Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Tunneling through region of reduced conductivity
Patent
1992-08-12
1993-10-05
Hille, Rolf
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Tunneling through region of reduced conductivity
257 36, 257 39, 505 1, 505702, 427 62, H01L 3922, B05D 512, H01B 1200
Patent
active
052508170
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a superconducting Josephson junction, circuit and method of manufacture and, more particularly, to an insulating barrier material having a chemical and structural composition generally compatible with that of the superconducting material.
2. Description of the Relevant Art
A superconductor-insulator-superconductor (SIS) Josephson junction is an electronic device consisting of two superconducting electrodes separated by a very thin insulating barrier. The thin insulating barrier permits the passage of supercurrents which tunnel through the barrier while maintaining zero voltage across the junction. Josephson devices switch very rapidly from zero voltage to a finite voltage level when current levels exceed a critical threshold value I.sub.c of supercurrents. The switching voltage is dependent on the superconductor material used, and is typically a few mV, as compared to the switching voltages of silicon and gallium arsenide transistors which are generally greater than 200 mV to 500 mV.
Because of their low operating voltages, Josephson devices can operate as low power, fast digital switches. Once current exceeds the threshold value I.sub.c (usually less than 1 mA), the device will switch rapidly from, e.g., an "off" state to an "on" state. Since the threshold current is quite low and the device exhibits low switching voltage, very little power is dissipated during operation. These properties, along with the ability to detect small amounts of magnetic fields or field gradients, make Josephson devices potentially useful in high performance computers, sensors and communication systems.
Many superconductors operate according to the principles of superconductivity analyzed by the well known Bardeen, Cooper and Schrieffer theory ("BCS theory") which predicted how the forces between electrons and surrounding atoms in the superconducting matrix material can lead to a pairing of electrons, called "Cooper pairs" resulting in zero resistance. An important part of the BCS theory is the calculation of the coherence length. Coherence length is a measure of the size of the Cooper pair and is defined as the fundamental size scale of superconductivity. The degree of superconductivity does not change over an arbitrarily short distance in the superconductor, but can change only over the characteristic distance of the coherence length.
To maintain tunneling of supercurrents through a Josephson junction, it is important that the properties of the superconductor be maintained to within a coherence length of the barrier. If regions of non-superconductivity in the superconducting electrode at the barrier interface exceed the coherence length, then the supercurrents are significantly decayed and may cease. Likewise the finite voltage characteristics of the tunnel junction are also dependent on the quality of the superconducting electrode at the barrier interface. The non-zero voltage behavior of the tunnel junction is a characteristic of the electronic states of the electrode material at the barrier interface. For superconductors, a gap in the electronic states exists creating a gap in the current-voltage (I-V) behavior of the junction with no current flow in the sub-gap region between -Vg and +Vg, as shown in FIG. 1. If, however, the electrode material at the barrier interface is not superconducting to within the coherence length, then non-superconducting characteristics are sampled in the I-V behavior of the device and this gap structure will be degraded or be eliminated completely, and sub-gap current may exist.
One important property of a high quality SIS Josephson device is its hysteretic behavior. Hysteretic behavior is largely dictated by the previous state of the device, whether it was in a zero voltage or a finite voltage state. As stated previously, FIG. 1 is an exemplary hysteretic current-voltage behavior of an ideal SIS Josephson tunnel junction at absolute zero temperature. As shown, if the junction is in the zero voltage state, it will re
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The U.S. Government has a paid-up license in this invention and limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of contract No. N00014-90-C-0088 awarded by the Defense Advanced Research Projects Agency (DARPA).
Hille Rolf
Microelectronics and Computer Technology Corporation
Saadat Mahshid
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