SINGLE ELECTRON TRANSISTOR

Just like metal-oxide semiconductor field-effect transistors (MOSFETs) and MOS capacitors, to understand the fundamental principles of SETs, we need to start with a single-electron capacitor (SEC), which is the simplest known single-electron device. Also called a single-electron box, the capacitor usually consists of a quantum dot that is asymmetrically located between two electrodes. The capacitance between the quantum dot and the closer source electrode is the tunnel junction, and the other electrode is called the control gate capacitor. Once we apply voltage across two electrodes, electrons will be injected into quantum dots or, vice versa, ejected from quantum dots through the tunneling junction depending on the signs of the voltages. This is the same for any charge storage media between two electrodes. The difference in the case of the single-electron box is that because of the size of quantum dots, every time we try to inject an additional electron into quantum dots, excessive energy is needed due to Coulomb blockade effect. This unique property enables us to control the motion of a single electron through tunnel junctions. The Coulomb blockade is caused by an increase in the excessive charging energy due to the smaller size of quantum dots. Strictly speaking, it is not a quantum effect but rather a classic phenomenon under the effect of nanometer size.

The discovery of the transistor has clearly had enormous impact, both intellectually and commercially, upon our lives and work. It also led to the microminiaturization of electronics, which has permitted human to have powerful computers that communicate easily with each other via the Internet. Over the past 30 years, silicon technology has been dominated by Moore’s law: the density of transistors on a silicon integrated circuit doubles about every 18 months. To continue the increasing levels of integration new approaches and architectures are required. In today’s digital integrated circuit architectures, transistors serve as circuit switches to charge and discharge capacitors to the required logic voltage levels. Artificially structured single electron transistors studied to date operate only at low temperature, but molecular or atomic sized single electron transistors could function at room temperature.

The effects of charge quantization were first observed in tunnel junctions containing metal particles as early as 1968. Then the idea that the Coulomb blockade can be overcome with a gate electrode was proposed by a number of

researchers, and Kulik and Shekhter developed the theory of Coulomb-blockade oscillations, the periodic variation of conductance as a function of gate voltage. Their theory was classical, including charge quantization but not energy quantization. However, it was not until 1987 that Fulton and Dolan made the first SET, entirely out of metals, and observed the predicted oscillations. They made a metal particle connected to two metal leads by tunnel junctions, all on top of an

insulator with a gate electrode underneath. Since then, the capacitances of such metal SETs have been reduced to produce very precise charge quantization.

The first semiconductor SET was fabricated accidentally in 1989 by Scott-Thomas in narrow Si field effect transistors. Single Electron Transistor [SET] have been made with critical dimensions of just a few nanometer using metal, semiconductor, carbon nanotubes or individual molecules. A SET consist of a small conducting island [Quantum Dot] coupled to source and drain leads by tunnel junctions and capacitively coupled to one or more gate. Unlike Field Effect transistor, Single electron device is based on an intrinsically quantum phenomenon, the tunnel effect. The electrical behaviour of the tunnel junction depends on how effectively barrier transmits the electron wave, which decreases exponentially with the thickness, which is given by the area of tunnel junction divided by the square of wave length. Quantum dot [QD] is a mesoscopic system in which the addition or removal of a single electron can cause a change in the electrostatic energy or Coulomb energy that is greater than the thermal energy and can control the electron transport into and out of the QD. The nanoparticle is separated from the electrodes by vacuum or insulation layer such as oxide or organic molecules so that only tunneling is allowed between them. So we can model each of the nanoparticles-electrode junctions with a resistor in parallel with capacitor. The resistance is determined by the electron tunneling and the capacitance depends on the size of the particle.

Essential Vocabulary

capacitor конденсатор

capacitance емкостное сопротивление

source electrode электрод истока

tunnel junction туннельный переход

gate capacitance емкость затвора

inject инжектировать, вводить

quantum effect квантовый эффект

integrated circuit интегральная схема

charge quantization квантование заряда

gate electrode управляющий электрод, электрод затвора

gate voltage напряжение управления, затворное напр.

lead вывод

source исток, источник

drain сток

intrinsic собственный

Exercise 1. Answer the following questions:

1. What does single-electron transistor consist of?

2. What is the tunnel junction?

3. What happens when voltage is applied across two electrodes?

4. What enables us to control the motion of a single electron through tunnel junctions?

5. How did the discovery of the transistor change the world?

6. What does the Moor’s law state?

7. What is the function of transistor in today’s digital integrated circuit architecture?

8. When were the effects of charge quantization first observed?

9. When and by whom was the first SET made? Describe its arrangement.

10. Who and when fabricated the first semiconductor SET? Describe it.

Exercise 2. Read and translate the following international words:

Principle, quantum, asymmetrically, locate, tunnel, voltage, media, energy, blockade, effect, phenomenon, microminiaturization, communicate, silicon, technology, dominate, integration, theory, critical, barrier, resistor, model.

Exercise 3. Match each word with its definition:

1. Source a. of or relating to the essential nature of a thing;

inherent.

2. Drain b. a transition region between regions of differing

electrical properties in a semiconductor

3. Junction c. the electrode region in a field-effect transistor

into which majority carriers flow from the

interelectrode conductivity channel

4. Capacitor d. the electrode region in a field-effect transistor

from which majority carriers flow into the

interelectrode conductivity channel

5. Intrinsic e. a device for accumulating electric charge, usually

consisting of two conducting surfaces separated

by a dielectric

Exercise 4. Fill in the blanks using terms given below:

Insulating polymers are a category of … materials widely used in electrical apparatus, exhibiting low … concentration and mobility, thus low electrical … , even at high fields. Carriers in insulating polymers are introduced by … and … , as well as by charges injected from the electrode-insulation … . When the electrical field applied to the polymer is higher than the threshold for … accumulation, charge would be … into the bulk material from the interface of polymer-electrode and … , forming homocharge or … . Based on this traditional conduction mechanism, the … is a continuous flow of carriers, which gradually approaches a steady state … at a given time after the onset of voltage application. Recently, a brand new phenomenon of … was discovered in some insulating polymers (e.g., minicable with polyethylene insulation and nanostructured epoxy-based material) at relatively low … , through the use of an ultra fast space charge acquisition system and a conduction current … apparatus endowed of low-pass filter. Charge pulses (packets) are observed to travel very rapidly through the insulation bulk with mobility 4 to 5 orders of magnitude higher than that for conventional conduction.

Terms:charge carrier, contaminants, interface, injected, heterocharge, current, value, electrical fields, measurement, conductivity, accumulate, space charge, impurities, organic, , charge transport.

Exercise 5. Find in the text constructions noun + noun + noun… and translate them .

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