Title: Single Electron Transistors (SET)
1Single Electron Transistors (SET)
- EE 240
- Group 6
- 05-06-05
- Adit Gupta, Sandeep Kotak,
- Ana MartinezMarrosu, Erik Stegall
2Overview
- Summary
- Background
- Creation
- Formulas
- Problems
- Devices and Applications
- Future/Closing
-
3Introduction
- 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.
4Background
- 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.
5Background
- 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
6Creation
- 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.
7Creation
- 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.
8Formulas
- 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.
9Problems
- 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.
10Device 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
11Applications
- 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
12Solutions
- 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.
- Is a picture of a charged lock loop that will
automatically tune away background charge. - Is a schematic of an SET with an FET output stage
with a voltage graph, solid that of the SET,
dotted of the FET
13The 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.
14The 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.Â
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. Â
15Work 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
16AndScene
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