Prashant Pal College of Technology GBPUA&T Pantnagar

Single Electron Transistor

1

Overview

What is SET?

How it works?

Logic Implementation

Programmable Logic Implementation

Fabrication

Applications

Limitations of SETs


2

What is a Single Electron Transistor (SET)?

Device based on ‘Quantum mechanical principles’

Exploits ‘Quantum effect of tunneling’. Tunneling on purpose. one electron sufficient to define a logic

state. Tunneling : a discrete process

3


The Tunneling Phenomenon

•Tunneling is possible because of the wave-like properties of matter.

•Quantum mechanics allows a small particle, such as an electron, to overcome a potential barrier larger than its kinetic energy.

•Transmission Probability: T ≈ 16ε(1 – ε)e-2κL

• Tunnelling on purpose: Make ‘T ‘ very high => Adjust device parameters

4


Introduction to Single Electron Transistor:

A Quantum Dot(QD) or Island.

Two tunnels Junctions

A Gate electrode

Gate capacitor

( optional ) 2nd Gate electrode

Symmetric device : S and D interchangeable

5


http://en.wikipedia.org/wiki/Image:Set_schematic.svg

What Happens in SET..?

A SET is similar to a normal MOS transistor, except

1) the channel is replaced by a nano dot.

2) the dot is separated from source and drain by thin insulators (SiO2). An electron tunnels in two steps: source → dot → drain

3) The gate voltage Vg is used to control the charge on the gate-dot capacitor Cg .

6


COULOMB BLOCKADE

Ec =e^2 /2Cg is the electrostatic energy needed for one electron of the "source" electrode to tunnel across the metal island and reach the "drain" electrode.

If this energy barrier is appreciably higher than the thermal fluctuation energy KBT, the island remains sensitive to the addition of just one extra electron to the millions it already contains.

And precisely one extra electron can be added in certain polarization conditions of the SET device.

If there is not enough electron energy, the transfer will be blocked.

This phenomenon is called the Coulomb blockade.


7

COULOMB BLOCKADE

Electron transfers on island : based on Coulomb interaction.

“ Island’s electrostatic potential increases with the addition of an electron and addition of further electrons becomes more difficult”

For electron to hop onto the island:

Its Energy =Charging energy, Ec =e^2 /2Cg

Provide ext. bias voltage…


8

V-I Characteristics

At Vg = e/2C ,current rises

Additional voltage ‘e/C’ (Coulomb gap voltage), for further increase.

Periodic Id-Vg chara. (Coulomb oscillations)


9

CONDITIONS FOR TUNNELLING


10

On Increasing Vg…..

Single electron tunnelling

Two electron tunneling Current rises..


11

On Increasing Vg…..

Electron gets trapped..Current drops


12

Both electrons trapped..Current=0

Conductance v/s Vg plot


13

LOGIC IMPLEMENTATION

nMOS nSET

( ON when Vg=‘1’)

pMOS pSET

( ON when Vg=‘0’)

To realise pSET :

=>Use 2nd Gate electrode..!!

=> Apply reqd. voltage to shift I-V characteristics


14

BASIC INVERTER..


15

PROGRAMMABLE LOGIC

With the help of Non Volatile Memory (NVM)

function. (implemented using SETs)

NVM node is capacitively coupled to the ISLAND

SET I-V characteristics programmed via NVM.

i.e SET can dynamically change to pSET or nSET.


16


17


18

SPICE Simulation O/p waveform


19

ADVANTAGES OF PROGRAMMABLE SET LOGIC

High degree of programmability with low device count.

A single logic ckt can implement many logic

functions..!

Impossible with CMOS tech.


20

Initial device structure of SET

SIMOX wafer. A type of

SOI.

Conductance oscillations as a function of the gatevoltage measured at 300 K at a drain voltage of 10 mV.

21

Conductance oscillations as a

function of the gate voltage measured at

40 K and at adrain voltage of 10

mV.

Initial structure of the twin SETs before

22

SET


23

Structure of SET24


Supersensitive electrometry:-Supersensitive electrometry:-If the source-drain voltage to a single- electron transistor is slightly above its Coulomb blockade threshold, source-drain current through the device is extremely sensitive to the gate voltage.

Supersensitive electrometry:-Supersensitive electrometry:-If the source-drain voltage to a single- electron transistor is slightly above its Coulomb blockade threshold, source-drain current through the device is extremely sensitive to the gate voltage.

Microwave detection:-Microwave detection:-The videoresponse ("photoresponse") of single-electron systems to electromagnetic radiation with frequency f=Ec/h. Microwave has low frequency, and so low energy, so detection would not have been possible without SET.

Microwave detection:-Microwave detection:-The videoresponse ("photoresponse") of single-electron systems to electromagnetic radiation with frequency f=Ec/h. Microwave has low frequency, and so low energy, so detection would not have been possible without SET.

26

LIMITATIONS OF SET

Susceptible to Back ground charge. High bit error rates. Room temperature operation. Fabrication. Low fanout (High o/p impedance).


27

Conclusion

The future of SETs looks very bright. Instead of working with millions of electrons in today’s MOS tech., one can realize the limit of calculating with single electrons. No matter how good SET tech. might turn out to be, it is hard to imagine that it will replace MOS tech. completely. The biggest benefits seem to lie in the clever combination of both.

Moore’s Law will sustain......!!


28

REFERENCES

1. H. Pothier, P. Lafarge, D. Esteve, C. Urbina, and M. H. Devoret, “Passing electrons one by one: Is a 8 accuracy achievable?,” IEEE Trans. Instrum. Meas., vol. 42, pp. 324–330, Apr. 1993.

2. Josh Clark, "How Quantum Suicide Works"

3. (http://science.howstuffworks.com/quantum-suicide.htm)

4. M.A. Kastner, Physics Today 46(1993) 24

5. R.H. Chen, A.N. Korotkov, K.K. Likharev: Single- Electron Transistor Logic, Appl. Phys.Lett., Vol. 68, No. 14, April 1996, pp. 1954 – 1956Single Electron Transistor

29

http://science.howstuffworks.com/quantum-suicide.htm


30


31

Prashant Pal College of Technology GBPUA&T Pantnagar

Engineering

Transcript of Prashant Pal College of Technology GBPUA&T Pantnagar