Single Electron Transistors (SET)

1
Single Electron Transistors (SET)

• EE 240
• Group 6
• 05-06-05
• Adit Gupta, Sandeep Kotak,
• Ana MartinezMarrosu, Erik Stegall

2
Overview

• Summary
• Background
• Creation
• Formulas
• Problems
• Devices and Applications
• Future/Closing

3
Introduction

• Summary
• Definition
• An ultra-small device, that transfers one
  electron at a time, based on Coulomb interaction.
  This occurs on a tiny conducting layer know as
  an island. This islands electrostatic potential
  increases significantly with the introduction of
  just one electron.
• Single-electron transistors SET's are considered
  to be the elements of the future. In this
  future, integrated circuits will be highly dense
  and low powered. These ultra-low powered
  circuits will be of a nanometer scale electronic
  and they will be able to detect the motion of
  individual electrons. Problems, however, are that
  SET's have low voltage gain, high output
  impedances, and are sensitive to random
  background charges. Also, for SETs to be useful
  in practical applications they must be able to
  operate in room temperature. SETs are required
  to be no larger than 10 nm. This is why its
  highly unlikely that single-electron transistors
  would ever replace field-effect transistors
  (FET's) which work better in applications where
  large voltage gain or low output impedance is
  necessary.

4
Background

• The start of the SET transistor began in 1985
  thanks to Dmitri Averin and Konstantin Likharev.
  They proposed the idea of a new three-terminal
  device called a single-electron tunneling (SET)
  transistor. Two years later Theodore Fulton and
  Gerald Dolan at Bell Labs in the US, created such
  a device and demonstrated how it would operate.

• What are SET transistor made from
• Single-electron transistors have been made with
  just a few nanometers using
• Metals
• Semiconductors
• Carbon nanotubes
• Individual molecules. 5-7

The charging of electrons for a tunnel junction
with an Capacitance C and an Charge Q has been
the bases for how SETs would function. The
electric current. I?Q/ ?t, that is associated
with tunneling of single electrons is Ie/t
e3/2phC
Dmitri Averin, currently a professor at Suny
Stony Brook.
5
Background

• General Information about SET Transistors.
• A single-electron transistor consists of a small
  conducting island connected to an source and
  drain leads by tunnel junctions and connected to
  one or more gates.
• Generally two gates are used, one used as an
  input for the SET while the second is used to
  tune the background charge, a common obstacle
  needed to be overcome.
• There are still several problems, to be discussed
  later, that have slowed the main stream use of
  SETs.
• The two more common versions of the 4 stated
  before are the metallic and semiconductor
  versions.
• Coulomb Blockade -gt

6
Creation

• Procedure
• For Metallic
• The first metallic version created by Fulton and
  Dolan, a material such as a thin aluminum film is
  used to make all of the electrodes. Beginning
  with metal being evaporated through a shadow mask
  that will form the source, drain and gate
  electrodes. Next the tunnel junctions are formed
  by adding oxygen to chamber so the metal becomes
  coated by a thin layer of its natural oxide.
  Finally, a second layer of the metal, that is
  shifted from the first by rotating the sample, is
  evaporated to form the island.
• For semiconductors the source, drain and
  island are obtained by "cutting" regions in a
  two-dimensional electron gas formed at the
  interface between two layers of semiconductors
  such as gallium aluminum arsenide and gallium
  arsenide. The conducting regions have metallic
  electrodes patterned on the top semiconductor
  layer. Negative voltages applied to these
  electrodes deplete the electron gas just beneath
  them, and the depleted regions can be made
  sufficiently narrow to allow tunneling between
  the source, island and drain. Electrodes that
  shape the islands can be used as the gate
  electrode.

7
Creation

• Another way to form an SET is using a scanning
  tunneling microscope (STM) which can avoid the
  control problems in self- organized structures.
  Using this technique an SET can be created that
  operates at room temperature, showing a clear
  Coulomb staircase with a 150 mV period at 300 K.
• The Process
• A 3 nm titanium (Ti) metal film is deposited on
  a 100 nm thermally oxidized SiO2/n-Si substrate.
  The Ti surface is oxidized by through the water
  on the surface via the atmosphere. By using the
  STM tip as a cathode nanometer size Ti oxide
  (TiOx) lines can be formed. The barrier height of
  a TiOx/Ti junction has been found to be 285 meV
  for the electron from the temperature dependence
  of the current.
• Picture of an titanium SET-gt
• Picture of SET using an ATM.

8
Formulas

• Formula for calculating voltage in an island. For
  n electrons. V(n) (-ne Q0 C1V1 C2V2
  Cg1Vg1 Cg2Vg2)/CS. 
• The charging energy, Ec e2/(2C), sets the
  energy scale for single-electron effects. The
  charging energy is typically in the range 1 - 100
  meV.
• A single electron passing through a junction has
  a change in electrostatic energy ?Ec
  -e(2Q-e)/2C
• Quantum Conductance Goe2/h
• Wc e2/2CgtgtKbT when this true the electron is
  blocked, called Coulomb blockade, when not true
  electrons can be travel through the junction.
• Rt gtgt h/e225.8 kO tunneling resistance must be
  greater then resistance quantum along with the a
  charged electron energy is greater than the
  thermal energy is required for tunneling to
  occur.
• Polarization charge Qt/Ct Qg/Cg Vg, Qt is
  polarization charge of tunnel junction and Qg is
  the gates.
• 1/(CsCg)(-nee/2_CgVg) gt Vd gt
  1/(CsCg)(-ne-e/2CgVg) using Thevenims
  theorem. Used for the relationship of the drain
  voltage Vd and the gate voltage Vg.
• Using Thevenims theorem in a circuit.
• Is a picture of a circuit connected to a source,
• Is a picture of a circuit connected to a drain.

9
Problems

• Limiting factors
• Most SETs with functional uses need to be at
  extremely low temperatures around 100 mK.
• Background charge problem, is an issue that is
  the greatest inhibiter of the widespread use of
  SET's. The cause of the background charge problem
  is the extreme charge sensitivity of SET's. A
  single charged vacancy or an ion in the oxide
  layers near a SET can be enough to switch the
  transistor from the being conducting to being
  non-conducting.
• Voltage gain decreases as the size of the device
  decreases, because voltage decreasing with gate
  capacitance. This requires an 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.

This 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.
10
Device App

• Radio-Frequency SET
• - fast-response and high-sensitivity
  electrometer

RF-SET has the sensitivity and speed to count
electrons at frequencies gt10 MHz (that is,
measure a current on the order of pico-amperes,
electron by electron) where the 1/f noise due to
background charge motion is completely negligible.

• Applications of the RF-SET
• Fast and accurate counting of electrons on
  nanosecond
• time scales, for electrical metrology.
• Detection and analysis of charge Qubit
  imperfections.
• Directly probing the Hamiltonian of high
  impedance electrical circuits, such as molecular
  nanowires, through
• ultra-sensitive polarized measurements.

500 nm
A comparison of the performance of different SET
and conventional memory technologies
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Applications

• SETs used to increase battery life in portable
  electronics
• Blick's transistor the "island" is connected to
  a tiny nanopillar that oscillates.
• This new mechanical model can operate at room
  temperature.
• This research will result in smaller and less
  power-greedy electronic items

• Single-Electron MOS Memory (SEMM)

• Coulomb Blockade
• Miniature Flash Memory
• Yano Type Memory

12
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 at least provides a building
  block and provides a more efficient way to use
  circuits with FETs.

1. Is a picture of a charged lock loop that will
   automatically tune away background charge.
2. Is a schematic of an SET with an FET output stage
   with a voltage graph, solid that of the SET,
   dotted of the FET

13
The Future, Logic and Electrometers

• 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 smaller scale 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.

Whats to come, quantum computers.
14
The Future
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.
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.  
15
Work Cited

• Stevenson T. R, Pellerano F.A, Stahle C.M, Aidala
  K, Schoelkopf R.J. 2002,
• Applied Physics Letters, 80, 16.
• Bladh K, Gunnarsson D, Johansson G, Käck 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, Ashoori R.C, Smith H,
  Melloch M, 1997, American
• Vacuum Society, 2844.
• Schoelopf R. J, Wahlgren P, Kozhevnikov A,
  Delsing P, Prober D. 1998, Science,
• 280, 1238.
• http//www.princeton.edu/chouweb/newproject/resea
  rch/SEM/SelfLimitChargProc.html
• Guo L, Leobandung E, Chou S. Y. 1997, Science,
  275, 649.
• http//homepages.cae.wisc.edu/wiscengr/feb05/tran
  sitioningelecfrontiers.shtml

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AndScene
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