SET Report.doc

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Dept. of E&C A.I.T, Chikmagalur Page 1

ABSTRACT

The chief problems that are faced by chip designers are regarding the size of the chip.

According to Moores Law, the number of transistors on a chip will approximately double

every 18 to 24 months. This Law works largely through shrinking transistors-the circuits that

carry electrical signals. By shrinking transistors, designers can squeeze more transistors into a

chip. However, more transistors means more electricity and heat compressed into an even

smaller space. Furthermore, smaller chips increase performance but also compound the

problem of complexity. To solve this problem, the single-electron tunneling transistor (SET) -

a device that exploits the quantum effect of tunneling to control and measure the movement of

single electron was devised. Experiments have shown that, charge does not flow continuously

in these devices but in a quantized way.

Although it is unlikely that SETs will replace FETs in conventional electronics, they should

prove useful in ultra-low-noise analog applications. Moreover, because it is not affected by the

same technological limitations as the FET, the SET can approach closely the quantum limit of

sensitivity. It might also be a useful read-out device for a solid-state quantum computer. In

future when quantum technology replaces the current computer technology, SET will find

immense applications.

This report discusses the principle of operation of SET, its fabrication and its applications. It

also deals with the merits and demerits of SET compared to MOSFET.


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CONTENTS

1 Introduction

1.1 What is a Transistor ?...1.2 What is a Single Electron Transistor ?.1.3 What problem does the SET help solve?.............................

2 The Single Electron Transistor2.1 About the Single Electron Transistor...2.2 How is the SET formed?......

3 Operation of the Single Electron Transistor3.1 How does the SET work?.....3.2 What is the Coulomb island?................................................3.3 Coulomb Blockade3.4 Requirements for function

4 Applications of the Single Electron Transistor5 Merits and Demerits

5.1 Merits..

5.2 Demerits..

6 Solutions7 Future of SETs8 Conclusion9 References


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CHAPTER 1

INTRODUCTION

During the 1980s, the main discoveries in Mesoscopic physics were the tunneling of singleelectrons and the Coulomb blockade phenomena, from which many scientists predicted that if

the size of the quantum dots were to be reduced to several nanometers, it is highly possible to

produce an applicable Single Electron Transistor which will be capable of working aboveliquid nitrogen temperature thus bringing about a revolution in field of electronics. Since then,

SET has been a popular research area. The breakthrough in nanotechnology as well as its

successful combination with semiconductor technologies gives hope to the Single Electron

Transistor, and some are of the opinion that it will be a mature technique in the coming decade.

1.1 What is a transistor?

A transistor is a solid-state semiconductor device that functions only in one direction, in

which it draws current from its load resistor. It can be used for numerous purposes such as

amplification, switching, voltage stabilization, signal modulation and many other functions.

The transistor acts as a variable valve which, based on its input current (as in the case of

BJT) or input voltage (as in the case of FET), allows a precise amount of current to flow

through it from the circuit's voltage supply.

Figure 1.a): NPN Transistor using two diodes and connecting both anodes together


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In an NPN transistor, as shown in the figure, one cathode (the emitter) is tied to common; the

other cathode (the collector) goes to a load resistor tied to the positive supply. For

understanding, the transistor is configured to have the diode signal start up unimpeded until it

reaches ~ 0.6 volts peak. At this point the base voltage will stop increasing. No matter how

much the voltage applied from the generator increases, the "base" voltage appears to not

increase. However, the current into the junction of the two anodes increases linearly, which is

given as, I = [E - 0.6]/R.

Figure 1.b): Graph of how the base voltage acts with increasing input voltage.

The above figure 1.2 depicts how base voltage acts with increasing input voltage. It shows

that as the voltage increases from 0 to 0.5 volts there is no current. However, at 0.6V a smallcurrent starts to show which is drawn by the base. The voltage at the base stops increasing

and remains at 0.6 volts, and the current starts to increase along with the collector current.

The collector current will slow down at some point until it stops increasing. This is where

saturation occurs. If this transistor was being used as a switch or as part of a logic element,

then it would be considered to be switched on.


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1.2 What is a Single Electron Transistor?

The single electron transistor or SET is a new type of switching device that uses controlled

electron tunneling to amplify current. It is an ultra-small device which transfers one electron at a

time, based on Coulomb interaction.

Coulomb interaction occurs on a tiny conducting layer know as the Coulomb Island. The

electrostatic potential of this island increases significantly with the introduction of just one

electron.

SET's are considered to be the elements of the future, wherein integrated circuits will be highly

dense and low powered. These ultra-low powered circuits will be sized at the nanometer scale

and will be able to detect the motion of individual electrons.

A SET is used mainly where low-powered devices are needed.

1.3 What problem does the SET help solve?

The Problem of Making More Powerful Chips:

Intel co-founder, Gordon Moore was the first to observe that the number of transistors on a

chip will approximately double every 18 to 24 months. This observation refers to what is

known as Moores Law.

This law has given chip designers greater incentives to incorporate new features on silicon

chips.

The chief problem facing designers comes down to size. Moore's Law works largely through

shrinking transistors, the circuits that carry electrical signals. By shrinking transistors,

designers can squeeze more transistors into a chip. However, more transistors means more

electricity and heat compressed into an even smaller space. Furthermore, smaller chips

increase performance but also compound the problem of complexity.

To solve this problem, the single-electron transistor - a device that exploits the quantum

effect of tunneling to control and measure the movement of single electrons was devised.

Experiments have shown that charge does not flow continuously in these devices but in aquantized way.


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CHAPTER 2

THE SINGLE ELECTRON TRANSISTOR

2.1 About the Single electron transistor

The Single Electron Transistor or SETis a new type of switching device that uses controlled

electron tunneling to amplify current. Using controlled electron tunneling, SETs can transfer

electrons from source to drain one by one.

The Single Electron Transistor is a three-terminal device. It consists of three electrodes-the drain, gate

and source. The drain and gate are connected through two tunnel junctions to one common electrode with

a low self-capacitance, known as the Coulomb Island or the quantum dot. The electrical potential of the

island can be tuned by the third electrode, called the gate. This gate electrode, electrostatically

influences electrons traveling between the source and drain electrodes.

The electrons in the SET need to compulsorily cross the two tunnel junctions that form the

Coulomb Island. Electrons passing through the island will charge and discharge it, and the

relative energies of systems containing 0 or 1 extra electrons depend on the gate voltage. At a

low source-drain voltage, a current will only flow through the SET if these two charge

configurations have the same energy. The SET transistor comes in two versions that have been

nicknamed metallic and semiconducting.

Figure 2.a): The schematic diagram of the three-terminal Single Electron Transistor


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A single tunnel junction is depicted schematically in below figure 2.b).

The tunnel junction consists of two pieces of metal separated by a very thin (~1nm) insulator.

The only way for electrons in one of the metal electrodes to travel to the other electrode is to

tunnel through the insulator.

Since tunneling is a discrete process, the electric charge that flows through the tunnel

junction flows in multiples of the charge of electrons, e.

Figure 2.b): A single tunnel junction

2.2 How is SET formed?

The SET is formed by placing two tunnel junctions in series, as shown in the belowschematic in figure 2.c)

The two tunnel junctions create what is known as a Coulomb Island that electrons can only

enter by tunneling through one of the insulators.

The gate capacitor may seem like a third tunnel junction, but is much thicker than the others

so that no electrons can tunnel through it.

It simply serves as a way of setting the electric charge on the Coulomb Island.

Figure 2.c): SET is formed by placing two tunnel junctions in series


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CHAPTER 3

OPERATION OF THE SINGLE ELECTRON TRANSISTOR

Figure 3.a): Energy levels of source, island and drain in a SET for both blocking state (upper

part) and transmitting state (lower part).


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3.1 How does the SET work?

The above figure 3.a) shows the energy levels of the source, island and drain in a SET for

both blocking state and transmitting state.

In the blocking state, no accessible energy levels are within tunnelling range of the electron(shown in red) on the source contact. All energy levels on the island electrode with lower

energies are occupied. Hence, the electron cannot tunnel through from source to drain via the

island.

When a positive voltage is applied to the gate electrode the energy levels of the island

electrode are lowered and the SET enters into the transmitting state.

The electron (green 1) can tunnel onto the island (green 2), occupying a previously vacant

energy level. From there, it can tunnel onto the drain electrode (green 3) where it scatters and

reaches the drain electrode Fermi level (green 4).

The energy levels of the island electrode are evenly spaced with a separation of E.

This gives rise to a self-capacitance, C of the island, given as, C = (e^2) /E.

A key point in the SETs operation is that the charge passes through the island in quantized

units. For an electron to hop onto the island, its energy must equal the coulomb energy,

(e^2/2) Cg.

When both the gate and the bias voltages are zero, electrons do not have enough energy to

enter the island and current does not flow. As the bias voltage between the source and drain

is increased, an electron can pass through the island when the energy in the system reaches

the coulomb energy. This effect is known as the coulomb blockade, and the critical voltage

needed to transfer an electron onto the island equal to e/C, is called the coulomb gap energy.


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3.2 What is the Coulomb island?

The Coulomb Island is a tiny layer formed between the source electrode and the gate electrode

through which a single electron tunnels, due to the phenomenon of Coulomb blockade.

To understand the Coulomb island, consider the following schematic diagrams.

Figure a) shows that when a capacitor is charged through a resistor, the charge on the capacitor is

proportional to the applied voltage and shows no sign of quantization.

Figure 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 island increases in steps

as the voltage is increased.

In the above figure c), we see that the steps are sharper for more resistive barriers and at lowertemperatures.


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3.3 Coulomb Blockade

The simplest device in which the effect of Coulomb blockade can be observed is the single

electron transistor.

Coulomb blockade is defined as the increased resistance at small bias voltages of an electronicdevice comprising at least one low-capacitance tunnel junction.

Because of the Coulomb Blockade, the resistances of devices are not constant at low bias

voltages, but increase to infinity for zero bias (i.e. no current flows).

The effect in which electron cannot pass through the island unless the energy in the system is

equal to the coulomb energy e^2/Cg.

Coulomb blockade tries to alleviate any leak by current during the off state of the SET.

To achieve Coulomb blockade, three criteria have to be met:

1. The bias voltage must be lower than the elementary charge divided by the self-capacitance of the island: ;

2. The thermal energy in the source contact plus the thermal energy in the island, i.e.must be below the charging energy: or else the electron will be able to passthe QD via thermal excitation; and

3. The tunneling resistance, should be greater than which is derived fromHeisenberg's uncertainty principle.

3.4 Requirements For Function

For the Single Electron Transistor to function normally, the following conditions must be

satisfied:

1) Capacitance of the island must be less than 10^(-17) Farad.2) The size of the island must be smaller than 10 nm.3) The wavelength of the electrons is comparable with the size of the dot, which means that

their confinement energy makes a significant contribution to the coulomb energy.


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CHAPTER 4

APPLICATIONS OF THE SET

Supersensitive Electrometer:

The high sensitivity of single-electron transistors have enabled them as electrometers in unique

physical experiments. For example, they have made possible unambiguous observations of theparity effects in superconductors. Absolute measurements of extremely low dc currents

(~10-20A) have been demonstrated. A modified version of the SET has been used for the first

proof of the existence of fractional-charge excitations in the fractional quantum hall effect.

Single-Electron Spectroscopy:

One of the most important applications of single-electron electrometry is the possibility ofmeasuring the electron addition energies (and hence the energy level distribution) in quantumdots and other nanoscale objects.

Detection of Infrared Radiation:

The calculations of the photo response of single-electron systems to electromagnetic radiation

have shown that generally the response differs from that the well-known Tien-Gordon theory of

photon-assisted tunneling. In fact, this is based on the assumption of independent (uncorrelated)tunneling events, while in single-electron systems the electron transfer is typically correlated.

This fact implies that single-electron devices, especially 1D multi-junction array with their low

co-tunneling rate, may be used for ultra-sensitive video- and heterodyne detection of high

frequency electromagnetic radiation, similar to the superconductor-insulator-superconductor(SIS) junctions and arrays. The Single electron array devices have advantages over their SIS

counterparts: Firstly lower shot noise and secondly the adjustment of the threshold voltage. This

opportunity is especially promising for detection in the few-terahertz frequency region, where nobackground-radiation-limited detectors are yet available.

Charge State Logics:The problem of leakage current is solved by the use of another logic device name charge state

logic in which single bits of information are presented by the presence/absence of single

electrons at certain conducting islands throughout the whole circuit. In these circuits the static

currents and power vanish, since there is no dc current in any static state.

Programmable Single Electron Transistor Logic:An SET having non volatile memory function is a key for the programmable SET logic. The

half period phase shift makes the function of SET complimentary to the conventional SETs. As

a result SETs having non-volatile memory function have the functionality of both theconventional (n-MOS like) SETs and the complementary (p-MOS like) SETs. By utilising this

fact the function of SET circuit can be programmed, on the basis of function stored by the

memory function. The charged around the QD of the SET namely an SET island shift the phase


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of coulomb oscillation, the writing/erasing operation of memory function which inject/eject

charge to/from the memory node near the SET island , makes it possible to tune the phase ofcoulomb oscillation. If the injected charge is adequate the phase shift is half period of the

coulomb oscillation.

Radio-frequency SET:Using SETs it is possible to make a fast-response and high-sensitivity electrometer to perform

fast and accurate counting of electrons on nanosecond time scales, for electrical metrology.

The RF-SET has the sensitivity and speed to count electrons at frequencies >10 MHz (that is,

measure a current on the order of pico-amperes, electron by electron) where the 1/fnoise due to

background charge motion is completely negligible.

There are also other applications using the RF-SET:

Detection and analysis of charge Qubit imperfections. Directly probing the Hamiltonian of high impedance electrical circuits, such as molecularnanowires, through ultra-sensitive polarized measurements.


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CHAPTER 5

MERITS AND DEMERITS

5.1 Merits of the SET

Using SETs it is possible to manufacture highly dense and low powered ICs.

The ICs will be sized at the nanometer scale.

Battery life of portable electronic devices will significantly be increased.

5.2 Demerits of the SET

Lithography Techniques:

The first biggest problem with all single-electron logic devices is the requirement Ec~100kBT,which in practice means sub-nanometer island size for room temperature operation. In VLSI

circuits, this fabrication technology level is very difficult. Moreover, even if these islands are

fabricated by any sort of nanolithography, their shape will hardly be absolutely regular. Evensmall variations in island shape will lead to unpredictable and rather substantial variations in the

spectrum of energy levels and hence in the device switching thresholds.

Background Charge:The second major problem with single-electron logic circuits is the infamous randomness of the

background charge. A single charged impurity trapped in the insulating environment polarizesthe island thereby switching the SET from the being conducting to being non-conducting.

Room Temperature Operation:

Most SETs with functional uses need to be at extremely low temperatures around 100 mK.

As with lithography techniques, a big problem is the requirementEc ~ 100 kBT, which in

practice means sub-nanometer island size for room temperature operation. In such small

conductors the quantum kinetic energy gives a dominant contribution to the electron addition

energy. Even small variations in island shape will lead to unpredictable and rather substantial

variations in the spectrum of energy levels and hence in the device switching thresholds.

Voltage Gain:

The voltage gain decreases as the size of the device decreases, due to voltage decreasing with

gate capacitance. This requires a few extra volts having to be applied to an output of a few mV.

But there is a limit to the voltage increase and it is connected to gate capacitance.The voltage

increases until the charging energy is of order kbT, then it drops.


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The below graph shows that it is very difficult make SETs with voltages greater than those that

operate at room temperature. This is even harder for dense integrated circuits that operate at 400

K.


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CHAPTER 6

SOLUTIONS

Currently, the best way to use SETs is in an hybrid setting. Most commonly is that of an FET/SET

combination. FETs would be used to speed the charge measurement and should be placed as close as

possible to the SET. The FET can also buffer the high output impedance produced by the SET. While

this method weakens the case for all SET circuits, it atleast provides a building block and provides a

more efficient way to use circuits with FETs.

The above figure (a) shows a circuit of a charged lock loop that will automatically tune away

background charge.

The figure (b) is a schematic of an SET with an FET output stage with a voltage graph, solidthat of the SET, dotted of the FET


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CHAPTER 7

FUTURE OF SETS

Using a hybrid combination, similar to that of SET and FET, the combination of SETs and

CMOS transistors in SETMOS devices can provide enough gain and current drive to perform

logic functions on a much smaller scale than possible with just CMOS transistors. 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 a logic state. It works on the notation of Coulomb Blockade oscillations,

but operates at a much faster current-driving capability.

More uses of electrometers:

Electrometers based on SET transistors could also be used to measure the quantum superposition

of charge states in a island connected by a tunnel junction to a superconductor. Islands could

therefore provide a means for implementing the quantum bits needed for a quantum computer.


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Professor Daniel Prober and Professor Robert Schoelkopf from the Department of Applied

Physics at Yale, have created an ultra fast, single-electron transistor which could lead to the

development of "quantum" computers with supercomputer powers and the size of a thumbtack.

The breakthrough involves inducing a small part of the transistor that will "resonate" with the

arrival of each electron.

This resonance creates a way for tracking each electron and also gives an extra bit of energy to

push the electrons as they are moving through the switch, this makes it 1,000 times faster than

any previous device.


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CHAPTER 8

CONCLUSION

Single Electronic Transistors have proved their value as tool in scientific research. Researchers

may someday assemble these transistors into molecular versions of silicon chips. The current

starts to flow through the junction when applied voltage is just sufficient to raise the energy of

electron above the coulomb blocked, this is called threshold voltage.

SETs could be used for memory device, but even the latest SETs suffer from offset charges,

which means that the gate voltage needed to achieve maximum current varies randomly from

device to device. Such fluctuations make it impossible to build complex circuits.The concept of

single electron logic suggested so far face sturdy challenges: either removing background charge

or providing continuous charge transfer in nanoscale. The development of room-temperaturesingle-electron devices may provide some important spin-offs for the digital world even at the

low-integration-scale stage.SET provide the potential for low-power, intelligent LSI chips,

appropriate for ubiquitous application. SETs are in still their infancy and new ideas and

technologies will surface in time. The future does look very bright for these devices because of

its much needed applications in present era.


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REFERENCES

Stevenson T. R, Pellerano F.A, Stahle C.M, Aidala K, Schoelkopf R.J. 2002, AppliedPhysics Letters, 80, 16.

Bladh K, Gunnarsson D, Johansson G, Kck A, wendin G, Delsing P, Aassime A,TaslakovM. Reading out Charge Qubits with a Radio Frequency Single Electron Transistor,

2002.

Berman D, Zhitenev N. B, Ashoori R.C, Smith H, Melloch M, 1997, American VacuumSociety, 2844.

Schoelopf R. J, Wahlgren P, Kozhevnikov A, Delsing P, Prober D. 1998, Science, 280,1238.

http://www.princeton.edu/~chouweb/newproject/research/SEM/SelfLimitChargProc.html Guo L, Leobandung E, Chou S. Y. 1997, Science, 275, 649. http://homepages.cae.wisc.edu/~wiscengr/feb05/transitioningelecfrontiers.shtml http://en.wikipedia.org/wiki/Coulomb_blockade http://www.smartplanet.com/blog/science-scope/scientists-create-a-single-electron-

transistor-a-big-step-for-quantum-computing/7923

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