Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
2000-12-13
2002-12-24
Jackson, Jerome (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
C257S017000, C257S020000
Reexamination Certificate
active
06498354
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an improved structure for a semi-conductor device and methods for using such a device.
2. Discussion of Prior Art
Over the last two decades there has been much interest in semiconductor devices which operate by restricting the motion of current carriers in one or more directions. In such devices the carriers can only occupy a discrete set of energy levels or sub-bands in one or more dimensions. The motion of the carriers is said to be quantised in the direction of confinement.
In heterojunctions, formed by the joining together of two semiconductor compounds of different band gaps, the carriers are confined to a potential or quantum well. A two dimensional electron gas is formed if the carriers are electrons (or a two dimensional hole gas is formed if the majority carriers are holes).
One particular type of semiconductor device which has been fabricated, typically from GaAs, is the single electron transistor which was invented in 1987. In this device the potential well is of such a size that it can hold only a few electrons (typically between 0 and 20). Furthermore, once this number is fixed (by an external contact potential) it does not fluctuate in time by more than one electron.
Such devices are confined to operate at low temperatures (typically less than liquid nitrogen temperatures) due to the physics which allows them to function. The devices rely on the fact that the potential well has a small capacitance, and the energy that it takes for electrons to charge this well is quite large. If the device is cooled to low temperatures the electron thermal energy becomes less than the charging energy. Without a significant source-drain voltage bias the electrons cannot travel through the potential well. This is known as Coulomb blockade.
SUMMARY OF THE INVENTION
According to a first aspect of the invention there is provided a field effect single electron transistor fabricated from a narrow band gap semiconductor.
Single Electron Transistors (SET) have the highest charge sensitivity of any man-made device. The SET is suited for applications where it is necessary to measure small fluctuations of charge without disturbing the system under study, or for providing low power transistor action. They also have potential for sensitive detectors of pressure, acceleration and temperature at least. Other detectors may be envisaged.
The transistor of the first aspect of the invention may be referred to as a Zener single electron transistor (Zener SET). Prior art transistors can be referred to as unipolar single electron transistors.
Zener SETs are advantageous because they are potentially simpler to fabricate and control, they may operate at higher temperatures than prior art devices, both n-type and p-type devices may be fabricated, and confinement may be enhanced due to the low effective mass of conduction electrons in p-type devices.
Further advantages of single electron transistors are that they are physically small (e.g. nanoscale) when compared to conventional field effect transistors resulting in a higher packaging density though lower power density.
The transistor preferably contains a heterojunction between layers of a first and a second material. The first material may be InSb or Cd
x
Hg
1−x
Te.
The second material may be InAISb or CdTe or Cd
x
Hg
1−x
Te. The heterojunction may be provided as a single layer of first material adjacent to a single layer of second material. Alternatively, the heterojunction may be provided as a single layer of first material between two layers of second material.
Should the heterojunction be provided as a single layer of first material adjacent to a second material, the second material may be an oxide (i.e. may be an insulator) or may be a semi-conductor. The first material may be considered a narrow band gap semiconductor. In the cases wherein the second material is a semi-conductor it may be considered a wide band gap semi-conductor. Other materials may be suitable for the first and second materials.
The skilled person will realise that should the first material be Cd
x
Hg
1−x
Te the band gap can be tailored to any desired value by adjusting the value of x. As x tends to ≅0.15 the band gap of the material tends to zero. However, x may be chosen to be an optimal value.
Should the second material be Cd
x
Hg
1−x
Te the value of x may be chosen to tend to one (that is CdTe). CdTe is preferred for its electronic properties but may not be achievable in view of other physical considerations: for example crystal growth considerations and lattice mismatch.
There may be provided on a first side of the heterojunction a third material which may be provided as a layer. The third material may function as a first gate electrode. The third material may be a metal. It may be Al or Au, or may be any other suitable conductor. Such a structure is advantageous because the presence of the gate electrode allows the electron/hole gas to be controlled within the heterojunction.
A second gate electrode may be provided on a second side of the heterojunction which is on the opposite side of the heterojunction from the first side. The second gate electrode may be fabricated from a metal. Such a structure in combination with the first gate electrode allows the electron/hole gas to be controlled.
The second gate electrode may be insulated from the materials forming the heterojunction by at least a single layer of insulation. An insulation layer is advantageous in that it modifies the interaction between the second gate electrode and the heterojunction in such a way as to give the desired functionality.
At least one (and preferably two) side gate may be provided. These may help to control the electron/hole gas in the desired manner.
The side gates may be insulated from the materials forming the heterojunction by a layer of insulation.
The side gate may comprise an elongate area along one side of the first or second material above the heterojunction. Preferably when two side gates are provided each forms an area along a side of the first or second material which sides are opposite each other. The areas may be rectangles. Most preferably the two side gates are in the same plane and there exists a gap within that plane between the two areas of side gates. The side gates preferably extend generally parallel to each other.
Preferably, the second gate electrode is provided above and may be insulated from the side gate. Again such a structure allows the electron/ hole gas to be controlled in the desired manner.
The insulation may be silicon dioxide SiO
2
or any other suitable insulation material. Indeed, different types of insulation material may be used for different layers of insulation. Or indeed, the layers of insulation separating the materials of the heterojunction and the side gate may be the same as the layers of insulation separating the side gate from the second gate electrode.
Preferably the second gate electrode comprises a primary portion which extends over the gap between the side gates. Such a structure may have a large influence in the electrons in the electron/hole gas.
The second gate electrode may have a first broad region connected to a second broad region via a narrower waist region. The second gate electrode may be a bow tie shape possibly with the central, waist, portion of the bow tie extending over the gap between the side gates.
The skilled person will appreciate that the effect of the side gate electrodes may be thought of as creating a quantum wire wherein electrons or holes are held by an applied electric field in a narrow strip within the electron sheet. The use of electrodes to form the quantum wire may be thought of as soft confinement.
An alternative, or additional, way of forming the quantum wire may be with hard confinement as opposed to through the provision of side gates (soft confinement).
In one embodiment the heterojunction may be provided between a strip of first material and a layer of second material. That is the width of the first mater
Jefferson John H.
Phillips Timothy J.
Jackson Jerome
Nixon & Vanderhye P.C.
Qinetiq Limited
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