Single Electron Transistor

1
Single Electron Transistor

• M. Eric Fombah

2
outline

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

3
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 circuits voltage supply.

4
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

5
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.

6
How is the SET formed

• Fig 4. A tunnel Junction and its schematic diagram

7
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.

8
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 e2/2Cg.
• 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.

9
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.

10
The Coulomb Island
11
Coulomb Island

• 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.
• 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.

12
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 e2/Cg.
• Coulomb blockade tries to alleviate any leak by
  current during the off state of the SET.

13
Application of SETs

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

14
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.