Single Electron Transistor [SET]

SINGLE ELECTRON TRANSISTOR [SET]

Definition

A transistor made from a quantum dot that controls the current from source to drain one electron at a time is called single electron transistor.

Difference between ordinary and single electron transistor

The single electron transistor [SET] is built like a conventional FET. The difference is that instead of a semiconductor channel between the source and drain electrodes, there is a quantum dot.

This dot can be a particle on an insulating surface, a disk sandwiched between insulators or even just a section of semiconducting material where electric fields effectively isolate electrons.

Working of SET

A generalized schematic of such a device and its operation is shown in Fig 3.12.

1. The purpose of SET is to individually control the tunneling of electrons into and out of the quantum dot. To do this, we must first stop random tunneling by choosing the right circuit geometry and materials. If an electron comes or goes from the dot, it will on purpose.

2. To control tunneling, we apply a voltage bias to the gate electrode. There is also a voltage difference between the source and the drain that dictates the direction for the current. Here we say that current and electron flow in the same direction and we will consider the electrode from which the electrons orginate.

3. This is similar to the working of an FET, where the gate voltage creates an electric field that alters the conductivity of the semiconducting channel below it, enabling current to flow in form source to drain.

4. Applying a voltage to the gate in an SET creates an electic field and change the potential energy of the dot with respect to the source and drain. This gate voltage‒controlled potential difference can make electrons in the source attracted to the dot and, simultaneously electrons in the dot attracted to the drain.

For current to flow, this potential difference must be atleast large enough to overcome the energy of the coulomb blockade.

The energy 'E' needed to move a charge, Q, across a potential energy difference, V, is given by

 E = VQ

Here Q → e → charge of an electron.

Hence, we get the energy needed equal to the energy of the coulomb blockade [E = Ec] and determine the voltage that will move an electron onto or off the dot:

 ………….. (1)

With this voltage applied to it, an electron can tunnel though coulomb blockade of the quantum dot.

Working for single‒electron transistor (SET) in nutshell

A single‒electron transistor (SET) is shown in Fig. 3.12. As opposed to the semiconductor channel in a field‒effect transistor, the SET has an electrically isolated quantum dot located between the source and drain.

1. The SET in "OFF" mode. The corresponding potential energy diagram shows that it is not energetically favorable for electrons in the source to tunnel to the dot as shown in Fig. 3.12 (a).

2. The SET in "ON" mode. At the lowest setting, electrons tunnel one at a time, via the dot, from source to drain as shown in Fig. 3.12 (b).

3. This is made possible by first applying the proper gate voltage, V_gate = e / 2C_dot, so that the potential energy of the dot is made low enough to encourage an electron to tunnel through the Coulomb blockade energy barrier to the quantum dot.

4. Once the electron is on it, the dot's potential energy rises as shown in Fig. 3.12 (c).

5. The electron then tunnels through the Coulomb blockade on the other side to reach the lower potential energy at the drain as shown in Fig. 3.16 (d).

6. With the dot empty and the potential lower again, the process repeats as shown in Fig. 3.12 (e).