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 circuit’s 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 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.

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 e^2/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.