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Single Electron Transistors (SET)

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Title: 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
11
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

16
AndScene
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