Seminar 2010
SET
ABSTRACT
Single electron transistor (SET) is a novel idea and has been intensively studied. This review gives a general picture of SET, such as its mechanism, fabrication, application and problems faced. During 1980s, the main discoveries discoveries in mesoscopic mesoscopic physics are the tunneling of single electron and Coulomb blockade phenomena, which make many scie scient ntis ists ts pred predic ictt that that if the the size size of the the quan quantu tum m dots dots is redu reduce ced d to seve severa rall nanometers, it is highly possible to produce applicable single electron transistor (SET (SET)) whic which h works works abov abovee liqui liquid d nitr nitroge ogen n temp temper erat atur ure, e, and and this this will will brin bring g a revolution revolution to electroni electronicc science. Since then SET has been a hot research area. The breakthrough of nanotech as well as its successful combination with semiconductor technologie technologiess gives hope to SET, and some think that it will be a mature mature technique in the coming decade.
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SET
1. INTRODUCTION
A conventional field-effect transistor, the kind that makes all modern electronics work, is a switch that turns on when electrons are added to a semiconductor and turns off when they are removed. These on and off states give the ones and zeros that digital computers need for calculation. One then has a transistor that turns on and off again every time one electron is added to it; we call it a single electron transistor (SET). Furthermore, the behavior of the device is entirely quantum mechanical.
Electron transport properties of individual molecules have received considerable attention over the last several years due to the introduction of singleelectron transistor (SET) devices which allow the experimenter to probe electronic, vibrational or magnetic excitations in an individual molecule. In a three-terminal molecular SET the molecule is situated between the source and drain leads with an insulated gate electrode underneath. Current can flow between the source and drain leads via a sequential tunneling process through the molecular charge levels, which the gate electrode is used to tune.
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Seminar 2010
SET
2.HISTORY OF SET The effects of charge quantization were first observed in tunnel junctions containing metal particles as early as 1968. Later, the idea that the Coulomb blockade can be overcome with a gate electrode was proposed by a number of authors, 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-Thomasetal. In narrow Si field effect transistors. In this case the tunnel barriers were produced by interface charges.
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3.
SET
SET SCHEMATIC REPRESENTATION
A model of SET is shown in Fig.1,(b) is the simplified model.
Figure(1) A schematic circuit of SET
The two areas filled with patched pattern are tunneling junctions; there are some discrete Coulomb islands between them. R1, C1 and R2, C2 are the resistance and capacitance of the junctions. The junctions form the source and drain of the transistor, 2 / V voltages are applied to them through conductive wires, the tunneling current pass through the islands is I. A layer of insulating media separates DEPT OF ECE
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SET
the islands from the gate; the capacitance between them is Cg. A voltage of Vg is applied on the gate and controls the open or close of the SET. Because of its unique structure, SET has many prospective characteristics such as low power consumption, high sensitivity, high switching speed, high packet density, etc. So much attention has been attracted on their fabrication and industrial realization.
Fig (2) Schematic diagram of SET
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SET
4.
WORKING OF SET
The single electron transistor is a new type of switching device that uses controlled electron tunneling to amplify current.
Conduction through a molecular
SET only occurs when a molecular electronic level lies between the Fermi energies of the leads. A bias voltage, V bias, applied between the source and the drain, changes the electrostatic potential of one of the leads by an energy |eV|. For small bias voltages, |eV| < Ec + ∆E where Ec is the Coulomb charging energy and ∆E is the energy difference between consecutive charge states of the molecule being measured, current cannot flow though the device because the excited molecular levels are not available to conduct charges between the electrodes. This is known as the Coulomb blockade regime.
Fig (3) Schematic of a single electron transistor
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Seminar 2010
SET
If bias voltages, |eV| > Ec + ∆E where Ec is the Coulomb charging energy and ∆E is the energy difference between consecutive charge states of the molecule being measured, current can flow though the device. Usually electrons move continuously in the common transistors, but as the size of the system goes down to nanoscale (for example, the size of metal atoms can be several nm, and the size of semi-conductive particles can be several tens nm), the energy of the system is quantumized, that is, the process of charging and discharging is discontinuous. The energy for one electron to move into the system is: EC=e2/2C where C is the capacitance of this system. This Ec is called Coulomb blockade energy, which is the repelling energy of the previous electron to the next electron. For a tiny system, the capacitance C is very small, thus Ec can be very high, and the electrons cannot move simultaneously, but must pass through one by one. This phenomenon is called "Coulomb blockade". If two quantum dots(QD) are joined at a point and form a channel, it is possible for an electron to pass from one dot over the energy barrier and move to the other dot, this is called "tunneling phenomenon". In order to overcome the barrier (Ec), the applied voltage on the quantum dots (V/2) should be V > e/C
Quantum tunnelling It refers to the quantum mechanical phenomenon where a particle tunnels through a barrier that it classically could not surmount because its total mechanical energy is lower than the potential energy of the barrier. This tunnelling plays an essential role in several physical phenomena, including radioactive decay and has important applications to modern devices such as the tunneling diode and the scanning tunnelling microscope.
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SET
COULOMB ISLAND
Fig (4) Coulomb Island
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SET
(a)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.
(b) 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.
A signature of this phenomenon is commonly seen at low temperatures as an absence of current for low bias voltages. As the bias voltage across the device increases, excited states will provide conduction channels in the device. As a result, discrete changes in the current through the SET will be obtained every time a new molecular level falls within the bias window. The simplest device in which the effect of Coulomb blockade can be observed is the so-called single electron transistor. It consists of two tunnel junctions sharing one common electrode with a low self-capacitance, known as the island . The electrical potential of the island can be tuned by a third electrode (the gate), capacitively coupled to the island. In the blocking state no accessible energy levels are within tunneling range of the electron (red) on the source contact. All energy levels on the island electrode with lower energies are occupied. When a positive voltage is applied to the gate electrode the energy levels of the island electrode are lowered. The electron (green 1.) can tunnel onto the island (2.), occupying a previously vacant energy level. From there it can tunnel onto the drain electrode (3.) where it in elastically scatters and reaches the drain electrode Fermi level (4.).
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Seminar 2010
SET
The energy levels of the island electrode are evenly spaced with a separation of Δ E . Δ E is the energy needed to each subsequent electron to the island, which acts as a self-capacitance C . The lower C the bigger Δ E gets. To achieve the Coulomb blockade, three criteria have to be met: •
The bias voltage can't exceed the charging energy divided by the capacitance
V bias =
•
;
The thermal energy k BT must be below the charging energy E C =
, or else
the electron will be able to pass the QB via thermal excitation; •
The bias voltage can't exceed the charging energy divided by the
capacitance V bias =
•
;
The thermal energy k BT must be below the charging energy E C =
, or else
the electron will be able to pass the QB via thermal excitation.
The single-electron transistor shows that its dc I-V curves may be quite complex [50]. However, the situation at small source-drain voltage is much simpler. In fact, Fig. 6c shows that on each period of the Coulomb blockade oscillations there is one special point Qe = e(n + 1/2), at which the Coulomb blockade is completely suppressed, and the I-V curve has a finite slope at low voltages Fig. 6b). Another way to express the same property is to say that the linear conductance G º dI/dV½V=0 of the transistor as a function of the gate U voltage exhibits sharp peaks. Theory shows that even if the electron quantization effects are substantial, the peak position may be found from a very natural "resonance tunneling" an energy level inside the island (with the account of the
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gate field potential)should be aligned with the Fermi levels in source and drain, which coincide at VÆ0. This rule yields a simple equation for the gate voltage distance between the neighboring Coulomb blockade peaks
If the charging effects dominate (Ec >> Ek) this relation is reduced,but at strong quantization (Ek >> Ec) the distance between the Coulomb blockade oscillation peaks, as well as the heiGmax of the peaks, may vary from level to level.
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Seminar 2010
SET
5. FABRICATION OF SET The fabrication of SET promotes many difficulties. For SET to be used in a large scale industrially and position. Basically the fabrication methods can be divided as physical or chemical techniques according to the main procedures. The physical methods often utilize the combination of thin film and lithographic technologies. Devices with carefully tailored geometries and electron density are got. For example, quantum dots or quasi-zero-dimensional puddles of electrons with weak coupling to simultaneously patterned electrical leads are fabricated to form a SET. However, lithographic and materials limitations restrict the minimum size and composition of such dots (100nm), and studies are typically limited to sub-Kelvin temperatures. Another approach is to grow nanostructures chemically. This approach is prosperous for its low cost and good controllability of the size of Coulomb islands, and it is possible to be a prospective technique. Though this technique is not mature industrially, the SET s fabricated in laboratories show fascinating results. Generally there are three most important steps: first, the fabrication of Coulomb islands as well as the control of their size and dispersity; second, the formation of tunneling junctions at the joint of electrodes and Coulomb island; third, the formation of gate between substrate and Coulomb islands.
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SET
6. APPLICATIONS
SET has found many applications in many areas. What's most exciting is the potential to fabricate them in large scale and use them as common units in modern computer and electronic industry.
SINGLE ELECTRON MEMORY Scientists have long been endeavored to enhance the capacity of memory devices. If single electron memory can be realized, the memory capacity is possible to reach its utmost limit. SET can be used as memory cell since the state of Coulomb island can be changed by the existence of one electron. Chou and Chan [5] first pointed out the possibility of using SET as memories in which information is stored as the presence or absence of a single electron on the cluster. They fabricated a SET by embedding one or several nano Si powder in a thin insulating layer of SiO2, then arranging the source and drain as well as gate around this Coulomb island. The read/write time of Chan's structure is about 20ns, lifetime is more than 109 cycles, and retention time (during which the electron trapped in the island will not leak out) can be several days to several weeks. These parameters would satisfy the standards of computer industry, so SET can be developed to be a candidate of basic computer units. If a SET stands for one bit, then an array of 4~7 SETs will be substantial to memorize different states. The properties of the memory unit composed of SETs are far more advantageous than that of CMOS. But the disadvantage is the practical difficulty in fabrication. When the time comes for the large scale integration of SETs to form logic gates, the full advantages of single electron memory will show. This is the threshold of quantum computing.
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Seminar 2010
SET
HIGH SENSITIVITY ELECTROMETER The most advanced practical application currently for SETs is probably the extremely precise solid-state electrometers (a device used to measure charge). The SET electrometer is operated by capacitively coupling the external charge source to be measured to the gate. Changes in the SET source-drain current are then measured. Since the amplification coefficient is very big, this device can be used to measure very small change of current. Experiments showed that if there is a charge change of e/2 on the gate, the current through the Coulomb island is about 109 e/sec. This sensitivity is many orders of magnitude better than common electrometers made by MOSFIT. SETs have already been used in metrological applications as well as a tool for imaging localized individual changes in semiconductors. Recent demonstration of single photon detection and RF operation of SETs make them exciting for new applications ranging from astronomy to quantum computer read-out circuitry. The SET electrometer is in principle not limited to the detection of charge sites on a surface, but can also be applicable to a wide range of sensitive chemical signal transduction events as well. For example, the gate can be made coupling with some molecules, thus can measure other chemical properties during the process. If the device under test has a large capacitance, it is not advantageous to use SETs as an electrometer. Since for a typical SET, F CSET m 1 <, the suppression factor becomes unacceptable when the macroscopic device has a capacitance in the pF or nF range. Therefore, SET amplifiers are not currently used for measuring real macroscopic devices. Other low-capacitance electrometers such as a recently proposed quantum point contact electrometer also suffer from a similar capacitance mismatch problem. But it is believed that if the capacitance mismatch can be solved efficiently, SETs may find many new ultralow-noise analog applications.
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Seminar 2010
SET
MICROWAVE DETECTION If a SET is attacked black body radiation, the photon-aided tunneling will affect the charge transfer of the system. Experiments show that the electric character of the system will be changed even by a tiny amount of radiation. The sensitivity of this equipment is about 100 times higher than the current best thermal radiation detector.
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SET
ADVANTAGES AND DISADVANTAGES ADVATAGES Small size. Low energy consumption. And high sensitivity
DISADVANTAGES Integration of SETs in a large scale
As has been mention above, to use SETs at room temperature, large quantities of monodispersed Nan particles less than 10nm in diameter must be synthesized. it is very hard to fabricate large quantities of SETs by traditional optical lithography and semiconducting process. Linking SETs with the outside environment Practical difficulty in fabrication
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SET
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.
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Seminar 2010
SET
REFERENCE M. N. Leuenberger and E. R. Mucciolo, Phys. Rev. Lett. 97, 126601 (2009). C. Romeike, M. R. Wegewijs, and H. Schoeller, Phys. Rev. Lett. 96, 196805 (2008). Seminar project.com www.wikipedia.com www.google.com
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