Single electron transistor (set)

SINGLE ELECTRON TRANSISTOR (SET)

Although a single-electron box can control the number of electrons in the quantum dot, it does not have the properties of a switching device.

Single switching devices are essential elements in Ultra large scale integrated circuits (ULSIs). The single-electron switching devices are required to utilize single electron devices in logic circuits.

Definition

SET is three-terminal switching device which can transfer electrons from source to drain one by one.

That is the purpose of the SET is to individually control the tunneling of electrons into and out of the quantum dot.

Construction & working

The structure of SET is shown in fig.5.11. It has the similar structure of a conventional Field Effect Transistor (FET).

It has tunneling junctions in place of pn-junctions and quantum dot in place of the channel region of the FET.

To control tunneling, a voltage bias to the gate electrode is applied. A separate voltage bias is applied between source and drain electrodes for the current direction.

For current to flow, gate bias voltage must be large enough to overcome the Coulomb blockade energy.

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

E = VQ.

Presently, the charge of an electron (Q= e). So, the voltage that will move an electron onto or off the quantum dot is given by

V = E/e = W_c / e

Since E = W_c

where, W_c - charging energy

With this voltage applied to the quantum dot, an electron can tunnel through Coulomb blockade of the quantum dot. The gate voltage as given in eqn.(1) can be used to tune the number of electrons on the dot at a time.

At the lowest setting, electrons tunnel one at a time, from source to drain through the dot. At the proper gate voltage, V = e/2C the potential energy of the dot is low enough to allow an electron to tunnel through Coulomb blockade energy barrier to the quantum dot.

Once the quantum dot gets an electron, its potential energy rises. Then, the electron tunnels through the Coulomb blockade on the other side reach the lower potential energy at the drain. Since, the dot is empty and potential is lower again, the process repeats.

Thus, when the gate voltage V_g is zero, no current flows. The first gate voltage which is large enough to move an electron through the Coulomb blockade is called V_Coulomb'

For single-electron tunneling, V_R = V_Coulomb'

It If the gate voltage equals V _Coulomb' + e/2C, then two electrons can be moved on the quantum dot at a time. If the gate voltage equals

V_Coulomb' + e/2C + e/2C = V _Coulomb'  + e/C’

then three electrons can be moved on the quantum dot at a time and so on.ten

Thus, number of electrons in the quantum dot is controlled using the gate voltage.

These ON and OFF states can be utilized to make an effective switch out of a SET.

The gate voltages for a SET are a few millivolts and source-to-drain currents are in the pico ampere range.

Advantages

• The fast information transfer speed between cells (almost amo near light speed) is carried carried out via via electrostatic sonsu interactions only.

• No wire is needed between arrays. The size of each cell Dau can be as small as 2.5 nm. This made them very suitable for high density memory.

• This can be used for the next generation quantum computer.

Limitations

• In order to operate SET circuit at room temperature, the lao size of the quantum dot should be smaller than 10 nm.

• It is very hard to fabricate by traditional optical lithography and semiconductor process.

• The methods must be developed for connecting the individual structures into logic circuits, and these circuits must be arranged into larger 2D patterns.

Applications

• The main fields of application of the single electron anotransistor is used in sensor technology and digital electronic circuits.

• A variety of digital logic functions, including AND or NOR gates, is obtained based on SET operating at room temperature.

• It is used for mass data storage.

• It is used in highly sensitive electrometer.

• SET can be used as a temperature probe, particularly in the range of very low temperatures.

• SET is a suitable measurement set-up for single electron spectroscopy.

• It is used for the fabrication of a homo-dyn receiver operating at frequencies between 10 and 300 MHz.