Single_Electron_Transistor_Aneesh_Raveendran

Single Electron Transistor
Aneesh Raveendran
INDIA
Aneesh Raveendran, INDIA,
aneeshr2020@gmail.com

outline
• What are Transistors
• What is a SET
• Operation of SETs
• Application of SETs
• How do SETs differ from Conventional

What is a Transistor
• A transistor is a solid state semiconductor device
which can be used for numerous purposes including
signal modulation, amplification, voltage
stabilization, and many others.
• Transistors act like a variable valve which, based on
its input current (BJT) or input voltage (FET), allow a
precise amount of current to flow through it from
the circuit’s voltage supply.

Transistor
• Transistors
– What are transistors?
– How do they work?
A transistor is a device that functions only in one direction,
in which it draws current from its load resistor. The
transistor is a solid state semiconductor device which can
be used for amplification, switching, voltage stabilization,
signal modulation and many other functions. It acts as a
variable valve which, based on its input current (BJT) or
input voltage (FET), allows a precise amount of current to
flow through it from the circuit's voltage supply.

Fig 1. NPN Transistor using two diodes and connecting both anodes together

• One cathode is tied to common (the emitter); 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 (within reason), the "base"
voltage appears to not increase. However, the current into
that junction (two anodes) increases linearly: I = [E - 0.6]/R.

Fig 2. Graph of how the base voltage acts with increasing input voltage.
As the voltage increases from 0 to 0.5 volts there is no current. However, at 0.6
a small current 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.

• Single-electron Transistor
- what problem does it help solve?
- what is its operation?

Problem of Making More Powerful Chips
• Intel co-founder Gordon Moore that the number of transistors on a chip will
approximately double every 18 to 24 months. This observation refers to what
is known as Moore’s Law.
• This law has given chip designers greater incentives to incorporate new
features on silicon.
• 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.

What is a SET
• The single electron transistor is a new type of
switching device that uses controlled electron
tunneling to amplify current.
• Fig 3. Single Electron Transistor

Operation
• 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.

How is the SET formed
• Fig 4. A tunnel Junction and its schematic
diagram

Dynamics
• The SET is made by placing 2 tunnel junctions in
series
• The 2 tunnel junction create what is known as a
“Coulomb Island” that electrons can only enter by
tunneling through one of the insulators.
• This device has 3 terminals like the FETs.
• The cap may seem like a third tunnel junction, but is
much thicker than the others so that no electrons
could tunnel through it.
• The cap simply serves as a way of setting the electric
charge on the coulomb island.

Procedure
• A key point is that charge passes through the island in
quantized units. For an electron to hop onto the island, its
energy must equal the coulomb energy e^2/2Cg.
• 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.

For Function
• Capacitance of the island must be less than
10^-17 Farads and therefore its size must be
smaller that 10 nm.
• 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.

The Coulomb Island

Coulomb Island
(a) 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.
(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
c) The steps are sharper for more resistive barriers and
at lower temperatures.

Coulomb Blockade
• The effect in which electron can not 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.

Application of SETs
• Quantum computers
-1000x Faster
• Microwave Detection
• High Sensitivity Electrometer

Conclusion
• Researchers may someday assemble these
transistors into molecular versions of silicon chips ,
but there are still formidable hurdles to cross.
• 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 future does look bright for these devices.

Thank you
Aneesh Raveendran
“A common Man”,
INDIA

Single_Electron_Transistor_Aneesh_Raveendran

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