Single-electron transistor

In physics, a Coulomb blockade (CB), named after Charles-Augustin de Coulomb's electrical force, is the increased resistance at small bias voltages of an electronic device comprising at least one low-capacitance tunnel junction. Because of the CB, the resistances of devices are not constant at low bias voltages, but increase to infinity for biases under a certain threshold (i.e. no current flows). When few electrons are involved and an external static magnetic field is applied, Coulomb blockade provides the ground for spin blockade (also called Pauli blockade) and valley blockade which includes quantum mechanical effects due to spin and orbital interactions respectively between the electrons.

A Coulomb blockade will also be observed when making the device very small (like a quantum dot). When the device is small enough, electrons inside the device will create a strong coulomb repulsion preventing other electrons to flow. The device will no longer follow Ohm's law. The current-voltage relation of the coulomb blockade looks like a staircase.

The tunnel junction is, in its simplest form, a thin insulating barrier between two conducting electrodes. If the electrodes are superconducting, Cooper pairs (with a charge of two elementary charges) carry the current. In the case that the electrodes are normalconducting, i.e. neither superconducting nor semiconducting, electrons (with a charge of one elementary charge) carry the current. The following reasoning is for the case of tunnel junctions with an insulating barrier between two normal conducting electrodes (NIN junctions).

According to the laws of classical electrodynamics, no current can flow through an insulating barrier. According to the laws of quantum mechanics, however, there is a nonvanishing (larger than zero) probability for an electron on one side of the barrier to reach the other side (see quantum tunnelling). When a bias voltage is applied, this means that there will be a current, and, neglecting additional effects, the tunnelling current will be proportional to the bias voltage. In electrical terms, the tunnel junction behaves as a resistor with a constant resistance, also known as an ohmic resistor. The resistance depends exponentially on the barrier thickness. Typical barrier thicknesses are on the order of one to several nanometers.

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