Single Electron Transistor

Presented By: Guided By:Pranoti Bachhav S.D.Pable

Contents:• History of transistor.• An introduction to SET.• Working of SET.• Types of SET.• Applications• Disadvantages.• References.

In 19th century Shockley, Brattain, Bardeen invented the information age “the transistor”.

“Moore’s law” states that transistor density on integrated circuit doubles at every two years.

History of transistor

An introduction to SET

• Single electron transistor (SET) is the most fundamental three-terminal single electron device (SED) which is capable of offering low power consumption and high Operating speed.

• A single electron transistor (SET) consist of a small, low capacitance (C), conducting Island [Quantum Dot] which is coupled to the source and drain leads by two tunnel junctions and capacitively coupled to one or more gate.

• Single electron device is based on an intrinsically quantum phenomenon known as the Tunnel effect. This single electron tunneling technology presents the ability to control the transfer of individual electrons.

Concept of single electrons:

• Single electronics is the fundamental concept behind single electronic devices like single electron transistors (SETs) that can be explained by assuming a small metallic sphere.

• The concept behind the single electronics shows that the more accurate measurement of the strength of this tunneling effect is charging energy or electrostatic energy, which is given by: Ec = e2/2c.

• The phenomena of single-electron transfer may be easily understood, if the concept about the movement of electronic charge through a conductor is clear.

Concept of single electrons:

• The charging and discharging of tunnel junction and thermal fluctuations are closely related to each other.

• Therefore, to avoid this problem, coulomb energy must be greater than the thermal fluctuations. Thus, the required condition to observe the single electron phenomenon is as follows: Ec = e2/2c > KBT where kB is Boltzmann‟s constant and T is temperature in Kelvin.

Concept of single electrons:

Concept of single electrons:• There are two fundamental conditions to observe the charging effect at room

temperature. 1)The capacitance C must be smaller than 3 af. 2)The quantum fluctuations of number of electrons must be negligible.

• This required condition of opacity can be maintained if tunnel resistance RT is larger than the quantum resistance. i.e.

RT > h/e2 = 25.813 KΩ

where h is plank‟s constant and e is electronic charge.

Working of SET.• A single-electron transistors are based on an

intrinsically quantum phenomenon which is known as „Tunnel effect‟.

• This tunnel effect may be observed when two metallic electrodes are separated by an insulating barrier known as tunnel junction.

• These three terminal switching devices can transfer electrons form source to drain one by one

Working of SET.• As SET is composed of two tunnel junctions along with one island which are

capacitively coupled to gate electrode. The island is located in between two tunnel junctions with capacities c1 and c2

• SET can electrostatically be controlled by the gate capacitance . Thus the total capacitance of the island is given by, cs = c1+ c2 + cg. Thus, such a set up is called single electron transistor.

Single Electron Tunneling & Coulomb Blockade.

• The transfer of electrons through the barriers between the quantum dots would result in charging of the neighboring quantum dots. Now, this would result in an increase of the electrostatic energy which is given by

Ec= e2/2c.where C is the effective capacitance of the island. Later on, this electrostatic energy became known as Coulomb charging energy or coulomb blockade energy

I-V Characteristic of SET

• The sequential entrance and leaving of an electron from one junction to another is generally known as “Correlated tunneling of electrons‟.

Advantages of SET.

• Low energy consumption • High sensitivity • Compact size • High operating speed • Simplified circuit • Feature of reproducibility • Simple principle of operation • Straight forward co-integration with traditional CMOS

circuits.

Disadvantages of SET.• Integration of SETs in a large scale. • It is difficult to link SETs with the outside environment. • Practically difficult to fabricate Single electron transistors (SETs).• Cotunneling• Background charge.• Room temperature operations.