Superconducting input interface circuit for superconducting circuit

Background of the Invention

Field of the invention

The present invention relates to a superconducting input interface circuit for a superconducting circuit, and more specifically to a superconducting interface circuit which transform a signal sent by an ordinary electronic circuit to a suitable signal for a superconducting circuit.

Description of related art

When a superconducting device or a superconducting integrated circuit is adopted to an electronic equipment, it needs to be associated with a conventional electronic circuit operating at an ordinary temperature. Since it is still impossible for the superconducting device or the superconducting integrated circuit to complete the entire electronic equipment.

Owing to the real nature of superconducting phenomena, such as problems of cooling etc., superconducting devices utilized in a conventional electronic equipment are limited to a switching device and a sensor device including a Josephson junction, a resister, a capacitor and an inductor. Therefore, a complicated circuit still can not be composed of only the superconducting devices alone. Moreover, the superconducting circuit has a small voltage swing range less than millivolts. By this, a weak signal of a few millivolts or a few microvolts are transferred through the superconducting circuit.

In the prior art, in order to transmit a signal of several volts from an ordinary electronic circuit to the superconducting circuit, the signal should be converted to have a voltage swing of a few millivolts or a few microvolts by a normal conducting interface circuit, and then this small voltage swing signal is sent to the superconducting circuit cooled by liquid nitrogen or liquid helium.

This small voltage swing signal sent to the superconducting circuit from the interface circuit is easily affected and disturbed by external electromagnetic waves. Therefore, multiplexing and coding of the small voltage swing signal are restricted so as to decrease the possibility of introducing an error from these processing. This restriction of the multiplexing and coding of the small voltage swing signal leads to an increase the number of signal lines. In addition, since the above mentioned superconducting circuit has a small voltage/current gain, it has insufficient drivability. In order to compensate for the drivability of the superconducting circuit, the number of the signal lines further increases.

The stable transmission of the small voltage swing signal also requires an complete electromagnetic shield around the signal lines. The electromagnetic shield needs additional space.

The increase of the number of the signal lines and the attachment of the electromagnetic shield bring difficulties for realizing a highly integrated superconducting circuit.

The document EP - A - 0 480 814 describes a superconducting FET having a superconductor channel of an extremely thin oxide supraconductor thin film, a superconducting source region and a superconducting drain region of oxide superconductor thin films positioned at both ends of the superconducting channel, and a gate electrode on the superconducting channel through a gate insulator.

The document EP -A -0 344 925 and the article published in IBM Technical Disclosure Bulletin, Vol.19, N°4, September 1976, p.1463 disclose the use of superconducting field effect devices in an inverter. In addition, the document US - 5, 024, 993 discloses a superconducting-semiconducting electrical circuit element comprising a superconducting FET as an interface circuit between a normal-conducting circuit and a superconducting circuit using superconducting wirings. However, these documents do not describe the structure of the interface circuit and particularly the connections between the super-FET devices, the normal-conducting circuit and the superconducting circuit.

Summary of the Invention

Accordingly, it is an object of the present invention to provide an input interface circuit for a superconducting circuit which has overcome the above mentioned defect of the conventional one.

Another object of the present invention is to provide a superconducting interface circuit which transform a signal sent by a normal circuit to a suitable signal for a superconducting circuit.

The above and other objects of the present invention are achieved in accordance with the present invention by a superconducting interface circuit, between a normal-conducting circuit and a superconducting circuit, converting a signal sent from the normal conducting circuit into a small voltage swing signal suitable for the superconducting circuit, comprising two superconducting field effect devices, each of them having a superconducting channel of an extremely thin oxide superconductor thin film, a superconducting source region and a superconducting drain region of oxide superconductor thin films positioned at the both ends of the superconducting channel, and a gate electrode on the superconducting channel through a gate insulator so as to control superconducting current flowing through the superconducting channel by a voltage applied to the gate electrode, wherein the gate electrode of the first superconducting field effect device is connected to a signal line which transmits a voltage signal from the normal conducting circuit, the superconducting source of the first superconducting field effect device is connected to a superconducting bus through a resistor, and the superconducting drain of this first superconducting field effect device is connected to a ground line; and the superconducting drain and the gate of the second superconducting field effect device are connected to the superconducting bus through the resistor, so as to connect the superconducting interface circuit to the superconducting circuit through the superconducting bus,and the superconducting source of this second field effect device is connected to a power line.

The superconducting interface circuit in accordance with the present invention can directly receive a signal of a several volts sent by an ordinary electronic circuit, for example a CMOS circuit. Therefore, by using the superconducting interface circuit in accordance with the present invention, the signal transmission from the ordinary electronic circuit section to the superconducting section becomes stable and is hard to be affected by an environmental electromagnetic field. In addition, multiplexing and coding of the signals become much easier.

By all of these, the number of signal lines between the ordinary electronic circuit section and the superconducting section can be reduced and an electromagnetic shield for the signal lines can be omitted so that a high packing arrangement becomes possible.

The super-FET utilized in the superconducting interface circuit has a superconducting channel of an extremely thin oxide superconductor. Superconducting current flowing trough the superconducting channel is controlled by a signal voltage of several volts applied to a gate electrode arranged on the superconducting channel through a gate insulator. For example, if the superconducting channel of an oxide superconductor has a thickness of 5 nanometers, the gate can be closed by a voltage of 5 volts and superconducting current is completely shut off. Therefore, a signal voltage of 5 or 3.3 volts from a CMOS device can be directly sent to the gate electrode of the super-FET and can directly drive the super-FET. In addition, superconducting current of 10 mA can flow through the superconducting channel having a width of 100 µm.

This large current capability of the super-FET makes it possible that plural superconducting circuit is connected to the superconducting interface circuit through a superconducting bus.

It is preferable that the superconducting channel, superconducting source region and superconducting drain region of the above super-FET are formed of for example Y₁Ba₂Cu₃O_7-δ oxide superconductor material. In this connection, these superconducting regions are preferably formed of not only Y₁Ba₂Cu₃O_7-δ oxide superconductor material but also a high-T_c (high critical temperature) oxide superconductor material, particularly a high-T_c copper-oxide type compound oxide superconductor material, for example a Bi-Sr-Ca-Cu-O compound oxide superconductor material, and a Tl-Ba-Ca-Cu-O compound oxide superconductor material. By using one of these oxide superconductor, a super-FET even operating under liquid nitrogen cooling can be realized.

The above and other objects, features and advantages of the present invention will be apparent from the following description of preferred embodiments of the invention with reference to the accompanying drawings.

Brief Description of the Drawings

• Figure 1 is a block diagram of an embodiment of the electronic equipment including the superconducting interface circuit in accordance with the present invention;
• Figure 2 is a schematic wiring diagram of an embodiment of the superconducting interface circuit in accordance with the present invention used in the electronic equipment shown in Figure 1;
• Figure 3 is a schematic sectional view of a superconducting field effect device (super-FET) utilized in the superconducting interface circuit in accordance with the present invention.

Description of the Preferred embodiments

Referring to Figure 1, there is shown a diagram of an embodiment of an electronic equipment including the superconducting interface circuit in accordance with the present invention. The electric equipment includes an ordinary semiconductor circuit 4 and a superconducting section 5 which are connected to each other by a signal line 6. The superconducting section 5 includes a superconducting interface circuit 1 in accordance with the present invention and a superconducting circuit 2, for example a superconducting signal processor, utilizing an oxide superconductor which are connected through a superconducting bus 7. As explained hereinafter, the superconducting bus 7 is driven by the superconducting interface circuit in accordance with the present invention. The entire superconducting section 5 is cooled by liquid nitrogen.

The ordinary circuit 4 includes an output circuit 3 utilizing a CMOS semiconductor device, which are connected to the superconducting interface circuit 1 by the signal line 6, precisely. A signal transmission along the signal line 6 has an output voltage level of 5.0 volts or 3.3 volts, which is produced by the CMOS semiconductor device of the output circuit 3. This ordinary voltage level signal is difficult to be disturbed by external electromagnetic fields. The signal is converted to a small voltage swing signal which is suitable for the superconducting circuit 2 by the superconducting interface circuit and is sent to the superconducting circuit 2 through the superconducting bus 7. Therefore, in the electronic equipment including the superconducting interface circuit 1 in accordance with the present invention, the small voltage swing signal is transmitted only in the superconducting section 5. Since it is easy to form a magnetic shield which covers the entire superconducting section 5 and this magnetic shield does not need so large space, stable signal transmission from the ordinary circuit 4 to the superconducting section 5 and high density integration becomes possible in the electronic equipment.

Figure 2 shows a wiring diagram of an embodiment of the superconducting interface circuit which is used in the electronic equipment shown in Figure 1. The superconducting interface circuit includes a first super-FET 11 which has a gate connected to the signal line 6, a source connected to the superconducting bus through a resister 13 and a drain connected to the ground line 16, and a second super-FET 12 which has a drain and gate connected to the superconducting bus through the resister 13 and source connected to a power line 15. As explained in detail hereinafter, the super-FETs 11 and 12 have a large superconducting current capability and the flow of the superconducting current is controlled by a signal voltage of several voltages applied to their gates. Therefore, the signal transmitted along the signal line 6 of 5.0 volts or 3.3 volts can be converted to a small voltage swing signal suitable for the superconducting circuit 2 by this superconducting interface circuit.

Figure 3 shows a schematic sectional view of an embodiment of a super-FET which is used in the superconducting interface circuit in accordance with the present invention. The super-FET 20 includes a substrate 25, a superconducting channel 21 of an oxide superconductor thin film having a thickness in the order of 5 nanometers formed on the substrate, a superconducting source region 22 and a superconducting drain region 23 of an oxide superconductor thin film having a thickness of 200 nanometers formed at the both ends of the superconducting channel 21. The super-FET further includes a gate electrode 24 on the superconducting channel 21 through a gate insulator 27. In the super-FET, superconducting current flowing through the superconducting channel 21 is controlled by a signal voltage of several volts applied to the gate electrode 24.

The super-FET has large current capability and current driving ability so that it can send superconducting signals to plural superconducting circuits through a superconducting bus or magnetic coupling.

It is preferable that the superconducting channel, superconducting source region and superconducting drain region of the above super-FET are formed of for example Y₁Ba₂Cu₃O_7-δ oxide superconductor material. In this connection, these superconducting regions are preferably formed of not only Y₁Ba₂CU₃O_7-δ oxide superconductor material but also a high-T_c (high critical temperature) oxide superconductor material, particularly a high-T_c copper-oxide type compound oxide superconductor material, for example a Bi-Sr-Ca-Cu-O compound oxide superconductor material, and a Tl-Ba-Ca-Cu-O compound oxide superconductor material.

As mentioned above, superconducting current flowing through the superconducting channel of the super-FET is controlled by a signal voltage of several volts applied to the gate electrode. Therefore, the signal can be directly sent from a CMOS device to the gate electrode of the super-FET. By using the superconducting interface circuit in accordance with the present invention, it becomes unnecessary to convert a signal into a small voltage swing signal suitable for a superconducting circuit before the signal is sent from a normal conducting circuit, so that the signal sent from the normal conducting circuit has an ordinary level. This makes it easy to send a multiplexed and coded signal from a normal conducting circuit to a superconducting circuit.

The invention has thus been shown and described with reference to the specific embodiments. However, it should be noted that the present invention is in no way limited to the details of the illustrated structures but changes and modifications may be made within the scope of the appended claims.