Operation of a SET - Single Electron Transistor:
The SET transistor can be viewed as in the figure that it has has two separate junctions for the entrance and exit of single electrons. It can also be viewed as a field-effect transistor in which the channel is replaced by two tunnel junctions forming a metallic island. The voltage applied to the gate electrode affects the amount of energy needed to change the number of electrons on the island.
The single electron transistor is a new type of switching device that uses controlled electron tunneling to amplify current. The key point is that charge passes through the island in quantized units. For an electron to hop onto the island,
The energy for one electron to hop must be equal to Ec=e2/2C
where C is the capacitance of this system
Ec is called Coulomb blockade energy, which is the repelling energy of the previous electron to the next electron.
For a tiny system, the capacitance C is very small, thus Ec can be very high, and the electrons cannot move simultaneously, but must pass through one by one. This phenomenon is called "Coulomb blockade".
The SET transistor can be viewed as in the figure that it has has two separate junctions for the entrance and exit of single electrons. It can also be viewed as a field-effect transistor in which the channel is replaced by two tunnel junctions forming a metallic island. The voltage applied to the gate electrode affects the amount of energy needed to change the number of electrons on the island.
The single electron transistor is a new type of switching device that uses controlled electron tunneling to amplify current. The key point is that charge passes through the island in quantized units. For an electron to hop onto the island,
The energy for one electron to hop must be equal to Ec=e2/2C
where C is the capacitance of this system
Ec is called Coulomb blockade energy, which is the repelling energy of the previous electron to the next electron.
For a tiny system, the capacitance C is very small, thus Ec can be very high, and the electrons cannot move simultaneously, but must pass through one by one. This phenomenon is called "Coulomb blockade".
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SINGLE ELECTRON TRANSISTOR
Our world is without doubt built on the power of the transistor, a microscopic electronic switch used to perform digital logic. Right now, we are able to fit enough of these tiny devices onto a microchip to allow us to perform several billion operations in a single second.
Chief problem that is faced by the chip designers today is regarding the size of the chip. According to Moore's Law - the number of transistors on a chip will approximately double every 18 to 24 months. Moore's Law works largely through shrinking transistors and due to this designers can squeeze in many transistors into a small chip. Which leads to more Electricity and more Heat in an even smaller space in the chip.
To solve this problem scientists designed SET - Single Electron Transistor.
The Single Electron Tunneling transistor(SET) - A device that exploits the quantum effect of tunneling to control and measure the movement of a single electron. Its a three-terminal electronic device that transfers electrons from the source to the drain one by one. The charge does not flow continuously in a Quantized way.
Electron transport properties of individual molecules have received considerable attention over the last several years due to the introduction of single-electron transistor (SET) devices which allow the experimenter to probe electronic, vibrational or magnetic excitations in an individual molecule. In a three-terminal molecular SET the molecule is situated between the source and the drain leads with an insulated gate electrode underneath. Current can flow between the source and the drain leads via a sequential tunneling process through the molecular charge levels, which the gate electrode is used to tune.
Chief problem that is faced by the chip designers today is regarding the size of the chip. According to Moore's Law - the number of transistors on a chip will approximately double every 18 to 24 months. Moore's Law works largely through shrinking transistors and due to this designers can squeeze in many transistors into a small chip. Which leads to more Electricity and more Heat in an even smaller space in the chip.
To solve this problem scientists designed SET - Single Electron Transistor.
The Single Electron Tunneling transistor(SET) - A device that exploits the quantum effect of tunneling to control and measure the movement of a single electron. Its a three-terminal electronic device that transfers electrons from the source to the drain one by one. The charge does not flow continuously in a Quantized way.
Electron transport properties of individual molecules have received considerable attention over the last several years due to the introduction of single-electron transistor (SET) devices which allow the experimenter to probe electronic, vibrational or magnetic excitations in an individual molecule. In a three-terminal molecular SET the molecule is situated between the source and the drain leads with an insulated gate electrode underneath. Current can flow between the source and the drain leads via a sequential tunneling process through the molecular charge levels, which the gate electrode is used to tune.
When both the gate and bias voltages are zero, electrons do not have enough energy to enter the island and current does not flow. As the bias voltage between the source and the drain is increased, an electron can pass through the island when the energy in the system reaches the Coulomb energy. This effect is known as the Coulomb blockade, and the critical voltage needed to transfer an electron onto the island, V=e/C, is called the Coulomb gap voltage. Now imagine that the bias voltage is kept below the Coulomb gap voltage. If the gate voltage is increased, the energy of the initial system (with no electrons on the island) gradually increases, while the energy of the system with one excess electron on the island gradually decreases. At the gate voltage corresponding to the point of maximum slope on the Coulomb staircase, both of these configurations equally qualify as the lowest energy states of the system. This lifts the Coulomb blockade, allowing electrons to tunnel into and out of the island.
The Coulomb blockade is lifted when the gate capacitance is charged with exactly minus half an electron, which is not as surprising as it may seem. The island is surrounded by insulators, which means that the charge on it must be quantized in units of e, but the gate is a metallic electrode connected to a plentiful supply of electrons. The charge on the gate capacitor merely represents a displacement of electrons relative to a background of positive ions.
If we further increase the gate voltage so that the gate capacitor becomes charged with -e, the island again has only one stable configuration separated from the next-lowest-energy states by the Coulomb energy. The Coulomb blockade is set up again, but the island now contains a single excess electron. The conductance of the SET transistor therefore oscillates between minima for gate charges that are integer multiples of e, and maxima for half-integer multiples of e.
APPLICATIONS Of SET
1. Quantum computers
-1000x Faster
2. Microwave Detection
-Photon Aided Tunneling
If a SET is attacked black body radiation, the photon-aided tunneling will affect the charge transfer of the system. Experiments show that the electric character of the system will be changed even by a tiny amount of radiation. The sensitivity of this equipment is about 100 times higher than the current best thermal radiation detector.
3. High Sensitivity Electrometer
-Radio-Frequency SET
The most advanced practical application currently for SETs is probably the extremely precise solid-state electrometers (a device used to measure charge). The SET electrometer is operated by capacitively coupling the external charge source to be measured to the gate. Changes in the SET source-drain current are then measured. Since the amplification coefficient is very big, this device can be used to measure very small change of current.
CONCLUSION
Researchers have long considered whether SET transistors could be used for digital electronics. Although the current varies periodically with gate voltage - in contrast to the threshold behaviour of the field-effect transistor - a SET could still form a compact and efficient memory device. However, even the latest SET transistors suffer from "offset charges", which means that the gate voltage needed to achieve maximum current varies randomly from device to device. Such fluctuations make it impossible to build complex circuits. One way to overcome this problem might be to combine the island, two tunnel junctions and the gate capacitor that comprise a single-electron transistor in a single molecule - after all, the intrinsically quantum behaviour of a SET transistor should not be affected at the molecular scale.
It is not yet clear whether electronics based on individual molecules and single-electron effects will replace conventional circuits based on scaled-down versions of field-effect transistors. Only one thing is certain: if the pace of miniaturization continues unabated, the quantum properties of electrons will become crucial in determining the design of electronic devices before the end of the next decade.
REFERENCES
1. M. N. Leuenberger and E. R. Mucciolo, Phys. Rev. Lett. 97, 126601 (2009).
2. C. Romeike, M. R. Wegewijs, and H. Schoeller, Phys. Rev. Lett. 96, 196805 (2008).
3. Michel H Devoret and Christian Glattli (1998), “Single-electron transistors.” Physics World
4. Liboff, Richard L. (2003). Introduction to Quantum Mechanics. Pearson Education Inc.
5. Silicon single-electron quantum-dot transistor switch operating at room temperature Appl. Phys. Lett. 72 1205
1893 L L Sohn, L P Kouwenhoven and G Schoen 1997.
6. Mesoscopic Electron Transport (Nato series, Kluwer, Dordrecht) M Tinkham 1996
7. Introduction to Superconductivity (McGraw-Hill, New York) L Zhuang, L Guo and S Y Chou 1998
The Coulomb blockade is lifted when the gate capacitance is charged with exactly minus half an electron, which is not as surprising as it may seem. The island is surrounded by insulators, which means that the charge on it must be quantized in units of e, but the gate is a metallic electrode connected to a plentiful supply of electrons. The charge on the gate capacitor merely represents a displacement of electrons relative to a background of positive ions.
If we further increase the gate voltage so that the gate capacitor becomes charged with -e, the island again has only one stable configuration separated from the next-lowest-energy states by the Coulomb energy. The Coulomb blockade is set up again, but the island now contains a single excess electron. The conductance of the SET transistor therefore oscillates between minima for gate charges that are integer multiples of e, and maxima for half-integer multiples of e.
APPLICATIONS Of SET
1. Quantum computers
-1000x Faster
2. Microwave Detection
-Photon Aided Tunneling
If a SET is attacked black body radiation, the photon-aided tunneling will affect the charge transfer of the system. Experiments show that the electric character of the system will be changed even by a tiny amount of radiation. The sensitivity of this equipment is about 100 times higher than the current best thermal radiation detector.
3. High Sensitivity Electrometer
-Radio-Frequency SET
The most advanced practical application currently for SETs is probably the extremely precise solid-state electrometers (a device used to measure charge). The SET electrometer is operated by capacitively coupling the external charge source to be measured to the gate. Changes in the SET source-drain current are then measured. Since the amplification coefficient is very big, this device can be used to measure very small change of current.
CONCLUSION
Researchers have long considered whether SET transistors could be used for digital electronics. Although the current varies periodically with gate voltage - in contrast to the threshold behaviour of the field-effect transistor - a SET could still form a compact and efficient memory device. However, even the latest SET transistors suffer from "offset charges", which means that the gate voltage needed to achieve maximum current varies randomly from device to device. Such fluctuations make it impossible to build complex circuits. One way to overcome this problem might be to combine the island, two tunnel junctions and the gate capacitor that comprise a single-electron transistor in a single molecule - after all, the intrinsically quantum behaviour of a SET transistor should not be affected at the molecular scale.
It is not yet clear whether electronics based on individual molecules and single-electron effects will replace conventional circuits based on scaled-down versions of field-effect transistors. Only one thing is certain: if the pace of miniaturization continues unabated, the quantum properties of electrons will become crucial in determining the design of electronic devices before the end of the next decade.
REFERENCES
1. M. N. Leuenberger and E. R. Mucciolo, Phys. Rev. Lett. 97, 126601 (2009).
2. C. Romeike, M. R. Wegewijs, and H. Schoeller, Phys. Rev. Lett. 96, 196805 (2008).
3. Michel H Devoret and Christian Glattli (1998), “Single-electron transistors.” Physics World
4. Liboff, Richard L. (2003). Introduction to Quantum Mechanics. Pearson Education Inc.
5. Silicon single-electron quantum-dot transistor switch operating at room temperature Appl. Phys. Lett. 72 1205
1893 L L Sohn, L P Kouwenhoven and G Schoen 1997.
6. Mesoscopic Electron Transport (Nato series, Kluwer, Dordrecht) M Tinkham 1996
7. Introduction to Superconductivity (McGraw-Hill, New York) L Zhuang, L Guo and S Y Chou 1998