A single-electron transistor (SET) is a sensitive electronic device based on the Coulomb blockade effect. In this device the electrons flow through a tunnel junction between source/drain to a quantum dot (conductive island). Moreover, the electrical potential of the island can be tuned by a third electrode, known as the gate, which is capacitively coupled to the island. The conductive island is sandwiched between two tunnel junctions modeled by capacitors, C D {\displaystyle C_{\rm {D}}} and C S {\displaystyle C_{\rm {S}}} , and resistors, R D {\displaystyle R_{\rm {D}}} and R S {\displaystyle R_{\rm {S}}} , in parallel.
History A new subfield of condensed matter physics began in 1977 when David Thouless pointed out that, when made small enough, the size of a conductor affects its electronic properties. This was followed by mesoscopic physics research in the 1980s based on the submicron-size of systems investigated. Thus began research related to the single-electron transistor. The first single-electron transistor based on the phenomenon of Coulomb blockade was reported in 1986 by Soviet scientists K. K. Likharev and D. V. Averin. A couple years later, T. Fulton and G. Dolan at Bell Labs in the US fabricated and demonstrated how such a device works. In 1992 Marc A. Kastner demonstrated the importance of the energy levels of the quantum dot. In the late 1990s and early 2000s, Russian physicists S. P. Gubin, V. V. Kolesov, E. S. Soldatov, A. S. Trifonov, V. V. Khanin, G. B. Khomutov, and S. A. Yakovenko were the first ones to demonstrate a molecule-based SET operational at room temperature.
Relevance The increasing relevance of the Internet of things and the healthcare applications give more relevant impact to the electronic device power consumption. For this purpose, ultra-low power consumption is one of the main research topics into the current electronics world. The amazing number of tiny computers used in the day-to-day world (e.g. mobile phones and home electronics) requires a significant power consumption level of the implemented devices. In this scenario, the SET has appeared as a suitable candidate to achieve this low power range with high level of device integration. Applicable areas include: super-sensitive electrometers, single-electron spectroscopy, DC current standards, temperature standards, detection of infrared radiation, voltage state logics, charge state logics, programmable single-electron transistor logic.
Device
Principle
The SET has, like the FET, three electrodes: source, drain, and a gate. The main technological difference between the transistor types is in the channel concept. While the channel changes from insulated to conductive with applied gate voltage in the FET, the SET is always insulated. The source and drain are coupled through two tunnel junctions, separated by a metallic or semiconductor-based quantum nanodot (QD), also known as the "island". The electrical potential of the QD can be tuned with the capacitively coupled gate electrode to alter the resistance, by applying a positive voltage the QD will change from blocking to non-blocking state and electrons will start tunnelling to the QD. This phenomenon is known as the Coulomb blockade. The current, I , {\displaystyle I,} from source to drain follows Ohm's law when V S D {\displaystyle V_{\rm {SD}}} is applied, and it equals V S D R , {\displaystyle {\tfrac {V_{\rm {SD}}}{R}},} where the main contribution of the resistance, R , {\displaystyle R,} comes from the tunnelling effects when electrons move from source to QD, and from QD to drain. V G {\displaystyle V_{\rm {G}}} regulates the resistance of the QD, which regulates the current. This is the exact same behaviour as in regular FETs. However, when moving away from the macroscopic scale, the quantum effects will affect the current, I . {\displaystyle I.}
In the blocking state all lower energy levels are occupied at the QD and no unoccupied level is within tunnelling range of electrons originating from the source (green 1.). When an electron arrives at the QD (2.) in the non-blocking state it will fill the lowest available vacant energy level, which will raise the energy barrier of the QD, taking it out of tunnelling distance once again. The electron will continue to tunnel through the second tunnel junction (3.), after which it scatters inelastically and reaches the drain electrode Fermi level (4.). The energy levels of the QD are evenly spaced with a separation of Δ E . {\displaystyle \Delta E.} This gives rise to a self-capacitance C {\displaystyle C} of the island, defined as: C = e 2 Δ E . {\displaystyle C={\tfrac {e^{2}}{\Delta E}}.} To achieve the Coulomb blockade, three criteria need to be met:
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