Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Tunneling through region of reduced conductivity
Reexamination Certificate
2001-03-06
2002-04-02
Loke, Steven (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Tunneling through region of reduced conductivity
C257S030000, C257S032000, C257S036000
Reexamination Certificate
active
06365912
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a superconductive tunnel junction device which is capable of both current and voltage amplification of low-level signals.
The search for a superconductive 3-terminal device with amplifying capability and good isolation of the input from the output has been in progress for more than 20 years. The advantages of superconductive electronics over semiconductor electronics are in two broad areas: digital applications based on the high switching speed and low power dissipation of Josephson junctions, and analog applications based on extremely high sensitivity and response to electromagnetic phenomena over a very wide frequency spectrum. This spectrum ranges from dc and low frequency magnetic fields (where SQUID devices are used), to microwaves and mm waves, infrared, optical, and UV radiation, through to the detection and spectroscopy of X-rays and &ggr;-rays. Also lowering the temperature of electronic circuitry tends to reduce its inherent noise. However, without a complementing 3-terminal transistor-like device, these advantages are limited to very specialised applications. In the immediate future, particularly in the application of superconducting photon counting spectrometers to astrophysics, many detector pixels and analogue electronics channels will be required.
In order to exploit these advantages a 3-terminal transistor-like device should fulfil a stringent list of properties including:
current gain;
voltage gain;
isolation between input and output;
high speed;
potential for very large scale integration (VLSI);
low power,
impedances compatible with other devices and with transmission lines;
manufacturability.
For ease of digital design 3-terminal devices should also be inverting and non-latching.
At least three different devices based on non-equilibrium superconductivity and quasiparticle dynamics have been proposed and studied: the Gray effect transistor, the Quiteron, and the quasiparticle multiplier. Each of these will be described briefly below but all of these devices utilise quasiparticle tunnelling.
The Gray effect transistor is described in U.S. Pat. No. 4,157,555 and relies on the fact that there are two tunnelling processes (and their inverses) for quasiparticles in superconductor-insulator-superconductor (SIS) junctions, and under certain conditions multiple tunnelling can occur. The Gray effect transistor is formed by disposing three thin films of superconductive material in a planar parallel arrangement and insulating the films from each other by layers of insulating oxides to form two tunnel junctions. One junction is biased above twice the superconductive energy gap and the other is biassed at less than twice the superconductive energy gap. Injection of quasiparticles into the centre film by one junction provides a current gain in the second junction.
This multiple tunnelling effect when combined with quasiparticle trapping has been used to amplify low-level signals from the absorption of single X-ray quanta. More recently the same scheme with multiple tunnelling amplification factors of up to about 200 has been used to count and measure the energy of individual optical quanta. Although this amplification scheme has proven to be extremely useful, it produces charge amplification, not current amplification, that is to say the length of the current pulses is extended, not their amplitude.
The Quiteron is described in EP-A1-0081007 and produces gain by suppression of the energy gap &Dgr; of the central electrode of a double SIS tunnel junction structure. In this device the superconductive energy gap of the central superconductive layer is greatly altered by over-injection of energetic quasiparticles so that the energy gap changes greatly with respect to its thermal equilibrium value, and in most cases is made to vanish. In one example of this device a three electrode device is constructed with tunnel barriers between the electrodes. A first of the tunnel junctions is used to heavily inject energetic quasiparticles into the central superconductive electrode to change its superconductive energy gap drastically. In turn, this greatly modifies the current-voltage characteristics of the second tunnel junction. This device appeared initially to be very promising, but never fulfilled all of the requirements for either analog or switching devices, particularly that of isolation of the input from the output. This problem can be visualised using the two-fluid (Landauer) model of transistor-like operation in which a 3-terminal device is viewed in analogy with a fluid-actuated “piston” in which a “control” fluid controls the flow of a second “moving” fluid. For proper operation it is important that the fluids be separate with little mixing between them. For bipolar transistors a small base current controls a much larger collector current and changes in the output have little effect on the input. Similar features apply to most other semiconductor 3-terminal devices. However, for the Quiteron the central superconductive electrode is shared between input and output with only a small degree of unidirectionality. Such a device is not very useful. Moreover, the gap suppression requires a large deviation from equilibrium.
The quasiparticle multiplier [N E Booth “Quasiparticle trapping and the quasiparticle multiplier”, Appl. Phys. Lett. 50, 293 (1987)] makes use of the quasiparticle trapping scheme. Like the Quiteron it consists of (a minimum of) three superconductive films separated by tunnelling barriers. The central film is a bilayer of two superconductors, a primary film S
1
of energy gap &Dgr;
1
and a trapping film S
2
of energy gap &Dgr;
2
. If &Dgr;
2
is less than one third of &Dgr;
1
, additional quasiparticles can be produced by phonons emitted in the trapping process where a quasiparticle diffusing from S
1
into S
2
relaxes to the lower energy gap. In contrast to the Quiteron, there is a high degree of directionality. Although, as originally proposed, several stages can be cascaded like a photomultiplier, the gain per stage for Nb—Al films is not more than 3, and this value is obtained only if all relaxation phonons are absorbed in the trap, which is highly unlikely.
It should also be said that all three of the devices above require the application of a small magnetic field to suppress Josephson effects.
Other devices which may have specific applications, such as devices based on vortex flow, have also been proposed and studied. Very recently a new device based on controlling a supercurrent by the current through a normal metal film has been proposed A F Morpurgo, T M Klapwijk and B J van Wees, “Hot Electron Tunable Supercurrent”, Appl. Phys. Lett. 72, 966 (1998)
Thus the need for a satisfactory superconductive transistor-like device has still not been met.
SUMMARY OF THE INVENTION
The present invention provides a device in which quasiparticles travelling from a superconductive region with energy gap &Dgr; to a normal region lose their potential energy in electron-electron interactions to increase the number of excited electrons in the normal region above the equilibrium thermally excited number. This increase is the basis for current amplification. The same effect can occur with holes. In the following, the term charge carriers will be used to refer to electrons or holes.
In more detail, according to the present invention there is provided a superconductive tunnel junction device comprising a first superconductive region in contact with a first normal region, wherein the potential energy of quasiparticles from the first superconductive region relaxing into the first normal region is converted into an increased number of charge carriers excited above the Fermi level of the first normal region.
The device may further comprise a second superconductive region separated from the first normal region by an insulating tunnel barrier to form a tunnel junction, across which the charge carriers tunnel into the second superconductive region to form quasiparticles therein. This second superconductive region
Booth Norman Ewart
Nahum Michael
Ullom Joel Nathan
Hu Shouxiang
Isis Innovation Limited
Loke Steven
Nixon & Vanderhye
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