layer)
, 2 is an oxide superconductor source region, 3 is an oxide superconductor drain region, 4 is a fine particle region made of an oxide superconductor, 5 is an oxide barrier layer, 6 is an oxide conductive layer, and 7 is a metal Reference numeral 8 indicates a source electrode, numeral 8 indicates a metal drain electrode, and numeral 9 indicates a metal gate electrode.

In this superconducting transistor, a tunnel medium substrate 1
is composed of the same oxide as the source region 2, drain region 3, and particulate region 4, but because the composition is different, the carrier density that carries the superconducting current is smaller than that of the source region 2, etc. , source region 2 and fine particle region 4
It acts as a barrier between the particulate region 4 and the drain region.

The source region 2 and drain region 3 are made of oxide superconductor and exhibit superconducting characteristics at the operating temperature.

The fine particle region 4 is composed of conductive fine particles made of the same oxide superconductor as the source region 2 and drain region 3, and has a diameter of, for example, about 10100 (n). Exhibits superconducting properties at different temperatures.

The barrier layer 5 is a gate insulating film, and is made of the same oxide as the source region 2, drain region 3, etc., but has a different composition, so the carrier concentration that carries the superconducting current is a small value. However, it does not exhibit superconductivity.

The oxide conductive layer 6 together with the barrier layer 5 constitutes a gate.

Each of the oxide layers ideally consists of a single single crystal;
Alternatively, they can be regarded as the same type of substance, except that they have at least a similar crystal structure and differ in carrier concentration due to the difference in composition. In other words, since they are all composed of essentially the same type of crystal structure substance,
For example, between the source region 2 and the particulate region 4, or
There is almost no possibility that an undesired electrical barrier will be generated between the fine particle region 4 and the drain region 3, and therefore a superconducting transistor with good characteristics can be realized with good reproducibility.

・Now, the illustrated superconducting transistor operates as follows.

Typically, carriers tunnel from the source region 2 to the particulate region 4 and from the particulate region 4 to the drain region 3.
It also appears through tunneling. That is, when carriers tunnel from the source region 2 to the particulate region 4, the potential of the particulate changes, and the probability of carrier tunneling from the particulate region 4 to the drain region 3 increases. Therefore, tunneling from source region 2 to fine particle region 4 to drain region 3 occurs continuously.

Here, in the fine particle region 4, as in the conventional superconducting transistor using fine metal particles explained next in FIG. If it is sufficiently large compared to the noise voltage kT/e, the potential change acts as a barrier against the movement of carriers, and applying a voltage to the gate electrode 9 effectively changes the height of the barrier. ,
The drain-source current is controlled. That is, the tunnel current changes depending on the application of the gate voltage, and a transistor effect appears.

From the above, in the superconducting transistor according to the present invention, the source portion ( For example, an oxide superconductor source region 2) and a drain region (for example, an oxide superconductor drain region 3) are interposed between the source region and the drain region, and are made of the same material as these and exhibit superconductivity. Carriers tunnel between a particulate part made of an oxide superconductor whose carrier concentration is maintained at a reasonably high level (for example, particulate region 4 made of oxide superconductor) and the particulate part and the source part and the drain part. The fine particle portion, the source portion, and the drain portion are made of the same material with different compositions and have a low carrier concentration to the extent that they do not exhibit superconductivity (for example, Oxide tunnel medium substrate 1
), a gate insulating film (for example, an oxide barrier layer) formed on the fine particle portion and made of the same material with a different composition and having a low carrier concentration to the extent that it does not exhibit superconductivity; A gate conductive layer (for example, an oxide conductive layer 6) is laminated on the fJA edge film, and is made of a material having the same composition as the fine particle portion, the source portion, and the drain portion, and has a high carrier concentration to the extent that it exhibits superconductivity. We are prepared.

[Effect]

By adopting the above-mentioned means, the movement of carriers from the source region 2 to the drain region 3 is realized by two consecutive tunnel phenomena.

The intervals between these tunnel phenomena are determined by the propagation of potential changes within the particle region 4 and the tunneling probability between the source region 2 - the particle region 4 - the drain region 3, and the carriers within the particle region 4. Independent of propagation speed.

Since the potential change within the particle region 4 propagates at the speed of light, its propagation time is extremely short, thus enabling high-speed operation.

By the way, what has a great influence on the operation of the superconducting transistor according to the present invention is the height of the barriers generated between the source region 2 and the particle region 4 and between the particle region 4 and the drain region 3. If there are variations in the barrier hide in terms of size and shape, the impedance level of the transistor will vary greatly, and variations in the barrier shape will also cause variations in characteristics.

In a superconducting transistor using metal fine particles as explained in Fig. 3, which utilizes an interface formed between a metal and its oxide, crystals at the metal-oxide interface It is difficult to control the shape of the barrier because of the poor matching of the barrier and the effects of surface states and fixed charges in the oxide.

The superconducting transistor according to the present invention described with reference to FIG.
etc. are essentially made of the same type of material, and there are no surface states or fixed charges that would impair transistor characteristics at the interface of each part, and junctions are created using amounts of materials with different crystal structures. There are no technical difficulties as in the case.

〔Example〕

FIG. 2 shows a cutaway side view of essential parts of an embodiment of the present invention.

In the figure, 21 is a substrate, 22 is a tunnel medium layer, 23
is a source layer, 24 is a drain layer, 25 is a fine particle layer, 26
27 is a gate insulating film, 27 is a gate conductive layer, 28 is a source electrode, 29 is a drain electrode, and 30 is a gate electrode.

Examples of main data of each part in this embodiment are as follows.

+11 Material for the substrate 21: S r T i O3 (2) Material for the tunnel medium layer 22: Insulating La 2 Cu O4 Thickness: 400 (nm) (3) Material for the source N23: (L a l-X S rg ) z Cu
OaX value: 0.075 Thickness: 100(n10 0(n) Material for drain layer 24: (Lad-, 5FX)z CubaX value: 0
．． 075 Thickness: 100(n10 0(n) Material for fine particle layer 25: (L a I-X Sr, t) z Cu
OaX value: 0.075 Thickness: 100 (n10 0 (n for the gate insulating film 26 Material: insulating La2C, u04 , )t Cu
baX value: 0.075 Thickness: 300 (nm) (6) Regarding the source electrode 28, drain electrode 29, and gate electrode 30 Material: Au Thickness: 1 [μm] In order to change the carrier concentration between layers that do not exhibit conduction, we varied the composition of the constituent elements.
It is possible to change the carrier concentration simply by changing .

Specifically, in Bat YCu, it becomes superconducting when the X value is about 6.7 to 6.8, and it becomes semiconducting when the begins to show characteristics.

In addition, when B15rCaCuO is used and the composition is 2:2:0:1:6, the critical temperature T is about 10 (K), and similarly, when the composition is 2:2:l:2:8. In this case, the critical temperature Tc is about 80 (K), and when the ratio is 2:2:2:3:10, the critical temperature Tc is about 110 (K).

By the way, the critical temperature Tc when Nb is used as a superconductor is 9
．． It is about 2 (K).

As can be understood from the above, the superconducting transistor according to the present invention has a great advantage in that the materials to be bonded are all oxides, and the crystal structure has many similarities, so it can be composed of almost the same material. For example,
In the B15rCaCuO described above, Ca=0 may be used, but other than that, the same material is used.

〔Effect of the invention〕

In the superconducting transistor according to the present invention, the source part, drain part, particulate part, tunnel medium part, gate, etc. are all made of substantially the same kind of material such as an oxide superconductor or an oxide. .

With this configuration, there is no need to consider the technical problems that occur when forming a bond between different materials, and there is no need to consider electrical barriers between each part. Because there is no particle, the current transfer rate is high, and the carrier movement between the source and drain is performed by two tunneling phenomena, and the effective carrier movement time is determined by the tunneling probability and the electromagnetic wave propagation time in the particulate part. The carrier movement time inside the oxide superconductor does not limit the operating speed, and therefore superconducting transistors with good device characteristics such as excellent frequency characteristics have been realized with good reproducibility. .

[Brief explanation of drawings]

Fig. 1 is a cutaway side view of the main part of a superconducting transistor for explaining the present invention in detail, Fig. 2 is a cutaway side view of the main part of an embodiment of the present invention, and Fig. 3 is a cutaway side view of the main part of a conventional example. 4 and 4 respectively represent cutaway side views of essential parts of a superconducting transistor according to the prior art. In the figure, 1 is an oxide tunnel medium substrate, 2 is an oxide superconducting source region, 3 is an oxide superconducting drain region,
Reference numeral 4 indicates a fine particle region made of an oxide superconductor, 5 an oxide barrier layer, 6 an oxide conductive layer, 7 a metal source electrode, 8 a metal drain electrode, and 9 a metal gate electrode. Patent applicant: Fujitsu Ltd. Representative Patent Attorney Shoji Aitani

Claims (1)

[Scope of Claims] A source portion and a drain portion made of an oxide superconductor whose carrier concentration is lower than that of a metal and whose carrier concentration is maintained high enough to exhibit superconductivity; A particulate part made of an oxide superconductor which is interposed in between and made of the same material as the above, and whose carrier concentration is maintained high enough to exhibit superconductivity, and between the particulate part and the source part and the drain part. A tunnel in which carriers are interposed in such a thickness that they can be tunneled, and the fine particle part, the source part, and the drain part are made of the same material with different compositions, and the carrier concentration is low enough not to exhibit superconductivity. a medium portion, a gate insulating film formed on the fine particle portion and made of the same material with a different composition and having a low carrier concentration to the extent that it does not exhibit superconductivity; A superconducting transistor characterized in that the fine particle portion, the source portion, and the drain portion are made of a substance having the same composition and include a gate conductive layer having a high carrier concentration to the extent that it exhibits superconductivity.