2CONTENTS Introduction Need for SET FET Vs. SET What is SET What is “island”?What is “Tunneling”?Principle of SETOperationTowards room temperatureApplicationsDrawbacksConclusion
3INTRODUCTIONAs transistors continue to shrink, a question naturally arises: will the quantum nature of electrons and atoms become important in determining how the devices are built? In other words, what will happen when a transistor is reduced to the size of a few atoms or a single molecule?Here arises concept SET.
4Problem of Making More Powerful Chips Intel co-founder Gordon Moore that the number of transistors on a chip will approximately double every 18 to 24 months. This observation refers to what is known as Moore’s Law.This law has given chip designers greater incentives to incorporate new features on silicon.The chief problem facing designers comes down to size. Moore's Law works largely through shrinking transistors, the circuits that carry electrical signals. By shrinking transistors, designers can squeeze more transistors into a chip. However, 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.
5Need for SETIn SET the electrons are confined within a small volume and communicate with the electrical leads by tunneling. One then has a transistor that turns on and off again every time one electron is added to it; we call it a single electron transistor (SET).
6Solution of the problem (set) the tunnel effect Whereas natural atoms are studied by adding, removing or exciting electrons with light, these artificial atoms typically have such small energy scales that they are best studied by measuring the voltage and current resulting from tunneling of an electron through the artificial atom
7SET v/s FETA conventional field-effect transistor, the kind that makes all modern electronics work,is a switch that turns on when electrons are added to a semiconductor and turns off when they are removed unlike SET .FET’s on and off states give the ones and zeros that digital computers need for calculation unlike SET.Interestingly, these transistors are almost completely classical in their physics but SET is quantum in physics.SET power consumption is less.SET’s Switching speed is faster.Fabrication density is more.
8What is “SET”? The single-electron tunneling transistor - a device that exploits the quantum effect of tunneling to control and measure the movement of single electrons was devised. Experiments have shown that charge does not flow continuously in these devices but in a quantized way
9What is this “island”?(a) 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. (b) 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.
11Principle of SETThe SET transistor can be viewed as an electron box that 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 SET transistor comes in two versions that have been nicknamed "metallic" and "semiconducting".
12So how does an SET transistor work? The 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 e2/2C.and the critical voltage needed to Transfer an electron onto the island,equal to e/C, is called the Coulomb gap voltage.
13Behavior Towards room temperature Until recently single-electron transistors had to be kept at temperatures of a few hundred millikelvin to maintain the thermal energy of the electrons below the Coulomb energy of the device.Most early devices had Coulomb energies of a few hundred microelectronvolts because they were fabricated using conventional electron-beam lithography, and the size and capacitance of the island were relatively largeFor a SET transistor to work at room temperature, the capacitance of the island must be less than 10-17pF and therefore its size must be smaller than 10 nm.
15Drawbacks 3)Operation at Room Temp 4)Fabrication 5)Charge Offset 1)The current varies periodically with gate voltage - in contrast to the threshold behavior of the field-effect transistor2) 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 .3)Operation at Room TempCapacitor Size4)FabricationChemical FabricationCoulomb IslandsTunneling junctionsGate between substrate and Coulomb islands5)Charge Offset1 electron at a time
16ConclusionIt 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.