07-01-2011, 03:25 PM
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OUTLINE
What are Transistors
What is a SET
How do SETs differ from the Conventional Transistors
Operation of SETs
Application of SETs
What is a Transistor
A transistor is a solid state semiconductor device which can be used for numerous purposes including signal modulation, signal amplification, voltage stabilization, and many other applications.
Transistors act like a variable valve which, based on its input current (BJT) or input voltage (FET), allow a precise amount of current to flow through it from the circuit’s Voltage supply.
Problems Associated With The Decrease In Size Of The Transistors
Moore’s Law
More transistors means more electricity and heat compressed into an even smaller space.
Furthermore, smaller chips increase performance but also compound the problem of complexity
What is a SET?
The single electron transistor is a new type of switching device that uses controlled electron tunneling to amplify the current.
It is an ultra-small device, that transfers one electron at a time, based on Coulomb interaction.
This occurs on a tiny conducting layer know as an island. This island’s electrostatic potential increases significantly with the introduction of just one electron.
Creation Of A SET
For semiconductors Construction:
The source, drain and island are obtained by "cutting" regions in a two-dimensional electron gas formed at the interface between two layers of semiconductors such as gallium aluminum arsenide and gallium arsenide. The conducting regions have metallic electrodes patterned on the top semiconductor layer. Negative voltages applied to these electrodes deplete the electron gas just beneath them, and the depleted regions can be made sufficiently narrow to allow tunneling between the source, island and drain. Electrodes that shape the islands can be used as the gate electrode.
Operation Of SETs
The tunnel junction consists of two pieces of metal separated by a very thin (~1nm) insulator.
The only way for electrons in one of the metal electrodes to travel to the other electrode is to tunnel through the insulator.
Since tunneling is a discrete process, the electric charge that flows through the tunnel junction flows in multiples of the charge of electrons ‘e’.
Operation Of SETs
A key point is that charge passes through the island in quantized units. For an electron to hop onto the island, its energy must equal the coulomb energy e²/2Cg.
When both the gate and the 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 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 equal to e/C, is called the coulomb gap energy.
Conditions for Coulumb Blockade
Capacitance of the island must be less than 10^-17 Farads and therefore its size must be smaller that 10 nm.
The wavelength of the electrons is comparable with the size of the dot, which means that their energy makes a significant contribution to the coulomb energy.
The Coulomb Island
Here n1 and n2 are the number of electrons passed through the tunnel barriers 1 and 2, respectively, so that n = n1 - n2, while the total island capacitance, C∑, is now a sum of CG, C1, C2, and whatever stray capacitance the island may have.
Coulomb Island
When a capacitor is charged through a resistor, the charge on the capacitor is proportional to the applied voltage and shows no sign of quantization.
When a tunnel junction replaces the resistor, a conducting island is formed between the junction and the capacitor plate. In this case the average charge on the island increases in steps as the voltage is increased
c) The steps are sharper for more resistive barriers and at lower temperatures.
Application of SETs
Quantum computers
1000x Faster
Microwave Detection
Photon Aided Tunneling
High Sensitivity Electrometer
Radio-Frequency SET
Conclusion
Researchers may someday assemble these transistors into molecular versions of silicon chips , but there are still formidable hurdles to cross.
SETs could be used for memory device, but even the latest SETs 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.
The future does look bright for these devices.
