JPH0237786A - Superconducting transistor - Google Patents

Abstract

PURPOSE:To make a transfer conductance large and to reproduce a characteristic well by constituting a source region, a channel region and a drain region of substances whose crystal structure is essentially identical. CONSTITUTION:A superconductor source region 26, a superconductor drain region 28, both being of a high concentration, and a channel region 24, of a low carrier concentration, which is sandwiched between the source region 26 and the drain region 28 are constituted of substances whose crystal structure is essentially identical except a difference in a carrier concentration because one part of their composition is different with each other. Accordingly, an electrical barrier which influences a characteristic adversely is not caused among the source region 26, the channel region 24 and the drain region 28. Thereby, a transfer conductance becomes large; the characteristic can be reproduced well.

Description

[Detailed Description of the Invention] [Summary] Regarding a superconducting transistor that uses a superconductor, the present invention aims to provide a superconducting transistor that can control the source current with a small gate voltage, that is, has a large transfer conductance. A source region and a drain region with a high carrier concentration and a superconductor, and a low carrier region sandwiched between these source and drain regions and having a similar crystal structure with a partially different composition from the source and drain regions. The semiconductor device is configured to have a channel region with a high concentration and a gate electrode provided on the channel region.

[Industrial Application Field] The present invention relates to a superconducting transistor, which is a transistor using a superconductor.

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

Josephson junctions, which have traditionally been used in the field of superconducting electronics, have the advantages of high speed and low power consumption, but because they are essentially two-terminal devices, it is difficult to realize them with circuit improvements alone. It had a function.

Therefore, if an element that operates like a transistor can be fabricated using a superconductor, it will be convenient for realizing various m functions. That is, by utilizing the properties of high-temperature superconductors, various active devices other than Josephson junctions can be fabricated, which has great industrial utility.

[Prior art] Semiconductor-coupled superconducting field-effect transistor (FET)
is shown in Figure 4.

For example, InAsJ? ? Semiconductor substrate 64 made of Si or the like
Above, niobium (Nb) poles 66 and 68 as source S[i and drain electrodes are formed by sputtering or the like. Nb, which is a high melting point metal, is a superconducting metal that exhibits superconductivity at a temperature Tc of 9.2K. In this way, the Nb electrodes 66, 68 as superconductors are coupled through the semiconductor substrate 64. Also, N b 'S 4 & 6 as source and drain electrodes
On the semiconductor substrate 64 between 6 and 68, a gate voltage f! is applied via an insulating layer 74, similar to a normal FET. 76 is formed.

Now, when a voltage is applied to the gate electrode 76, the strength of the coupling between the source and drain changes, and the source current changes. The distance between the Nb electrodes 66 and 68 is sufficiently short to have a coherent length of 0.1 μm or less, and the semiconductor substrate 6
4 and the Nb electrode 66.68 as superconductor metal, this superconducting FET operates as a Josephson device with critical current controlled by the gate voltage. .

In actually produced superconducting PET, it has been observed that the superconducting current changes with a change in gate voltage of several volts to several tens of square meters.

[Problems to be Solved by the Invention] However, in the conventional superconducting FET described above, N as a superconducting metal is formed on the semiconductor substrate 64 without an electrical barrier.
Bringing the b electrodes 66 and 68 into contact is an extremely difficult technical issue that has not been solved at this time, and there is also a technology to produce the electrical properties of contact between a semiconductor and a superconductor with sufficient reproducibility. has not yet been discovered.

Therefore, when an electrical barrier is formed between the semiconductor substrate 64 and the Nb electrodes 66 and 68 as superconducting metals, Nb electric '! The superconducting current flowing between 66 and 68 is very small, for example, several microamperes. That is, even with a gate voltage of several volts to several tens of volts, only a small current can be controlled in this manner, resulting in a problem that the transfer conductance is small.

Furthermore, since the electrical characteristics of the contact between the semiconductor and the fl conductor are not reproducible, there is a problem that the characteristics of the manufactured superconducting FET are also not reproducible.

For this reason, current FET-type superconducting transistors are not considered practical devices.

An object of the present invention is to provide a superconducting transistor whose source current can be controlled with a small gate voltage, that is, whose transfer conductance is large.

rMeans for Solving the Problem] The above problem is achieved by a source region and a drain region which are superconductors and have a high carrier concentration, and which are sandwiched between the source region and the drain region and have the same composition as the source region and the drain region. This is achieved by a superconducting transistor characterized by having a low carrier concentration channel region of similar crystal structure with different parts and a gate electrode provided on the channel region.

[Function] In other words, in the present invention, a source region and a drain region having a high carrier concentration and a channel region having a low carrier concentration sandwiched between the source region and the drain region, which are superconductors, have a partially different composition. They are essentially composed of substances with the same type of crystal structure, except for the difference in carrier concentration.

As a result, no electrical barrier is generated between the source region, channel region, and drain region that would adversely affect the characteristics.

[Example] The present invention will be specifically described below based on an illustrative example.

FIG. 1 is a sectional view showing a cross section of a superconducting transistor according to a first embodiment of the present invention.

On the S r T io 3 substrate 2, a Y layer with a thickness of 500 nm is
B a 2 Cu 30 x (x-6, 1 to 6.5
) layer 4 is formed. This Y B a 2 Cu
30x (x=6.1 to 6.5) layer 4 is grown by sputtering or electron beam evaporation of yttrium-based oxide superconductor YBa Cu307 at a temperature of 60°C.
Heat treatment was performed for 1 hour in a nitrogen atmosphere at 0°C, and oxygen O
It is formed by expelling a part of the Due to this composition change, YBa2Cu30X (x=6.1
~6.5) Layer 4 has a lower carrier concentration and has changed from a superconductor to a material with properties similar to a semiconductor.

YBa2Cu30x (x=6.8 to 7.0) regions 6. YBa2Cu30x (x=6.8 to 7.0) are formed as source and drain regions at predetermined positions on the YBa Cu30x (X=6.1 to 6.5) IgJ4 surface.
8 is formed. These YBa2Cu30X(x
=6.8 to 7.0> Region 6.8 uses a mask that opens only the source and drain regions, and the frequency is 13.56 MHz.
It is formed by exposing the YBa2Cu30X (x=6.1 to 6.5) layer 4 to oxygen plasma generated by high frequency for 20 minutes and selectively injecting oxygen O into the layer 4. And due to this compositional change, YBa2Cu30X (x=
6.8 to 7.0) Region 6.8 has a high carrier concentration and becomes a superconductor again.

On these YBa2Cu30X (x=6.8-7.0) regions IJIU6.8, gold (Au) poles 10.12 are formed as source and drain electrodes. Also, YBa2Cu30 as source and drain regions
YBa2Cu30X (x=6.1) as a channel region sandwiched between
~6.5) On layer 4, a 200 nm thick zirconium oxide (Z r O2) JWJ 14 is formed as an insulating layer by reactive vapor deposition. Furthermore, this Z
r On the O2 layer 14, there is an Au electrode 1 as a gate electrode.
6 is formed.

In this way, the source and drain regions consist of superconductor YBa Cu30X (x = 6.8 to 7.0) regions 6.8, and the channel region sandwiched between them has properties similar to those of a semiconductor. YBa2Cu30X (x=
6.1~6.5) A superconducting FET consisting of layer 4 is formed, but at this time YBa Cu30x (x-6,8~
7.0) Areas 6, 8 and Y B a Cu 30 x
< x-6, 1 to 6.5) Layer 4 is characterized in that it has essentially the same type of crystal structure, with the only difference being the composition of oxygen 0.

Next, the principle of operation of the present invention will be explained using FIG.

FIG. 2(a) is a schematic cross-sectional view of a superconducting transistor for explaining the operating principle of the present invention, and FIG. 2(b) is a schematic potential diaphragm thereof.

In FIG. 2(a), oxide superconductor regions 26. are used as the source and drain regions of the superconducting FET, respectively.
28 is formed. These superconductor regions 26.2
A region 24 having characteristics similar to those of a semiconductor is formed as a channel region sandwiched between the two regions. The region 24 having the same characteristics as this semiconductor is made of the same oxide as the superconductor regions 26 and 28 forming the source and drain regions, but due to a partial difference in composition, its carrier density is superconducting. This value is smaller than the carrier density that carries the superconducting '1 flow of the body 26.28.

In other words, the superconductor regions 26 and 28 as the source and drain regions and the region 24 as the channel region having properties similar to those of a semiconductor are ideally made of a single single crystal, or at least similar crystals. They are essentially composed of the same type of crystalline substance, except that there are differences in carrier concentration due to some differences in composition.

Furthermore, source and drain electrodes 30.32 are formed on the superconductor regions 26.28 serving as source and drain regions, respectively. Furthermore, a gate electrode 4f136 is formed via an insulating layer 34 on a region 24 having characteristics similar to those of a semiconductor as a channel region.

Now, when a predetermined voltage is applied between the source and drain electrodes 30 and 32, the potential from the source region through the channel region to the drain region becomes as shown in FIG. 2(b). In other words, the source region, the channel region, and the drain region are made of materials having essentially the same type of crystal structure, so that the potentials are continuously connected.

Also, the superconductor regions 26, 28 as source and drain regions have a higher carrier concentration, that is, in oxide superconductors, usually a higher hole concentration than the region 24, which has properties similar to the semiconductor as a channel region. Therefore, its potential is low. Conversely, a region 24 having properties similar to those of a semiconductor as a channel region
has low carrier density and high potential.

When no current is flowing through the element, the diffusion@flow caused by the difference in carrier concentration is balanced by the drift current caused by the difference in potential. Since the channel region has a high potential, it acts as a liu for carriers attempting to flow between the source and drain regions.

The carrier flow or current generated between the source and drain regions is controlled by changing the potential of the channel region. That is, an insulating layer 34 is formed on a region 24 having characteristics similar to that of a semiconductor as a channel region.
By applying a voltage to the gate electrode 36 provided through the channel region, the potential of the channel region changes,
Current control becomes possible.

Furthermore, the region 24, which has properties similar to those of a semiconductor as a channel region, has a crystal structure similar to that of the superconductor 26, 28, which serves as a source and drain region.
Superconductivity can be achieved if sufficient carriers are induced by the gate voltage. That is, a superconducting current can be induced by the gate voltage.

Furthermore, if the carrier density of the region 24, which has characteristics similar to those of a semiconductor as a channel region, is set to the limit where metal-nonmetal transition occurs, a slight change in carrier density can transform the channel into a superconducting-insulating state. It can be changed physically. That is, a small gate voltage can switch between superconductor and insulator between source and drain.

Thus, the superconducting transistor according to the first embodiment uses YBa2Cu30 as the source and drain regions.
Each junction of the substance Y
Since it is based on B a 2 Cu 30 Technical difficulties caused by this can be avoided.

Good electrical contact is therefore formed with sufficient reproducibility between the superconductor forming the source and drain regions and the material having similar properties to the semiconductor forming the channel region.

In the first example, the composition of oxygen 0 in the yttrium-based oxide superconductor YBa2Cu3O7 is changed.
0) Region 6.8 is the source and drain region, Y
Ba2Cu30X (x = 6, 1~
6,5) NJ4 is used as a channel region, but this yttrium Y is used as a lanthanide (Ln
) system elements such as erbium Er, holmium HO, cadrinium Gd, ytterbium Yb, etc.

Moreover, instead of the Zr02 layer 14 as an insulating layer provided between the YBa2Cu30x (x=6.1 to 6.5) layer 4 as a channel region and the Au electrode 16 as a gate electrode, an AI 203 layer is used. May be used.

Furthermore, without providing such a gate insulating layer,
YB a 2Cu 30 X (x = 6, 1~6
, 5) J17., in which a metal electrode as a gate electrode is formed directly on HA 4 and has a Schottky barrier.
It may be set to n.

Next, a second embodiment of the present invention will be described.

FIG. 3 is a sectional view showing a cross section of a superconducting transistor according to a second embodiment of the present invention.

A LaCuO4 layer 44 with a thickness of 500 nm is formed on the S r T i 03 substrate 42 . This La
A Cu 04 layer 44 is grown on the S r T i 03 substrate 42 by sputtering or e-beam evaporation. In addition, this L a Cu O4M 44 is
It is a substance that has properties similar to semiconductors.

On this La CuO4 layer 44, 400 nm thick (La S
r) A CuO4 (x=0.075)1-xx layer 46.48 is formed. These (La
Sr)CuO4(x=0.075)1-x
The x layer 46.48 has a thickness of 400 mm on the La CuO4 layer 44.
nm of (LaSr)CuO41-x
It is formed by growing a layer x (x=0.075) and then performing selective etching.

In addition, these (La Sr )CuO41−
x x (x=0.075)146.48 is a lanthanum-based oxide superconductor.

These (La Sr )CuO41-x
On the x (x=0.075) layers 46 and 48, Auti 50 and 52 are formed as source and drain electrodes. In addition, (L
On the La CuO4 layer 44 as a channel region sandwiched between the aSr ) CuO41-x It is formed by chemical vapor deposition. Furthermore, this Z r O
On the second layer 54, A U N W as a gate electrode is formed.
56 is formed.

In this way, the source and drain regions are superconducting (LaSr)CuO41-x
x (x=0.075) layers 46 and 48, and the channel region sandwiched between them has characteristics similar to that of a semiconductor.
A superconducting FET consisting of a CuO4 layer 44 is formed, but at this time (La Sr )CuO4 (x = 0.075)
The -x x layer 46, 48 and the L a Cu O 4 layer 44 are characterized in that they have essentially the same type of crystal structure, with the only difference being the presence or absence of a trace amount of strontium Sr.

The operation of this second embodiment is exactly the same as that described above using FIG.

Thus, the superconducting transistor according to the second embodiment is
Similar to the superconducting transistor shown in FIG. 1, (L a S r ) as the source and drain regions
CuO4(x-0,075)1-xx layer 46.48 and La2CuO4 as channel region
Since each junction with layer 44 has essentially the same crystal structure, there is no electrical barrier between each region that would adversely affect the properties, and the junctions are made with different amounts of materials with different crystal structures. This avoids the technical difficulties caused by the formation of

Good electrical contact is therefore formed with sufficient reproducibility between the superconductor forming the source and drain regions and the material having similar properties to the semiconductor forming the channel region.

Note that in the second embodiment, the L a 2 Cu O 4 layer 44 is used as the channel region;
Instead of this La 2 Cu O 4 layer 44, a trace amount of strontium Sr is added (La Sr
)CuO4(x<0102)1-xx layer may be used.

In addition, lanthanum-based oxide superconductor (La Sr )CuO4 (x=O-075)
1-x x layer 46 as source and drain regions, La2CuO
4 layer 44 or the composition of strontium Sr is changed (La1-x5rx)CuO4 (x < 0,
02) layer is configured as a channel region,
This strontium Sr may be replaced with barium Ba, calcium Ca, etc., for example.

Further, Z r as an insulating layer provided between the La2CuO4 layer 44 as a channel region and the gate electrode 56.
Instead of the 02 layer 54, an A I 203 layer may be used.

Furthermore, without providing such a gate insulating layer,
A metal electrode as a gate electrode may be formed directly on the La2CuO4 layer 44 as a channel region to form a structure having a Schottky barrier.

"Effects of the Invention" As described above, according to the present invention, since each of the source, channel, and drain regions is composed of a material having essentially the same type of crystal structure, the characteristics between each region are adversely affected. There is no electrical barrier that would give rise to the problem, and there are no technical problems caused by forming a junction with amounts of materials with different crystal structures.

This makes it possible to increase the transfer conductance of the superconducting transistor and improve its reproducibility.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing a superconducting transistor according to a first embodiment of the present invention, FIG. 2 is a diagram for explaining the operating principle of the present invention, and FIG. 3 is a cross-sectional view showing a superconducting transistor according to a second embodiment of the present invention. Cross-sectional view showing a superconducting transistor FIG. 4 is a cross-sectional view showing a conventional superconducting transistor. In the figure, 2.42...SrTiO3 substrate, 4...
・YBa2Cu3Ox (x=6.1~6.5) layer, 6.8...YBaCu30x (x=6.8~
7.0) Area, 10 12.16, 50, 52.56... Gold (
Au) electrode, 1.4.54...Zirconium oxide (Z r
02) layer, 24...A region 24 having similar characteristics to a semiconductor
．． 2628...Superconductor region, 30...Source electrode, 32...Drain as pole, 34.74...Insulating layer, 36.76... ...Gate electrode, 114...LaCuO4 layer, 46.48----= (La1-x5rx)CuO4
(x=0.075) layer, 64... Semiconductor substrate, 66.68... Niobium (Nb) electrode.

Claims (1)

[Scope of Claims] A source region and a drain region of high carrier concentration that are superconductors, and a similar crystal structure sandwiched between these source and drain regions and having a partially different composition from the source and drain regions. A superconducting transistor comprising: a channel region with a low carrier concentration; and a gate electrode provided on the channel region.