Superconducting field effect transistor with increased channel length

Disclosed herein is a superconducting field effect transistor (FET) which has at least an active region formed from a film of oxide normal conductor, a plurality of electrodes formed from a film of oxide superconductor, and a means to control the current which flows between the electrodes through the active region. Having a much greater electrode distance than the conventional superconducting device, it can be produced easily by lithography without resorting to special techniques.

BACKGROUND OF THE INVENTION 
The present invention relates to generally superconducting (FETS), more 
particularly, to a superconducting FET provided with electrodes of an 
oxide superconductor which can be produced easily. 
There are superconducting devices known as Josephson Junction devices, 
superconducting transistors, and superconducting quantum interference 
devices (SQUID). Despite their outstanding characteristic properties such 
as extremely high operating speed, low power dissipation, and high 
sensitivity for magnetic field detection, they have been considered to be 
impracticable because the conventional superconductors become 
superconducting only at an extremely low temperature close to the liquid 
helium temperature. A recent breakthrough in this field is the discovery 
of oxide superconductors that have their temperature higher than the 
temperature of liquid nitrogen. Superconducting devices based on an oxide 
superconductor are being put to practical use. 
An example of superconducting devices is a superconducting field effect 
transistor, which is explained in the following with reference to FIG. 1 
(sectional view). FIG. 1 shows two sets of identical devices formed on a 
single substrate. The principle on which this superconducting field effect 
transistor operates is as follows: A supercurrent induced by the 
superconducting proximity effect flows from source electrodes 1a, 1b of 
superconductor to drain electrodes 2a, 2b of superconductor through a 
semiconductor part 3. This supercurrent is controlled by the application 
of a gate voltage to gate electrodes 5a, 5b formed on insulators 4a, 4b. 
In the case where an oxide superconductor is used, it is theoretically 
known that the distance in which superconducting electrons diffuse from 
the superconductor to the semiconductor (normal layer) is extremely short. 
This has led one to believe that the distance between the source and drain 
electrodes should be shorter than 0.1 .mu.m in the case of superconducting 
field effect transistor based on an oxide superconductor. In other words, 
it is generally theorized that superconducting electronics devices (such 
as a Josphson junction device and superconductor three-terminal device) 
formed from an oxide superconductor with a high critical temperature 
should have a channel length corresponding to the coherence length of the 
oxide superconductor. For example, in the case of a superconducting device 
formed from a Y--Ba--Cu oxide, which is a typical oxide superconductor, 
the channel length (or the distance between the two electrodes) should be 
0.3-1.4 nm, which is equal to the coherence length of the oxide 
superconductor. An example of such a superconducting device is reported in 
Applied Physics Letters, vol. 51, p. 200, 1987. It is a Josephson junction 
device of Y--Ba--Cu oxide in which the crystal boundary of the oxide 
constitutes the junction. In other words, the crystal boundary in the 
narrow part of the Y--Ba--Cu oxide thin film functions as the Josephson 
junction. The thin film in a loop has two such junction areas to 
constitute a dc SQUID. 
As mentioned above, it has been considered that the Josephson junction and 
the channel length of a superconducting three-terminal device should have 
an extremely small dimension, say 1 nm, which equals the coherence length 
of the oxide superconductor. Unfortunately, it is impossible to form a 
pattern of oxide superconductor accurately on the order of a nanometer 
even with the most advanced fine pattern fabrication technique. In other 
words, there has been no method for producing superconducting devices by 
forming various fine patterns with high accuracy as designed. The 
conventional technology does not permit the production of devices with 
uniform characteristics as designed and the integration of many identical 
elements on a single substrate. It is very difficult to produce 
superconducting devices (such as three-terminal devices) which are more 
complicated in structure than conventional diodes. 
SUMMARY OF THE INVENTION 
The present invention is directed to solving the above-mentioned problems 
involved in the prior art technology. It is an object of the present 
invention to provide a superconducting device that can be produced easily 
by the currently available lithographic technology. 
It is another object of the present invention to provide a superconducting 
device which has electrodes of an oxide superconductor, wherein the 
distance between the electrodes (or the length of channel region) is not 
excessively short. 
According to the present invention, the electrodes are formed from an oxide 
superconductor and the channel region is formed from an normal oxide 
conductor and the distance between the electrodes is within a prescribed 
range. 
It has been considered that the distance between the electrodes (or the 
length of the channel region) should be as short as 0.1 .mu.m or less in 
the case where the electrodes are formed from an oxide superconductor. 
Contrary to this general belief, the present inventors found that the 
oxide superconductor permits supercurrent to flow a longer distance than 
expected from the known theory owing to the proximity effect. (This was 
reported in Extended Abstracts of the 9th Symposium on Future Electron 
Devices, pp. 85-90, 1990.) 
The present inventors experimented with a layered film of sandwich 
structure as shown in FIG. 2, which is composed of one middle layer 12 of 
La.sub.1.5 Ba.sub.1.5 Cu.sub.3 O.sub.y normal conductor (800 nm thick) and 
two outer layers 11 and 13 of YBa.sub.2 Cu.sub.3 O.sub.y superconductor, 
to test its electrical properties at the liquid helium temperature. 
Theoretically (as mentioned above), it is hardly possible that a 
supercurrent flows across the outer layers 11 and 13 of the superconductor 
in the case where the middle layer 12 of the normal conductor is as thick 
as 800 nm. Nevertheless, a supercurrent flowing across the outer layers 11 
and 13 of superconductor was detected. This is a new phenomenon which has 
never been observed. A similar phenomenon was also found when the middle 
layer of La.sub.1.5 Ba.sub.1.5 Cu.sub.3 O.sub.y normal conductor was 
replaced by a 100-nm thick layer of PrBa.sub.2 Cu.sub.3 O.sub.y normal 
conductor. It was found that supercurrent flows through the channel layer 
thicker than 100 mm only in the case where the channel layer is a normal 
conductor derived from an oxide superconductor, which has substituent 
cations, nonstoichiometric composition for cations, or oxygen deficiency 
or oxygen excess. If this specific normal conductor is replaced by other 
materials such as SiO.sub.2, CaF.sub.2, Y.sub.2 O.sub.3, Al.sub.2 O.sub.3, 
and ZnO, the phenomenon as mentioned above is not observed. 
According to the present invention, the two electrodes of oxide 
superconductor are arranged a certain distance apart from each other which 
is longer than the decay length of the probability amplitude of 
superconducting electron pairs which is determined by the clean limit, and 
shorter than 10 times the decay length. (The electrodes are connected by a 
Cu-based oxide normal conductor which forms the active region.) The length 
determined by the clean limit equals the decay length (.zeta.) of the 
probability amplitude of superconducting electron pairs in the normal 
conductor layer. The distance between the two electrodes should be equal 
to or longer than the decay length (.zeta.), as mentioned above. The decay 
length (.zeta.) in the clean limit is given by hv/4.pi..sup.2 kT (where h 
is Planck's constant, k is Boltzmann's constant, v is the Fermi velocity 
of the carrier, which is given by 
##EQU1## 
(where m is the mass of the carrier, h'=h/2.pi., and n is the density of 
free carriers), and T is the operating temperature.) The value calculated 
for La--Ba--Cu oxide (an oxide normal conductor) is 50 nm at 10K. This 
value varies depending on the temperature at which a particular device is 
operated. 
A preferred oxide normal conductor is one which has the same crystal 
structure (e.g., perovskite structure) as an oxide superconductor and the 
antiferromagnetic properties or semiconductive properties. It includes 
oxygen-deficient Y--Ba--Cu oxide, Pr--Ba--Cu oxide, and La--Ba--Cu oxide, 
and doped Y--Ba--Cu oxide containing metallic elements or ferromagnetic 
elements (such as Fe, Co, Al, and Ga). The length of the normal layer of 
these oxides (or the distance between the electrodes of the 
superconductor) should be greater than the decay length (which is 
determined individually by the Fermi velocity). 
The oxide normal conductor that can be used in the present invention 
include the following. Oxygen-deficient Y--Ba--Cu oxides having the ionic 
ratio of Y:Ba:Cu:O=1:2:3:X [X=6.2-6.6], with an allowable variation of 
.+-.20% for Y:Ba:Cu. Oxygen-deficient Pr--Ba--Cu oxides having the ionic 
ratio of Pr:Ba:Cu:O=Y:(3-Y):3:(6.5-7.3) [Y=1-1.5], with an allowable 
variation of .+-.20% for Pr:Ba:Cu. Oxygen-deficient La--Ba--Cu oxides 
having the ionic ratio of La:Ba:Cu:O=Z:(3-Z): 3:(6.5-7.3) [Z=1.3-1.6], 
with an allowable variation of .+-.20% for La:Ba:Cu. 3-50% of Cu in the 
Y--Ba--Cu oxides may be replaced by Fe, Co, Al, or Ga. 
The oxide superconductor that can be used in the present invention includes 
Y--Ba--Cu oxides, La--Sr--Cu oxides, Bi--Sr--Ca--Cu oxides, and 
Tl--Ba--Ca--Cu oxides. The Y--Ba--Cu oxides may have Y substituted by any 
of Eu, Gd, Dy, Er, Ho, and Sm. The Y--Ba--Cu oxides may have the ionic 
ratio of Y:Ba:Cu:O=1:2:3:(6.6-7.0). The La--Sr--Cu oxides may have the 
ionic ratio of La:Sr:Cu:O=(2-X):X:1:(3.8-4.3) [X=0.05-0.3]. The 
Bi--Sr--Ca--Cu oxides may have the ionic ratio of Bi:Sr:Ca:Cu:O=2:2:1:2:8 
or 2:2:2:3:10. The Ta--Ba--Ca--Cu oxides may have the ionic ratio of 
Ta:Ba:Ca:Cu:O=2:2:1:2:8 or 2:2:2:3:10. The ratios may vary within .+-.20%. 
The superconducting device of the present invention is a three-terminal 
device whose basic part is composed of one middle layer of the 
above-mentioned oxide normal conductor and two outer layers of the 
above-mentioned oxide superconductor, which are joined on top of the 
other. This basic part is provided with a gate electrode to control, by 
voltage application, the supercurrent flowing through the layer of oxide 
normal conductor. This gate electrode is formed on the layer of the oxide 
normal conductor with an insulating film interposed between them. 
At the interface between the superconducting layer and the normal 
conducting layer, the density of superconducting carriers is not 
continuously distributed on account of the discontinuity of atom 
arrangement and constituent elements. In general, the density of 
superconducting electrons at the interface discontinuously decreases up to 
the normal conducting layer. 
The rate of decrease depends on the combination of the superconductor and 
normal conductor. The rate of decrease is great when the two materials are 
of different species, as in the combination of an oxide superconductor and 
a metal normal conductor. In order that the density of superconducting 
electrons does not discontinuously decrease in the interface, it is 
desirable that the superconductor be combined with a normal conductor of 
the same crystal structure as the superconductor. It was found that a 
combination of a Cu-based oxide superconductor and a Cu-based oxide normal 
conductor is effective in reducing the decrease of the density of 
superconducting electrons. 
A preferred Cu-based oxide normal conductor is one which is given the 
antiferromagnetic properties or semiconductive properties by the reduction 
of carriers in a Cu-based oxide superconductor. In general, a metal 
superconductor does not possess magnetism and superconductivity 
simultaneously, and it loses its superconductivity when it contains 
magnetic impurities. 
On the other hand, in the case of oxide superconductor it is CuO.sub.2 
planes which are responsible for superconduction. Although CuO.sub.2 is 
inherently an antiferromagnetic material, its antiferromagnetism is weak 
in a Cu-based oxide superconductor because carriers (or holes) occur when 
Cu has a valence Treater than 2 and they cause the electronic wave 
functions to be less localized. This suggests that the superconductivity 
of an oxide superconductor depends not on the magnetic properties but on 
the presence of carriers. 
Very little is known about the microscopic mechanism of superconductivity 
in an oxide superconductor. Phenomenologically, it is assumed that 
CuO.sub.2 planes have something that causes carriers to attract to each 
other resulting in superconductivity. This is the reason why an oxide 
superconductor permits the coexistence of superconductivity and 
antiferromagnetism. It is concluded from the foregoing that the distance 
superconducting electron pairs travel from the oxide superconducting 
electrode to the oxide normal conducting layer is greater than the 
coherence length which is determined by the measurement of critical 
magnetic field. 
In the case of an oxide normal conductor (such as La--Ba--Cu oxide), the 
decay length to be determined by the dirty limit is about 1 nm according 
to calculations from the hall coefficient and resistivity. This decay 
length is apparently associated with the short mean free path of an oxide 
material. Resistance of an oxide material results not from the scattering 
due to lattice vibration but from the fact that there is no complete 
overlapping of wave functions for adjacent atoms. 
As mentioned above, a Cu-based oxide potentially possesses the 
superconducting properties although it is a normal conducting material. 
The superconducting electron pairs which travel from the superconducting 
layer to the normal conducting layer retain the superconducting properties 
even after apparent scattering. This leads one to conclude that the 
distance superconducting electron pairs travel is not the decay length 
which is determined by the dirty limit but it rather depends on the clean 
limit. The distance between the superconducting electrodes exponentially 
decreases with a constant determined by the clean limit (or the 
above-mentioned decay length .zeta.). Therefore, the electrode distance 
suitable for the Josephson coupling should be in the range from the decay 
length (.zeta.) to ten times the decay length (.zeta.). If the electrode 
distance is smaller than the decay length (.zeta.), the phase shift of 
superconducting wave functions does not take place between electrodes. If 
the electrode distance is greater than ten times the decay length, the 
supercurrent density of the device is smaller than 10.sup.5 A/m.sup.2 
which is necessary for practical use.

DETAILED DESCRIPTION OF THE INVENTION 
EXAMPLE 1 
A device of planar structure was prepared by forming on a SrTiO.sub.3 
substrate a normal conducting layer 22 of La--Ba--Cu oxide 
(La:Ba:Cu:O=1.5:1.5:3:(6.8-7.1)) and electrodes 23 of Y--Ba--Cu oxide 
(Y:Ba:Cu:O=1:2:3:(6.8-7.0)), as shown in FIG. 3. The device was tested for 
voltage-current characteristics, with the distance between the electrodes 
23 varied from 0.1 .mu.m to 0.8 .mu.m. The device wish an electrode 
distance of 0.2 .mu.m gave the voltage-current characteristics as shown in 
FIG. 4, which indicates the occurrence of superconductivity. The 
superconductivity was detected at temperatures up to about 70K. The same 
phenomenon as mentioned above was also observed with other devices with an 
electrode distance other than 0.2 .mu.m. 
The electrode distance (0.1-0.8 .mu.m) in this example is 100-1000 times 
the coherence length (about 1 nm) of an oxide superconductor. This 
suggests the possibility of reducing the electrode distance much more than 
believed before by using an oxide normal conductor having the same crystal 
structure as an oxide superconductor and also having the antiferromagnetic 
properties and semiconductive properties. 
EXAMPLE 2 
A superconducting three-terminal device was formed on a substrate 21 of 
SrTiO.sub.3 single crystal with (110) index, as shown in FIG. 5. On this 
substrate 21 was formed a normal conducting layer 22 of La--Ba--Cu oxide 
by RF magnetron sputtering under the following conditions. Atmosphere gas: 
argon-oxygen mixture (50:50%). 
Total pressure: 0.4 Pa. 
Target: Sintered disc of La--Ba--Cu oxide. 
Power source: 13.56 MHz high frequency. 
Electric power: 100 W 
Substrate temperature: 550.degree.-700.degree. C. 
The thus formed La--Ba--Cu oxide layer has the electric resistance similar 
to that of a semiconductor. 
Subsequently, on the La--Ba--Cu oxide layer 22 was formed a superconducting 
electrode layer 23 (80 nm thick) of Y--Ba--Cu oxide by the reactive vapor 
deposition method which consists of vaporizing Y, Ba, and Cu in an oxygen 
atmosphere. The Y--Ba--Cu oxide superconducting layer 23 has its critical 
temperature at 80K. 
The surface of the layer 23 was entirely coated with an organic resist (not 
shown) and a groove pattern (0.2 .mu.m wide) was formed in the resist by a 
known electron beam lithography method. Using this resist pattern as the 
mask, a groove was formed in the Y--Ba--Cu oxide layer 23 by reactive ion 
beam etching in an atmosphere of argon or argon-oxygen mixture. 
This groove divides the Y--Ba--Cu oxide layer into two superconducting 
electrodes 23 0.2 .mu.m apart. 
Then, a SrTiO.sub.3 layer (or a gate insulating layer 24) was formed by RF 
magnetron sputtering in an atmosphere of 50:50 argon-oxygen mixture. 
Further, a gate electrode 25 of Au layer was formed on the groove held 
between the superconducting electrodes 23. 
The thus formed three-terminal device of oxide superconductor permits 
supercurrent to flow between the superconducting electrodes 23. The 
supercurrent can be controlled by the application of a voltage to the Au 
gate electrode 25. In other words, when a positive voltage is applied to 
the gate electrode 25, the carrier concentration in the normal conducting 
layer 22 decreases. As a result, the effective decay length becomes short 
and the supercurrent decreases accordingly. In this way the device in this 
example functions as a superconducting three-terminal device. 
The device was tested for its characteristics with the electrode distance 
varied from 0.1 .mu.m to 1.0 .mu.m. The results indicate that the 
supercurrent can be controlled by the gate voltage as in the 
above-mentioned case where the electrode distance is 0.2 .mu.m. In the 
case of long electrode distance, the supercurrent is low when the gate 
voltage is zero. In such a case, a negative gate voltage is applied to 
form the accumulation layer in the normal conducting layer, thereby 
increasing the carrier density and supercurrent. 
In the same manner as mentioned above, another superconducting 
three-terminal device was prepared in which the superconducting electrode 
is Y--Ba--Cu oxide, the oxide normal conducting layer is Pr--Ba--Cu oxide 
(Pr:Ba:Cu:O=1:2:3:(6.8-7.1)), the gate insulation layer is SrTiO.sub.3, 
and the gate electrode is Au. This device, too, permits supercurrent to 
flow between the Y--Ba--Cu electrodes, which can be controlled by the 
application of a voltage to the Au gate electrode. Thus, this device 
functions satisfactorily as the three-terminal device. 
Superconducting three-terminal devices of the same structure as mentioned 
above were prepared in which La--Ba--Cu oxide or Pr--Ba--Cu oxide was 
replaced by La--Cu oxide or Bi--Sr--Cu oxide (of perovskite structure), 
oxygen-deficient Y--Ba--Cu oxide, or Y--Ba--Cu oxide containing as 
impurities ferromagnetic elements and metallic elements (such as Fe, Co, 
Al, and Ga). 
The oxide for the oxide normal conducting layer include the following. 
Oxygen-deficient Y--Ba--Cu oxides having the ionic ratio of 
Y:Ba:Cu:O=1:2:3:X [X=6.2-6.6], with an allowable variation of .+-.20% for 
Y:Ba:Cu. 
Oxygen-deficient Pr--Ba--Cu oxides having the ionic ratio of 
Pr:Ba:Cu:O=Y:(2-Y):3:(6.5-7.3) [Y=1-1.5], with an allowable variation of 
.+-.20% for Pr:Ba:Cu. 
Oxygen-deficient La--Ba--Cu oxides having the ionic ratio of 
La:Ba:Cu:O=Z:(2-Z):3:(6.5-7.3) [Z=1.3-1.6], with an allowable variation of 
.+-.20% for La:Ba:Cu. 3-50% of Cu in the Y--Ba--Cu oxides may be replaced 
by Fe, Co, Al, or Ga. 
The Y--Ba--Cu oxide for the superconducting electrode may be replaced by a 
Bi--Sr--Ca--Cu oxide or Tl--Ba--Ca--Cu oxide having the perovskite 
structure. 
Other oxides that can be used for the superconducting electrode include the 
following. 
Analogues of Y--Ba--Cu oxides in which Y is replaced by Eu, Gd, Dy, Er, Ho, 
or Sm. 
Y--Ba--Cu oxides having the ionic ratio of Y:Ba:Cu:O=1:2:3:(6.6-7.0), with 
an allowable variation of .+-.20%. 
La--Sr--Cu oxides having the ionic ratio of La:Sr:Cu:O=(2-X):X:1:(3.8-4.2) 
[X=0.05-0.3], with an allowable variation of .+-.20%. 
Bi--Sr--Ca--Cu oxides having the ionic ratio of Bi:Sr:Ca:Cu:O=2:2:1:2:8 or 
2:2:2:3:10, with an allowable variation of .+-.20%. 
Ta:Ba:Ca:Cu oxides having the ionic ratio of Ta:Ba:Ca:Cu:O=2:2:1:2:8 or 
2:2:2:3:10, with an allowable variation of .+-.20%. 
EXAMPLE 3 
A laminate structure as shown in FIG. 7 (plane view) was prepared which has 
the channel layer formed from La.sub.2 CuO.sub.4 which is a normal 
conducting compound. All the layers were formed by RF magnetron sputtering 
under the following conditions. 
Substrate temperature: 550.degree. C. 
Supplied power: 100 W. 
Atmosphere gas: argon-oxygen mixture (50:50%). 
Total pressure: 6.65 Pa. 
First, on a substrate 30 of SrTiO.sub.3 single crystal with (110) index was 
formed a thin film (10 mm square) 31 of La.sub.2-x Sr.sub.x CuO.sub.4 
(x=0.15) , with a stainless steel mask placed on the substrate. Then a 
thin film (5 mm square) 32 of La.sub.2-x Sr.sub.x CuO.sub.4 (x=0) 
(containing no Sr) was formed through a metal mask, with a part (3 mm 
square) thereof overlapping with the previously formed thin film 31, as 
shown in FIG. 7. Further, a thin film (10 mm square) 33 was formed from 
La.sub.2-x Sr.sub.x CuO.sub.4 (x=0.15), which is the same material as used 
for the lower thin film 31, with a part (3 mm square) thereof overlapping 
with the channel layer 31. The thus formed three layers have an overlapped 
part (1 mm square). The lower layer 31 is 700 nm thick, the channel layer 
32 is 800 nm thick, and the upper layer 33 is 400 nm thick. Finally, the 
laminate underwent heat treatment in argon at 400.degree. C. for 12 hours. 
After the heat treatment, the single layer of La.sub.2-x Sr.sub.x CuO.sub.4 
(x=0.15) exhibited superconductivity at a critical temperature of 32K. On 
the other hand, the single layer of La.sub.2-x Sr.sub.x CuO.sub.4 (x=0) 
showed no sign of superconduction in the current-voltage characteristics 
and produced no Meissner effect when measured with a SQUID magnetometer at 
4.2K. Apparently it remained to be a complete non-superconductor. 
The laminate structure prepared as mentioned above was tested for 
current-voltage characteristics at 4.2K according to the four-terminal 
probe method. The results are shown in FIG. 8. The test was carried out by 
measuring the voltage between the two terminals 36, and 37 which is 
induced when an electric current is applied to the terminals 34 and 35. 
The terminals are gold wires (50 .mu.m in diameter) electrically contacted 
with the sample with indium. The non-linearity of current in the 
neighborhood of zero voltage shown in FIG. 8 suggests that supercurrent 
flows between the two superconducting layers 31 and 33 through the normal 
conducting layer 32 (800 nm thick), with the direction of the supercurrent 
being perpendicular to the surface of the normal conducting layer 32. 
EXAMPLE 4 
The same procedure as in Example 3 was repeated except that the channel 
layer (normal conducting layer) was formed from La.sub.2-x 
Sr.sub.CuO.sub.4 (x=0.4) (which contains more Sr than La.sub.2-x Sr.sub.x 
CuO.sub.4 (x=0.15) for the superconducting layer) in place of La.sub.2-x 
Sr.sub.x CuO.sub.4 (x=0) which contains no Sr. The channel layer is 800 nm 
thick. It was found that the channel layer remained non-superconducting 
electrically and magnetically at 4.2K. The laminate structure was tested 
for current-voltage characteristics by the four-terminal probed method. 
The results indicate the non-linearity of current in the neighborhood of 
zero voltage, as in the case of Example 3. 
EXAMPLE 5 
A laminate structure as shown in FIG. 9 was prepared which has the channel 
layer formed from PrBa.sub.2 Cu.sub.3 O.sub.y which is a normal conducting 
compound. All the thin films were formed by RF magnetron sputtering under 
almost the same conditions as in Example 3. First, on a substrate 40 (10 
mm square) of SrTiO.sub.3 single crystal with (110) index was formed a 
layer 41 of YBa.sub.2 Cu.sub.3 O.sub.y. On the center of this layer was 
formed a layer 42 of PrBa.sub.2 Cu.sub.3 O.sub.y, with a metal mask (6 mm 
square) placed thereon, as shown in FIG. 9. Then on the center of the 
layer 42 was formed a layer 43 of YBa.sub.2 Cu.sub.3 O.sub.y (which is 
same material as used for the lower layer 41), with a metal mask (2 mm 
square) placed thereon. The lower layer 41 is 700 nm thick, the channel 
layer 42 is 100 nm thick, and the upper layer 43 is 400 nm thick. Finally, 
the laminate underwent heat treatment in argon at oxygen plasma at 
450.degree. C. for 30 minutes. 
After the heat treatment, the single layer of YBa.sub.2 Cu.sub.3 O.sub.y 
exhibited superconductivity at a critical temperature of 62K. On the other 
hand, the single layer of PrBa.sub.2 Cu.sub.3 O.sub.y showed no sign of 
superconductivity in the current-voltage characteristics and produced no 
Meissner effect when measured with a SQUID fluxmeter at 4.2K. Apparently 
it remained to be a complete non-superconductor. 
The laminate structure prepared as mentioned above was tested for 
current-voltage characteristics at 4.2K according to the four-terminal 
probe method. The results are shown in FIG. 10. The test was carried out 
by measuring the voltage between the two terminals 46 and 47 which is 
induced when an electric current is applied to the terminals 44 and 45. 
The terminals are gold wires (50 .mu.m in diameter) electrically contacted 
with the sample with indium. The non-linearity of current in the 
neighborhood of zero voltage shown in FIG. 10 suggests that supercurrent 
flows between the two superconducting layers 41 and 43 through the normal 
conducting layer 42 (100 nm thick). 
EXAMPLE 6 
A laminate structure as shown in FIG. 11 (plan view) was prepared which has 
the channel layer formed from La.sub.1.5 Ba.sub.1.5 Cu.sub.3 O.sub.y which 
is a normal conducting compound. First, on a substrate 50 of SrTiO.sub.3 
single crystal with (110) index was formed a thin film 51 (300 nm thick) 
of La.sub.1.5 Ba.sub.1.5 Cu.sub.3 O.sub.y by RF magnetron sputtering under 
almost the same conditions as in Example 3. On the center of this film was 
formed a thin film (1.times.5 mm) 52 of HoBa.sub.2 Cu.sub.3 O.sub.y by the 
reactive vapor deposition method that employs ECR oxygen plasma, with a 
metal mask placed thereon, as shown in FIG. 11. The vapor deposition was 
carried out under the following conditions. 
Microwave: 2.45 GHz generated by a 120 W magnetron. 
Magnetic field: 875 G. 
Atmosphere: Oxygen at 10.sup.-4 Torr. 
Rate of deposition: 0.6 nm/sec. 
Substrate temperature: 560.degree. C. 
Film thickness: 80 nm. 
After resist coating, a linear pattern (200 nm wide) crossing the thin film 
52 was formed by electron beam lithography. Using this resist pattern as 
the mask, a groove channel 53 was formed by reactive ion beam etching. The 
groove is 25 nm deep. Finally, the laminate underwent heat treatment in 
ECR oxygen plasma at 450.degree. C. for 30 minutes. 
After the heat treatment, the single layer of HoBa.sub.2 Cu.sub.3 O.sub.y 
exhibited superconduction at a critical temperature of 78K. On the other 
hand, the single layer of La.sub.1.5 Ba.sub.1.5 Cu.sub.3 O.sub.y showed no 
sign of superconductivity in the current-voltage characteristics and 
produced no Meissner effect when measured with a SQUID magnetometer at 
4.2K Apparently it remained to be a complete non-superconductor. 
The laminate structure prepared as mentioned above was tested for 
current-voltage characteristics at 4.2K according to the four-terminal 
probe method. The results are shown in FIG. 12. The Zest was carried out 
by measuring the voltage between the two terminals 56 and 57 which is 
induced when an electric current is applied to the terminals 54 and 55. 
The terminals are gold wires (50 .mu.m in diameter) electrically contacted 
with the sample with indium. The non-linearity of current is observed in 
the neighborhood of zero voltage. The temperature dependence of 
supercurrent is shown in FIG. 13. Supercurrent was detected at 
temperatures up to 76K, which is close to the critical temperature of the 
superconducting layer 52 of HoBa.sub.2 Cu.sub.3 O.sub.y. FIG.14 shows the 
Shapiro step observed at 4.2K and 2 GHz. The above-mentioned electrical 
properties indicate that supercurrent flows across the superconductor 52 
through the 200 nm wide groove channel 53. 
The devices in Examples 7 to 12 that follow are so designed as to control 
the supercurrent flowing between the source electrode and drain electrode 
by input signals applied to a plurality of gate electrodes formed on the 
channel. 
The current flowing between the source and drain electrodes varies 
depending on voltage applied to the gate electrode, as shown in FIG. 15. 
Therefore, if different voltages are applied to a plurality of gate 
electrodes formed on the channel, the output voltage to loads takes on a 
plurality of values. In addition, a multi-output circuit can be 
constructed by arranging the source electrodes, drain electrodes, and gate 
electrodes on a plane surface. Forming a plurality of gate electrodes on 
the channel is possible because the supercurrent flows over a long 
distance. 
EXAMPLE 7 
A three-terminal superconducting device was prepared by the process 
illustrated in FIGS. 16a to 16d. First, on a stress-free mirror-finished 
substrate 60 of SrTiO.sub.3 single crystal with (110) index was formed a 
thin film 61 (0.2 .mu.m thick) of La.sub.1.5 Ba.sub.1.5 Cu.sub.3 O.sub.y 
by magnetron sputtering, with the substrate temperature kept at 
700.degree. C. for epitaxial growth. On this film was formed a narrow 
rectangular film (0.1 mm wide, 5 mm long, and 0.08 .mu.m thick) 62 of 
YBa.sub.2 Cu.sub.3 O.sub.x by reactive vapor deposition, wish a metal mask 
place thereon, as shown in FIG. 16b. After resist coating, a linear 
pattern (0.2 .mu.m wide) was formed at the center of the narrow 
rectangular film 62 in the direction parallel to the short side, by 
electron beam lithography. Using this resist pattern as a mask, the film 
62 underwent reactive ion beam etching, to form a groove channel 63 as 
shown in FIG. 16c. On this groove channel 63 was formed a 0.1 .mu.m thick 
gate insulating film 64 of SrTiO.sub.3 by sputtering. After gold vapor 
deposition, unnecessary parts of the gold film were removed by electron 
beam lithography and etching to form two gate electrodes 65 and 66, a 
source electrode 67 and a drain electrode 68 on the gate insulating film 
64 and the superconducting film 62. 
The dependence of source-drain current on the gate voltage was measured at 
4.2K (liquid helium temperature) and 50K. The results are shown in FIG. 
17. It is noted that supercurrent flows between the source electrode 67 
and drain electrode 68 as voltages are applied to the gate electrodes 65 
and 66, and that the magnitude of the super-current can be controlled by 
varying the voltages applied to the gate electrodes 65 and 66. This device 
is capable of OR operation and AND operation at a properly established 
threshold value. 
EXAMPLE 8 
A device of the same structure as in Example 7 was prepared, except that 
the electron beam patterning was performed under different conditions so 
that the channel is 0.3 .mu.m wide and three electron gates were formed. 
The dependence of source-drain current on the gate voltage was measured at 
the liquid helium temperature. Because of the extended channel length, the 
supercurrent greatly decreased. Nevertheless, the magnitude of 
supercurrent varied depending on the voltage applied to the individual 
gates. Thus it was possible to control the output voltage to loads. 
EXAMPLE 9 
A device of the same structure as in Example 7 was prepared except that the 
oxide semiconductor layer was formed from Nd.sub.1.3 Ba.sub.1.7 Cu.sub.3 
O.sub.z or PrBa.sub.2 Cu.sub.3 O.sub.w and the superconductor layer was 
formed from HoBa.sub.2 Cu.sub.3 O.sub.m. The dependence of source-drain 
current on the gate voltage was measured. The results were the same as 
those in Example 7. 
Example 10 
A device of the same structure as in Example 7 was prepared except that the 
channel is 0.3 .mu.m long and the first gate electrode is 0.08 .mu.m wide 
and the second gate electrode is 0.16 .mu.m wide (so that the control 
signals applied to the gates vary). The dependence of source-drain current 
on the gate voltage was measured. The results were almost the same as 
those in Example 7. 
EXAMPLE 11 
A device as shown in FIG. 18a was prepared in the same manner as in Example 
7. On a substrate 70 was formed an oxide semiconductor layer 71, and then 
a film 72 of YBa.sub.2 Cu.sub.3 O.sub.x was formed. Two source electrodes 
73 and 74, two drain electrodes 75 and 76, and two gate electrodes 77 and 
78 as shown in FIG. 15b were formed by electron beam lithography and 
reactive ion beam etching. This device is capable of controlling two 
outputs simultaneously. 
EXAMPLE 12 
The same procedure as in Example 7 was repeated except that the substrate 
was replaced by a stress-free mirror-finished single crystal of magnesium 
oxide (MgO) with (100) index and the film on the substrate was re-placed 
by that of Y.sub.0.4 Pr.sub.0.6 Ba.sub.2 Cu.sub.3 O.sub.n. The substrate 
was kept at 600.degree. C. for epitaxial growth. The film thickness is 0.2 
.mu.m as in Example 7. On this film was formed a 0.08 .mu.m thick film of 
HoBa.sub.2 Cu.sub.3 O.sub.m by reactive vapor deposition. Two each of 
source electrodes and drain electrodes were formed in the perpendicular 
direction in the same manner as in Example 11. 
The dependence of source-drain current on the gate voltage was measured at 
the liquid helium temperature and liquid nitrogen temperature. It was 
possible to control the source-drain currents individually by the 
application of gate voltage. They were the same in magnitude. By contrast, 
in the case where the substrate is a single crystal of SrTiO.sub.3 with 
(110) index, the supercurrent in the [001] direction is one-third that in 
the [110] direction. Therefore, a substrate of SrTiO.sub.3 single crystal 
permits a single electrode to perform different controls. 
As mentioned above, the superconducting device according to the present 
invention is provided with a superconducting electrode of oxide 
superconductor and a channel region of oxide normal conductor. Therefore, 
it produces the superconducting proximity effect over a long distance. In 
other words, it permits the source and drain to be positioned much more 
apart than before. This makes it very easy to form the source, drain, and 
gate electrodes. 
The superconducting proximity effect over a long distance, however, poses 
some problems if many superconducting elements are densely integrated to 
such an extent as to shorten the distance between adjacent superconducting 
elements or superconducting wirings. In such a case the superconducting 
proximity effect brings about interactions between the superconducting 
elements and superconducting wirings, resulting in malfunctions and 
increased noise. For example, the device shown in FIG. 1 permits 
undesirable supercurrent (due to proximity effect) to flow between the 
drain electrode 2a and the source electrode 1b. This problem can be solved 
if the adjacent superconducting elements or superconducting wirings are 
electrically separated so that they do not affect each other. An example 
is shown in FIG. 19. In this case the adjacent drain electrode 2a and 
source electrode 1b are separated from each other by etching the 
semiconductor 3 between them. Etching may be replaced by ion implantation 
or chemical reaction which prevents supercurrent from flowing through the 
semiconductor. An alternative way is not to form the semiconductor between 
the superconducting devices or superconducting wirings from the first. 
EXAMPLE 13 
A planar-type superconducting field effect device was prepared which is 
shogun in FIG. 20 (sectional view) and FIG. 21 (plane view). The oxide 
superconductor is HoBa.sub.2 Cu.sub.3 O.sub.x (6.5&lt;.times.&lt;7.0). The oxide 
semiconductor is La.sub.1.5 Ba.sub.1.5 Cu.sub.3.sub.O.sub.x 
(6.5&lt;.times.&lt;7.0). The substrate 82 is a single crystal of SrTiO.sub.3 
(110). First, on the substrate 82 was formed a semiconductor film 81 of 
La.sub.1.5 Ba.sub.1.5 Cu.sub.3 O.sub.x (6.5&lt;.times.&lt;7.0) by RF magnetron 
sputtering under the following conditions. 
Target: sintered body of La.sub.1.5 Ba.sub.1.5 Cu.sub.4.5 O.sub.x. 
Substrate temperature: 650.degree. C. 
Sputtering gas: Ar--O.sub.2 (50:50%) mixture at 30 mTorr. 
RF power: 120 W. 
Film forming rate: 0.15 .mu.m/h. 
Film thickness: 0.6 .mu.m. 
Film forming step was followed by cooling to room temperature in oxygen at 
1 arm. 
On the semiconductor film 81 was epitaxially formed an 80 nm thick 
superconducting film of HoBa.sub.2 Cu.sub.3 O.sub.x (6.5&lt;.times.&lt;7.0) at a 
rate of 60 nm/h by the reactive vapor deposition method, which employs 
microwave oxygen plasma, in the following manner. The substrate surface 
was cleaned for 30 minutes by heating at 580.degree. C. with oxygen plasma 
generated by microwave (120 W) in oxygen (at 8.times.10.sup.-5 Torr). 
Then, vapors of three metals (Ho, Ba, and Cu) were generated from three 
Knudsen cells so as to establish a composition of Ho:Ba:Cu=1:2:3. The film 
forming step was followed by cooling to room temperature in oxygen at 1 
arm. 
The thus formed superconducting film underwent reactive ion beam etching 
with SF.sub.6 gas to form the patterns of source electrodes 88a and 88b 
and drain electrodes 80a and 80b. The gap at the channel between 88a and 
80b is 100 nm. On this was formed a 150 nm thick SrTiO.sub.3 insulating 
film 87a and 87b. On each insulating film was formed a 100 nm thick thin 
film of HoBa.sub.2 Cu.sub.3 O.sub.x (6.5&lt;.times.&lt;7.0), which was 
subsequently etched to form gate electrodes 89a and 89b. 
The thus formed device was tested for current-voltage characteristics. The 
results are shown in FIGS. 22a and 22b. It is noted that when a voltage of 
3 V is applied to the gate electrodes 89a and 89b, supercurrent is 10 mA, 
and when no voltage is applied to the gate electrodes (or in the off 
state), supercurrent decreases to 1 mA. This device is not satisfactory 
because supercurrent in the off state remains high. 
This device was tested again for current-voltage characteristics, with the 
adjacent devices separated by etching the middle part 83 of the 
semiconductor film 81. The results are shown in FIGS. 24a and 24b. It is 
noted that when the gate voltage is zero (or in the off state), 
supercurrent is less than 0.01 mA, which is small enough for the 
superconducting device to be practical. 
This example demonstrates the possibility of utilizing an oxide 
superconductor to make superconductor circuits (such as logic circuits and 
memory circuits) with superconducting field effect transistors, 
superconducting wirings, superconducting loops, etc. arranged less than 1 
.mu.m apart. 
EXAMPLE 14 
A device as shown in FIG. 25 was prepared, which differs from the one shown 
in FIG. 21 in that the adjacent superconducting devices were separated by 
implantation of iron (Fe) ions (10.sup.18 /cm.sup.2) into the 
semiconductor film 81 of La.sub.1.5 Ba.sub.1.5 Cu.sub.3 O.sub.x 
(6.5&lt;.times.&lt;7.0). The part of ion implantation is indicated by 85. This 
device exhibits the same current-voltage characteristics as that in 
Example 13. Thus this example also demonstrates the possibility of 
utilizing an oxide superconductor to make superconductor circuits (such as 
logic circuits and memory circuits) with superconducting field effect 
transistors, superconducting wirings, superconducting loops, etc. arranged 
less than 1 .mu.m apart. 
EXAMPLE 15 
A device was prepared which is similar to but different from the one shown 
in FIG. 21 in that the adjacent superconducting devices are separated by 
depositing silicon on the semiconductor film 81 of La.sub.1.5 Ba.sub.1.5 
Cu.sub.3 O.sub.x (6.5&lt;.times.&lt;7.0) and subsequently diffusing it into the 
semiconductor film by healing in oxygen at 500.degree. C. for 5 hours. 
This device exhibits the same current-voltage characteristics as that in 
Example 13. Thus this example also demonstrates the possibility of 
utilizing an oxide superconductor to make superconductor circuits (such as 
logic circuits and memory circuits) with superconducting field effect 
transistors, superconducting wirings, superconducting loops, etc. arranged 
less than 1 .mu.m apart. 
As mentioned above, the present invention produces the following effects. 
(1) The device does not require that the distance between superconducting 
electrodes be as narrow as 1 nm (which is extremely difficult to realize). 
Instead, it exhibits the superconducting characteristics even though the 
distance between superconducting electrodes is 0.1 .mu.m to 1 .mu.m. 
(2) The device can be formed in planar structure by lithography. The planar 
structure facilitates the formation of integrated and three-terminal 
devices. 
(3) The device (including switching circuit, microwave detector, and SQUID) 
capable of accurate control can be produced very easily according to 
desired specifications.