To avoid problems like power consumption challenges, high-performance, high-density chip design, and faster and more processing in transistors Mitri Averin and Konstantin Likharev proposed a new three-terminal device called single-electron tunneling transistor. Two years later Theodore Fulton and Gerald Dolan at Bells Lab in the US created this device and demonstrated its working and operation.
Single Electron Transistor (SET) is a three-terminal device which can transfer an electron from the source to drain one by one. SET consists of a quantum dot (conducting island) connected to the source and drain leads by tunnel junctions and connected to one or more gates. The quantum dot is separated from the source and drain by thin insulators. It has a tunneling junction instead of a PN junction compared to FET. In this device, the electron flow through the tunnel junction between the source/drain to the quantum dot, therefore, the purpose of this device is to control the tunneling of an electron to the quantum dot. The source and drain electrodes are attached to the island through a tunnel barrier. Furthermore, the electrical potential of the dot can be tuned by a third electrode, referred to as the gate, which is capacitively coupled to the dot.
SET is a sensitive electronic device that works based on the Coulombs blockade effect. Coulomb blockade is often observed by making a tool very small, such as a quantum dot. The electrons inside the quantum dot will create a powerful coulomb repulsion preventing other electrons to flow. Thus, the device will not obey Ohm’s law anymore. So even putting one more electron onto the quantum dot would cost too much coulomb energy. This is called the Coulomb blockade.
The Coulomb energy is Ec = e^2/2C
Where, e= charge on electron and
C=the total capacitance of the source, drain junction, and gate capacitor
The main purpose of SET is to individually control the random tunneling of electrons into and out of the quantum dot.
This is done by
- Choosing the correct geometry and materials.
- Applying bias voltage to the gate electrode located which is between source and drain.
To control tunneling, a voltage bias is given to the gate electrode. A separate voltage bias is applied between the source and drain electrode for the current direction. It creates an electric field and change in the potential energy of the quantum dot with respect to the source and drain. For the current to flow the potential difference should be at least large enough to overcome the energy of the Coulomb blockade.
The Energy(E) needed to move a charge(Q) across a potential energy difference(V) is given as E=VQ where Q=e
V=E/e=Wc/e since, E=Wc
With this voltage applied to the quantum dot, an electron can tunnel through the quantum dot.
While gate voltage Vg is zero, no current flows. For Single Electron tunneling Vg=Vcoulomb. When gate voltage Vg=Vcoulomb+e/2C then 2 electrons can be moved on the quantum dot at a time. When gate voltage Vg=Vcoulomb+e/2c+e/2c then 3 electrons can be moved on the quantum dot at a time. The number of electrons in the quantum dot is controlled using the gate voltage.
Working of Single Electron Transistor
When SET is in the ‘OFF’ mode the corresponding potential energy is not energetically favorable for electrons in the source to tunnel to the dot. When it is in ‘ON’ mode at the lowest setting, electrons tunnel one at a time, through the dot, from the source to drain. By applying the proper gate voltage the potential energy of the dot is made low enough to encourage an electron to tunnel the Coulomb blockade energy barriers to the quantum dot. Once, the electron is on it, the dot’s potential energy rises. Then the electron tunnels via the Coulomb blockade on the other side to reach the lower potential energy at the drain. Then with the dot empty and the potential energy lowering again the process continues.
Advantages of SET
- Low energy consumption
- High sensitivity
- Compact size
- High operating speed
- Simplified circuit
- Feature of reproducibility
- Simple principle of operation
Applications of SET
- Ultrasensitive Microwave Detector
- Single-Electron Spectroscopy
- Lithography Technique
- Quantum computers
- Single Electron transistor inverter
- Single Electron transistor scanning electrometer (SETSE)
Single-electron transistors have so many applications. However, they are not suitable for complex circuits owing to the fluctuations present in them. Other disadvantages in SET are randomness of the background charge, practical difficulty to fabricate SETs, cotunneling, and difficulty in maintaining the room temperature.