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SINGLE ELECTRON TRANSISTOR

K.Pooja Naga Sai B.Sirisha

([email protected]) ([email protected])

II/IV B.Tech, ECE II/IV B.Tech, ECE

KL University KL University

ABSTRACT

Our world is without doubt built on the power of the transistor, a microscopic electronic

switch used to perform digital logic. In order to keep up with this incredible rate of speed

increase, transistors are becoming smaller and smaller, to the point where in the very near

future, they will begin to not only feel the effects of quantum mechanics on their operation, but

will have to take quantum mechanics into account as the dominant force in their engineering.

Most transistors today are MOSFETs, where a semiconductor source and drain of one doping

type are separated by an oppositely doped bulk semiconductor. The bulk semiconductor is then

separated by a layer of oxide from a gate electrode between the source and the drain. As the gate

bias is changed, the bias causes the formation of a conducting channel in the bulk material

between the source and drain, allowing current to flow and thus turning the switch on. In asingle electron transistor, however, charge moves by utilizing the effect of quantum tunneling.

Instead of creating a channel of charge carriers between the source and drain electrodes, a

single electron transistor utilizes two junctions where tunneling is the dominant method of

electron transport to control the movement of single electrons through the device. The goal ofthis paper is to review in brief the basic physics of nanoelectronic device single-electron

transistor [SET] as well as prospective applications and problems in their applications. SET

functioning based on the controllable transfer of single electrons between small conducting

"islands".

Keywords:

Nanoelectronics; Single-electron transistor; Coulomb blockade, Coulomb oscillation, Quantum

dot
mailto:[email protected]:[email protected]:[email protected]:[email protected]


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1.INTRODUCTION

A 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. These on

and off states give the ones and zeros that

digital computers need for calculation. One

then has a transistor that turns on and offagain every time one electron is added to it;

we call it a single electron transistor (SET).

Furthermore, the behavior of the device is

entirely quantum mechanical.

Electron transport properties of

individual molecules have received

considerable attention over the last several

years due to the introduction of single-

electron transistor (SET) devices which

allow the experimenter to probe electronic,

vibrational or magnetic excitations in an

individual molecule. In a three-terminal

molecular SET the molecule is situated

between the source and drain leads with an

insulated gate electrode underneath. Current

can flow between the source and drain leads

via a sequential tunneling process through

the molecular charge levels, which the gate

electrode is used to tune.

2.HISTORY OF SET

The effects of charge

quantization were first observed in tunnel

junctions containing metal particles as early

as 1968. Later, the idea that the Coulomb

blockade can be overcome with a gate

electrode was proposed by a number of

authors, and Kulik and Shekhter developed

the theory of Coulomb-blockade

oscillations, the periodic variation of

conductance as a function of gate voltage.

Their theory was classical, including charge

quantization but not energy quantization.

However, it was not until 1987 that Fulton

and Dolan made the first SET, entirely out

of metals , and observed the predicted

oscillations. They made a metal particle

connected to two metal leads by tunneljunctions, all on top of an insulator with a

gate electrode underneath. Since then, the

capacitances of such metal SETs have been

reduced to produce very precise charge

quantization .The first semiconductor SET

was fabricated accidentally in 1989 by

Scott-Thomasetal. In narrow Si field effect

transistors. In this case the tunnel barriers

were produced by interface charges.

3. SET SCHEMATICREPRESENTATION

A model of SET is shown in Fig.1,(b)

is the simplified model.


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Figure(1)

A schematic circuit of SET

The two areas filled with patchedpattern are tunneling junctions; there are

some discrete Coulomb islands between

them. R1, C1 and R2, C2 are the resistance

and capacitance of the junctions. The

junctions form the source and drain of the

transistor, 2 / V voltages are applied to them

through conductive wires, the tunneling

current pass through the islands is I. A layer

of insulating media separates the islands

from the gate; the capacitance between themis Cg. A voltage of Vg is applied on the gate

and controls the open or close of the SET.

Because of its unique structure, SET has

many prospective characteristics such as low

power consumption, high sensitivity, high

switching speed, high packet density, etc. So

much attention has been attracted on their

fabrication and industrial realization.

Fig (2) Schematic diagram of SET

5.FABRICATION OF SET

The fabrication of SET promotes

many difficulties. For SET to be used in a

large scale industrially and position.

Basically the fabrication methods can be

divided as physical or chemical techniques

according to the main procedures.

The physical methods often utilize the

combination of thin film and lithographictechnologies. Devices with carefully

tailored geometries and electron density are

got. For example, quantum dots or quasi-

zero-dimensional puddles of electrons with

weak coupling to simultaneously patterned

electrical leads are fabricated to form a SET.

However, lithographic and materials

limitations restrict the minimum size and

composition of such dots (100nm), and

studies are typically limited to sub-Kelvin

temperatures.

Another approach is to grow nanostructures

chemically. This approach is prosperous for

its low cost and good controllability of the

size of Coulomb islands, and it is possible to

be a prospective technique. Though this

technique is not mature industrially, the SET

s fabricated in laboratories show fascinating

results. Generally there are three most

important steps: first, the fabrication of

Coulomb islands as well as the control of

their size and dispersity; second, the

formation of tunneling junctions at the joint

of electrodes and Coulomb island; third, the

formation of gate between substrate and

Coulomb islands.

4. WORKING OF SETThe single electron transistor is a

new type of switching device that uses

controlled electron tunneling to amplify

current. Conduction through a molecular

SET only occurs when a molecular

electronic level lies between the Fermi

energies of the leads. A bias voltage, V bias,


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applied between the source and the drain,

changes the electrostatic potential of one of

the leads by an energy |eV|. For small bias

voltages, |eV| < Ec + E where Ec is the

Coulomb charging energy and E is the

energy difference between consecutive

charge states of the molecule being

measured, current cannot flow though the

device because the excited molecular levels

are not available to conduct charges between

the electrodes. This is known as the

Coulomb blockade regime.

Fig (3) Schematic of a single electron

transistor

If bias voltages, |eV| > Ec + Ewhere Ec is the Coulomb charging energy

and E is the energy difference between

consecutive charge states of the molecule

being measured, current can flow though the

device.

Usually electrons move continuously

in the common transistors, but as the size of

the system goes down to nanoscale (for

example, the size of metal atoms can be

several nm, and the size of semi-conductive

particles can be several tens nm), the energy

of the system is quantumized, that is, the

process of charging and discharging is

discontinuous.

The energy for one electron to move into the

system is:

EC=e2/2C

where C is the capacitance of this system.

This Ec is called Coulomb blockade energy,

which is the repelling energy of the previous

electron to the next electron. For a tinysystem, the capacitance C is very small, thus

Ec can be very high, and the electrons

cannot move simultaneously, but must pass

through one by one. This phenomenon is

called "Coulomb blockade".

If two quantum dots(QD) are joined at

a point and form a channel, it is possible for

an electron to pass from one dot over the

energy barrier and move to the other dot,

this is called "tunneling phenomenon". In

order to overcome the barrier (Ec), the

applied voltage on the quantum dots (V/2)

should be V > e/C

Quantum tunnelling

It refers to the quantum mechanical

phenomenon where a particle tunnels

through a barrier that it classically could not

surmount because its total mechanical

energy is lower than the potential energy of

the barrier. This tunnelling plays an essential

role in several physical phenomena,

including radioactive decay and has

important applications to modern devices

such as the tunneling diode and the scanning

tunnelling microscope.

COULOMB ISLAND

(a)When a capacitor is charged through aresistor, the charge on the capacitor is

proportional to the applied voltage and

shows no sign of quantization.
http://en.wikipedia.org/wiki/File:Set_schematic.svg


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(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 islandincreases in steps as the voltage is increased.

c) The steps are sharper for more resistive

barriers and at lower temperatures.

A signature of this phenomenon is

commonly seen at low temperatures as an

absence of current for low bias voltages. As

the bias voltage across the device increases,

excited states will provide conduction

channels in the device. As a result, discrete

changes in the current through the SET willbe obtained every time a new molecular

level falls within the bias window.

The simplest device in which the

effect of Coulomb blockade can be observed

is the so-called single electron transistor. It

consists of two tunnel junctions sharing one

common electrode with a low self-

capacitance, known as the island. The

electrical potential of the island can be tuned

by a third electrode (thegate), capacitively

coupled to the island.

In the blocking state no accessible

energy levels are within tunneling range of

the electron (red) on the source contact. All

energy levels on the island electrode with

lower energies are occupied.

When a positive voltage is applied

to the gate electrode the energy levels of the

island electrode are lowered. The electron

(green 1.) can tunnel onto the island (2.),

occupying a previously vacant energy level.

From there it can tunnel onto the drain

electrode (3.) where it in elastically scatters

and reaches the drain electrode Fermi level

(4.).

The energy levels of the island electrode

are evenly spaced with a separation of E.

Eis the energy needed to each subsequent

electron to the island, which acts as a self-capacitance C.

The lowerCthe bigger Egets. To achieve

the Coulomb blockade, three criteria have to

be met:

The bias voltage can't exceed the charging

energy divided by the capacitance Vbias =

;The thermal energy kBTmust be below the

charging energy EC = , or else the

electron will be able to pass the QB via

thermal excitation; The bias voltage can't

exceed the charging energy divided by the

capacitance Vbias = ;

The thermal energy kBTmust be below

the charging energy EC = , or else theelectron will be able to pass the QB via

thermal excitation.

5.APPLICATIONS
http://en.wikipedia.org/wiki/Capacitance#Self-capacitancehttp://en.wikipedia.org/wiki/Capacitance#Self-capacitancehttp://en.wikipedia.org/wiki/Capacitance#Self-capacitancehttp://en.wikipedia.org/wiki/Capacitance#Self-capacitance


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SET has found many applications in many

areas.They are used in single electron memory,high sensitivity electrometer, microwave

detection, logic circuits design etc.

Advantages include small size, low energy

consumption and high sensitivity,

highcurrent density, good controllability, a

well defined tunnel barrier.

The main disadvantages are Integration

of SETs in a large scale is difficult, to use

SETs at room temperature, large quantitiesof monodispersed Nan particles less than

10nm in diameter must be synthesized. it is

very hard to fabricate large quantities of

SETs by traditional optical lithography and

semiconducting process. Linking SETs with

the outside environment Practical difficulty

in fabrication.

6.CONCLUSION

single-electron transistor which could lead

to the development of "quantum" computers

with supercomputer powers and the size of a

thumbtack.

SETMOS: Using a Hybrid combination,

similar to that of SET and FET, of SETs and

CMOS transistors in SETMOS devices can

provide enough gain and current drive to

perform logic functions on a much smallerscale than possible with just an CMOS. The

SETMOS device exhibits Coulomb

blockade oscillations similar to a traditional

SET but offers much higher current-driving

capability. Similar to a CMOS this

SETMOS uses a single electron to represent

an logic state. It works on the notation of

Coulomb Blockade oscillations, but operates

at a much faster current-driving capability.

REFERENCES:

Stevenson T. R, Pellerano F.A,Stahle C.M, Aidala K, Schoelkopf

R.J. 2002, Applied Physics Letters,

80, 16.

Bladh K, Gunnarsson D, JohanssonG, Kck A, wendin G, Delsing P,

Aassime A, Taslakov M. Reading

out Charge Qubits with a Radio

Frequency Single Electron

Transistor, 2002.

Berman D, Zhitenev N. B, AshooriR.C, Smith H, Melloch M, 1997,

American Vacuum Society, 2844.

http://homepages.cae.wisc.edu/~wiscengr/feb05/transitioningelecfrontiers.

shtml

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