Nanodevices. Principle and Applications

Table of Contents

CHAPTER 1: NANODEVICES-AN INTRODUCTION
1.1 Nanodevices
1.1.1 Classification of nanodevices
1.1.2 Nano ordered Material systems

CHAPTER 2: SEMICONDUCTOR NANODEVICES
2.1 Introduction to Semiconductors
2.2 P-N junction diode
2.3 Bipolar junction Transistors (BJTs): NPN and PNP transistors
2.4 Field effect transistor (FETs)
2.4.1 Junction FET
2.4.2 Metal Oxide Semiconductor FET (MOSFET)
2.5 Nanoscale MOSFET
2.5.1 Carbon nanotube FET (CNFET)
2.6 Metal-Semiconductor junctions: Ohmic and Schottky
2.6.1 Ohmic contact
2.6.2 Schottky contact
2.6.3 Schottky devices
2.7 Single Electron devices (SEDs)
2.7.1 Single Electron Transistor (SET)
2.7.2 Single Electron Dynamics
2.8 Resonant Tunnelling Transistor (RTT)
2.9 Micro and Nano electromechanical systems
2.9.1 Microelectromechanical systems (MEMS)
2.9.2 Nanoelectromechanical systems (NEMS)

CHAPTER 3: QUANTUM STRUCTURES AND DEVICES
3.1 Quantum Confined Structures
3.2 Quantum dots
3.3 Quantum wire
3.4 Quantum Well (QWs)

CHAPTER 4: NANOMACHINES AND MOLECULAR NANODEVICES
4.1 Introduction
4.2 Application of Nanomachines and Molecular nanodevices

CHAPTER 5: NANOSENSORS
5.1 Introduction
5.2 Types of Nanosensors
5.2.1 Catalytic type gas sensor
5.2.2 MOS type gas sensing device or sensor
5.2.3 Electrochemical gas sensing device or sensor
5.2.4 Protein based biosensors
5.3 Applications by function or operation

References

Preface

Nanotechnology is an interdisciplinary area. People who become interested in nanoscience and nanotechnology generally carry relatively diverse backgrounds towards the learning of the subject. They can be physicist, metallurgists, engineers, scientists, chemists, biologist, geologists, doctors, and technologists, etc.

In last two decades, nanotechnology has developed into the most significant and exciting field in various disciplines of science and technology. It has created immense promise to develop the advanced technology for various applications. The general outline and interpretation on nanotechnology and its applications should be provided to get in touch with large addressees. The present book hopes to achieve these aims.

The materials that have the particle size in between 1-100 nm known as nanomaterials. The properties of nanomaterials are quiet different compared to their bulk counter parts. The reason for this changeable behaviour is that the size of the particles are smaller than the characteristic length, which show the different behaviour based on the size of the particles, involving new physics and chemistry. This can allow to engineer the properties of nanomaterials and can be made nanodevices for various applications such as lighter materials, stronger materials, pharmaceuticals, magnetic storage, and faster switches for computers, etc.

A researcher in any area needs to reach further than their knowledge for evaluation the wider proposition of the subject such as nanotechnology. Technical managers, evaluators and funding providers have to understand the subject. The present book prepared to provide basic understanding, operational principles and applications of nanodevices.

This book can be useful for an academic course on nanoscience and nanotechnology. This book is very useful for the beginner in nanotechnology and nanoelectronics. The book is divided into seven chapters:

Chapter 1 contain the introduction of nanodevices, definition and classification of nanostructures materials and nanodevices.
Chapter 2 contains the detailed summary of the semiconductors and various semiconductor nanodevices. This will be helpful to study the changes occur at the nanoscale in bulk materials or bulk devices when their approach the nanoscale.
Chapter 3 contain the introduction, principles, and applications of various quantum confined structures and devices.
Chapter 4 gives the idea about the molecular junction, single molecular devices and their applications in other devices as an incorporated structures or hybrid applications. It contains the overview of natural and artificial nanodevices. It has given the knowledge of molecular nanoelectronics.
Chapter 5 contain the overview and advanced knowledge of natural and artificial nanosensors. It explains the various nanosensors and their applications.

The references are incorporated for further reading in the particular subjects. They include books, articles, reviews and patents. A list of books, papers, online resources are given in references section.

The real and promising applications of nanomaterials, fundamental principles and characteristics of the nanodevices have combined to give the interest in specific function. In modern days, the applications of nanomaterials and nanodevices are increased in many areas such as electronics, sports, space, medicine, smart displays, mobiles, computers, security, automobile, energy, etc.

My best thanks to all who have aided in my knowledge on nanotechnology and nanodevices.

J H Markna

India.

June, 2018

Acknowledgments

We are very thankful to the nanomission of Indian government that has motivated us to write a book on the special subject of nanotechnology. We also want to thank the editor of publishing group for keeping together the many detailed aspects of production.

We wish to thank the individuals who have discussed the subject with any of the authors. We also wish to thank the individuals who have shared their suggestions and comments of this text throughout the book.

CHAPTER 1: NANODEVICES-AN INTRODUCTION

1.1 Nanodevices

The devices that are designed with the size of 100 nm or less are known as nanodevices.

Semiconductor nanodevices: The structures fabricated from semiconducting materials with at least one dimension from 1 to 100 nm, called semiconductor nanostructures and devices designed with nanoscale size are called semiconductor nanodevices.

The scaling in devices has become challenging as the physical length of gate geometry of CMOS is reached up to nanoscale. The devices are influenced by the quantum effects at nanoscale. The nanoscale device offers a great miniaturization with high performance and density.

1.1.1 Classification of nanodevices

The operation of nanodevices depends on physical phenomena as like other electronic devices. The nanodevices can be classified into main three types:

Electrical nanodevices: These nanodevices are based on the ballistic transport, electrostatic and tunneling event. Electrons travel with no resistivity in a material in ballistic transport based nanodevices.

Magnetic nanodevices: For these devices such as magnetostatic nanodevice and spin transport nanodevice, two working phenomenon exist. The magnetic diple interactions are influenced to transmit the data and spin polarized electron transport is embarrassed by the use of the magnetic field, in magnetostatic and spin transport phenomena, respectively.

Mechanical nanodevices: For these types of nanodevices, the phenomenon is a restructuring of conductive polymers. The structure of polymer changes or moves while activated with input sources.

I. Electrical nanodevices

They are categorised into main three types, i.e. based on ballistic transport, tunnelling, and electrostatic.

Nanodevices based on ballistic transport

There are two types of electrical nanodevices that are operated on the basis of ballistic transport phenomenaon, for example, nanowire FET and carbon nanotube FET. Both have an almost same configuration as CMOS devices. The main and only difference between NW and CNT materials is the channel. CNT and nanowire have the practical characteristics like shape, electromigration and thermal. CNTFETs and NWFETs have applications in the programmable logic gate and non-volatile memory cell, respectively.

Nanodevices based on Tunneling

The nanodevices based on tunnelling are single electron tunnelling junction (SETJ) and resonant tunnelling diode (RTD). These nanodevices can be on and off. These nanodevices have advantages of being made very small and fast scalable. SETJs are used in encoded logic and RTDs have applications in memories.

Electrostatic nanodevices

Nanodevices such as EQCA (electrical quantum dot cellular automation) are example of electrostatic based nanodevices. These nanodevices need low temperature (cryogenic) and proper synchronization to work.

2. Magnetic nanodevices

These nanodevices utilize the magnetization to operate the devices. Spintronics (spin+electronics) has been developed after the giant magnetoresistance (GMR) discovery. The magneto-physical phenomenon has given the nanodevices based on spin and magnetostatic phenomena.

Magnetostatic nanodevices

Magnetostatic nanodevices have the example like MQCS or magnetic quantum dot cellular automaton. Their working principle is based on magnetostatic as a replacement for electrostatic relations. These nanodevices can be used to construct the basic logic function as well as used in military and space applications.

Spin nanodevices

These types of nanodevices use the spin state of electrons and hence they are known as spin based nanodevices, for example, spin field effect transistors. SpinFETs have the same structure as CMOS except the application of the ferromagnetic material. These nanodevices have the advantages as good as CMOS devices.

3. Mechanical nanodevices

These nanodevices restructure or reorganize the structure of material to execute an function. The mechanical nanodevices use mechanical force to move or to rotate its component. Molecular switches are the restructuring based nanodevices.

1.1.2 Nano ordered Material systems

Today, nanotechnology makes the manipulation possible at the molecular and atomic scale. The semiconductors with particle size of nanoscale behaves differently compare to their bulk counterparts. Nanomaterials have wide applications in many areas. Semiconductors can have a well-organized 0D, 1D, 2D and 3D structures at nanoscale.

A classification of nanomaterials is shown in figure 1.1.

3D, bulk materials
2D, thin film or well or sheet a few atoms thick,
1D, nanowire or nanotube
0D, quantum dot.

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Figure 1.1. Classification of nanomaterials (a) 3D-three dimensional, (b) 2D-two dimensional, (c) 1D-one dimensional, and (d) 0D-zero dimenisonal.

CHAPTER 2: SEMICONDUCTOR NANODEVICES

2.1 Introduction to Semiconductors

The word semiconductor gives an indication for its behaviour as the prefix semi is usually used to a midway range. A semiconductor has a value of conductivity between the insulator and conductors. The conductor is a material that supports a flow of charge when a voltage is applied between its connections. The insulator materials have extremely small value of conductivity.

Resistivity (ρ) is the inverse of the conductivity for a material and used to compare the resistance of various materials. It can be written as R = ρl/A, where R is resistance (ohm, Ω), l is length (cm), A is area (cm[2]) and ρ is resistivity (Ω • cm).

The conductivity (σ) is the basic electric property of a solid material. It is a capacity to flow the current. It is determined by the features of the interatomic bonds and decides the choice of electron to travel inside a solid material. Metals are very good conductors.

Metal: low resistivity (10-[2] – 10-[8] Ω m) or high conductivity.

Insulator: high resistivity (10[8] Ω m) or low conductivity.

Semiconductor: intermediate resistivity (10[5] Ω m) or conductivity between metals and insulators.

Semiconductors can be categorised into various types based on the basic physical properties of materials, chemical composition, and atom arrangements in 3D. Semiconductors can have a well-ordered 0D, 1D, 2D and 3D structures at nanoscale.

The most extensively used semiconductor is silicon (Si) for progressing microprocessor, memory integrated circuit technology, photovoltaic cell and MEMS/NEMS (micro/nano electromechanical systems) applications. The compound semiconductor materials such as GaAs, GaN, etc. are widely used for electronic and photonic applications.

Classification of semiconductors: chemical composition

Semiconductors are divided into main two catagories: (i) elemental, e.g. silicon and germanium and (ii) compound that have further classification as organic, inorganic and organic polymer semiconductors, e.g. anthracene, GaAs, and polyaniline, respectively.

Only group IV elements display semiconductor properties, for example, silicon (Si), germanium (Ge), and carbon (C), etc. Compound semiconductors can be synthesized with the use of groups II to VI elements. Organic compounds also display semiconductor properties.

Classification of semiconductors: nature of charge carriers

(i) Intrinsic semiconductors: Intrinsic semiconductors are the materials, which are developed to decrease the contaminations or impurities upto most pure form of material with modern technology. Here, the electrons (mobile –ve particles) and holes (+ve) both participate in the process of electrical conduction.
(ii) Extrinsic semiconductors or impure semiconductors: Extrinsic semiconductor materials are determined via the doping route by an external or extrinsic material. They can be prepared by doping or adding the pure semiconductor materials with a small amount of particular impurities by atoms with dissimilar valency from the parent material atoms. Here, the foreign atoms participate in the process of electrical conduction, means the extrinsic in nature.

Electrical conduction in semiconductor

Two ways of describing the phenomena of electrical conduction in semiconductors: (i) valence-bond model, (ii) Energy-band model.

In semiconductors, the bonding is such that under thermal excitation, the bonded electrons let themselves loose and can freely wander to give moderate values of conductivity. At low temperatures, the conductivity of most of the semiconductor is low and comparable to insulators.

Valence-Bond description of intrinsic semiconductors

The intrinsic semiconductor s refer to the pure materials in which the conductivity is because of its intrinsic property and it is not because of any impurities or foreign atoms, for example, Si and Ge.

For creation of a hole and making the bonded electron free, some ionisation energy Eg would be involved. The number of free electrons (ne) produced as result of such an ionisation has been found theoretically as equation 2.1,

ne = CT[3]/[2] exp (-Eg/2kT), (2.1)

where T = absolute temperature, k = Boltzaman constant, and C = constant.

For intrinsic semiconductors,

Number of free electrons (ne) = number of holes (nh) = ni

Total current (I) = electron current (Ie) + hole current (Ih)

Intrinsic semiconductors have several limitations when it comes to use for semiconductor based devices. The intrinsic charge carriers, electrons and holes, are in extremely small in numbers, about 10[16] m-[3] and generated as thermally. Therefore, their number can not be controlled and hence ne = nh. So, the conduction never becomes main only by electron or holes.

Valence-Bond description of extrinsic semiconductors

To overcome the limits of an intrinsic semiconductor, a small amount, nearly 1 ppm of an appropriate impurity is put in pure semiconductor material, are called impurity or extrinsic semiconductor materials. This addition of the impurities is known as doping and the atoms of impurities are known as dopants. Sometimes, such material is called as a doped semiconductor.

The main two kinds of dopants are used for doping the tetravalent (Ge or Si) element: (i) pentavalent (As, Sb, P, etc.), also known as donors, semiconductors as n-type and the electrons are majority charge carriers, ne>>nh. (ii) Trivalent (In, B, Al, etc., group-III elements), also known as acceptors, semiconductors as p-type and the majority chage carriers are holes, nh>>ne.

Energy band gap description in solids

This type of descriptive analysis is more closely associated with the energy and momentum concepts rather than the space localisation and velocity concepts. Valence band and conduction band is a lower band (it is fully filled) and upper band, respectively.

Energy band gap (Eg) is the energy gap between the top and bottom of the valence band and conduction band, respectively. Eg can exist as zero, smaller and larger, depending on the type of materials.

Metals have Eg ≈ 0, Insulator has Eg > 3 eV and Semiconductor: Eg < 3 eV.

An extra energy state, separately from Ec and Ev, lies in extrinsic semiconductors because of donor and acceptor impurities, ED and EA, respectively. Electrons require very small energy, about 0.1 eV to be free from the donor impurity in n-type semiconductor materials. A donor energy level (ED) exists near to the bottom of conduction band (Ec). Correspondingly, an acceptor energy level EA lies near to the top of valence band (Ev).

There are two types of semiconductors based on the shape of bandgap as a function of momentum (k): (i) direct bandgap semiconductors, electron and holes have same k value. (ii) Indirect bandgap semiconductors, minimum and maximum of conduction band and valence band, respectively, levels take place at dissimilar k value.

Semiconductor nanodevices

The structures fabricated from semiconducting materials with at least one dimension from 1 to 100 nm, called semiconductor nanostructures and devices designed with nanoscale size are called semiconductor nanodevices.

Semiconductor crystals are cut into thin slices called wafers. Semiconductor devices are made on wafers. Semiconductor nanodevices are capable to perform specific photonic, electronic and optical functions at nanoscale. The materials and methods are choiced based on the particular requirements, product and device such as p-n diode, BJT, MOSFET, ICs, LED, and solar cell.

Semiconductor devices are classified in main two catagories: planar and non-planar devices. (i) The word ‘planar’ is used as reference for 2D semiconductor devices. The depth or thickness of device is negligible compared to the width and length of device. The palanar and non-planar devices are fabricated with same methods. (ii) The non-planar word is used for 3D semiconductor devices. The practice of vertical surfaces and extremely etched characteristics are emphasized in 3D device. MEMS and MOSFET are the example of non-planar devices.

2.2 P-N junction diode

A p-n junction diode has a junction between two materials, responsible for the useful functions of a semiconductor device. The diode is a first electronic device. The diode is a simplest and the basic semiconductor devices, which plays an important role in electronic systems. They have the characteristics almost similar to a simple switch.

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Figure 2.1. Schematic of (a) p-n junction diode symbol, and (b) its configuration.

Figure 2.1 shows the symbol and configuration of p-n junction device, which is a basic unit of all semiconductor devices. A single p-n junction acts as a rectifying diode, two (p-n or n-p) as PNP or NPN transistor. Methods of making a p-n junction are (i) alloy junction, (ii) diffusion junction, (iii) vapor deposited junction, and (iv) rate grown junction.

If there is no applied bias voltage or it is zero, the current flow will be zero in one direction for the discussed p-n junction diode. In the presence of an applied voltage (bias) or p-n junction with externally applied voltage can be described as below. Figure 2.2a and 2.2b show the schematic represention of forward and reverse bias configuration, respectively.

Forward bias (VD > 0 V): Voltage is applied such that n-side has negative and p-side has a positive connection with applied bias. Here, the p-type and n-type of p-n junction diode are connected with positive and negative terminal of external supply (V), respectively. V and Vb (junction barrier potential) are opposite to each other. The flow of current is owing to the majority charge carriers and large, measured in mA.

Reverse bias (VD < 0 V): Here, the n-side and p-side of diode is connected with negative and positive of applied voltage, respectively. In a reverse bias condition, the current exists, which is called reverse saturation current (Is). V and Vb are in the same direction. The current flow is owing to only minority charge carriers, and it is small, measured in µA.

Current vs. Voltage or I-V characteristics

The Current vs. Voltage or I-V behaviour of a p-n junction diode is shown in figure 2.3. I-V characteristics of a semiconductor diode for both the bias regions can be defined from the equation 2.2:

ID = Is [e (kvD/Tk) – 1] (2.2)

Where Is is reverse saturation current, k is 11600/η; η = 1 (Ge), 2 (Si). Tk = Tc + 273°.

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Figure 2.2. (a) Forward and (b) Reverse bias configuration of the p-n junction diode

Applications of the p-n junction diode

A p-n diode is used as a rectifier in electrical circuits.

It can be used in fabrication of breakdown diodes such as Zener diode, avalanche diode, etc. The diode characteristics can be customized by adjusting doping levels, gradients and the geometry of a semiconductor crystal with specific capabilities. When the applied voltage in reverse bias goes above the limit of p-n junction diode, the breakdown will happen to resist the over limit. The avalanche breakdown and Zener break down occurs, when p-n junction is wide and narrow, respectively. The voltage across the p-n junction diode residue constant even in the condition of major changes in the current flow. The diodes can be fabricated with specific value of break down voltage, ranging from few volts to hundred volts, for the electronic circuits applications such as voltage regulators.

Photonic p-n junction devices. Photodetectors such as photoconductive cell and photodiode are widely used for the detection of an optical signal or light. The photonic devices such as photovoltaic devices are used for the conversion of the optical radiation to an electrical signal and electrical signal to light signal, for example, solar cell and light emitting diode, respectively.

Transistors are solid state semiconductor electronic device that do the functions only in one direction. They have the applications for switching, amplification, signal modulation, and voltage stabilization, etc. The transistor acts as a variable valve, which depends on the i/p current in BJTs and i/p voltage in FETs, allows flowing a current through the circuit.

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Figure 2.3. I-V characteristics of silicon semiconductor diode.

2.3 Bipolar junction Transistors (BJTs): NPN and PNP transistors

The transistor is a semiconductor device, which consists three layers of semiconductor materials as npn or pnp structure. The transistor has been classified into two types: (i) PNP transistor, where a thin n-type layer is kept in between the two p-type materials. The base, emitter and collector defined as B, E and C, respectively. (ii) NPN transistor, where a thin p-type layer is kept between two n-type materials and thin layer is expressed as a base.

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Figure 2.4. Circuit and symbols for the CB configuration: (a, c) PNP transistor, and (b, d) NPN transistor.

The symbol and circuit configuration for both types of transistors are shown in figure 2.4. The emitter, collector and base are expressed as E, C and B, respectively. Emitter supplies majority charge carriers to flow in a transistor. An emitter is a heavily doped region and has moderate size. A base is a lightly doped and very thin in size. Collector accumulates the majority charge carriers.

In NPN or PNP transistors, there are two junctions, one is forward biased where as other is reverse biased. The current passes more from the forward bias and less from the reverse bias due to low resistance and the high resistance, respectively. Thus, the ‘transistor’ word is derived from words resistor and transfer, as this device operates to transport the current with its inner resistance commencing one pathway to other.

The E-B (emitter-base) junction of a transistor is prepared as forward bias and C-B (collector-base) junction is prepared as reverse bias connection. The depletion region or layer will be wider and narrow for C-B and E-B junction, respectively. VEB voltage is small (0.5-1 V) and VCB voltage is large (5-15 V). The output current is controlled by input current in these transistors. So, it is called current operating device. The o/p Ic is controlled by i/p IB.

In PNP transistor, large numbers of majority charge carriers holes flow towards the base from emitter regions as the E-B is in forward bias condition. This represents the emitter current (IE) throughout the emitter region. The holes have a affinity with electrons in base region. A few holes (< 5%) can able to combine with electrons and provide a very small base current (IB) due to a very thin base layer and lightly doped. The holes observe the negative potential at the collector region and represent the collector current (Ic). An IE is an addition of Ic and IB; IE = IC + IB (IC >>IB) according to above argument and Kirchhoff law.

A similar mechanism can be prepared for the NPN transistor. Here, the majority charge carriers are electrons, which are supplied by the emitter region of n-type that pass through the p-type base region and capable to arrive at the collector region to provide Ic.

In any electronic circuit or device, input and output both have two terminals. The i/p or o/p connections in a circuit have chosen with the purpose of one of connection among E, B and C is common. Therefore, the transistors are connected as C-E, C-B or C-C configuration.

IB vs. VBE behaviour is recognized as the input characteristics and Ic vs. VCE is recognized as the output characteristics. To select Q point means at i/p and o/p side, arranging the dc voltage and current thus it decides a middle point in the active region. This arrangement of battery and resistance is called Biasing.

BJT is used in many sectors. It is known as biplar because it involves conduction by both holes and electrons. In unipolar devices, only one charge carrier (hole or electron) is engaged, for example, Schottky diode. The transistor is used as an amplifier, as an oscillator and also used ICs.

2.4 Field effect transistor (FETs)

The FETs have three terminals in its device structure. It is used in various applications. FET is also called UJT (Unijunction transistor) because, in FET, the current flows only due to majority charge carriers. FETs and BJTs are voltage and current controlled semiconductor device, respectively, as shown in figure 2.5. The current Ic is a function of IB and ID is a function of Vgs (gate to source voltage) in BJT and FET, respectively. The o/p current is controlled by the i/p current in BJT and by an applied voltage in FET.

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Figure 2.5. (a) Current controlled and (b) Voltage controlled devices.

FET is classified as p channel FET and n channel FET. In BJT, a prefix bi is representing the conduction by hole and electron as the charge carriers and hence it is called bipolar device. In FET, conduction depends either only electrons or holes, hence it is called unipolar device.

An electric field, established by the present charge carriers in FET, controls the conduction pathway of o/p circuit and there is no direct contact among the controlled and controlling quantity. The most important characteristics of the field effect transistor are high input impedance of several hundred megohms level. This behaviour is used to design and construct the linear AC amplifiers.

BJTs are more sensitive to change signal than FETs. FETs have higher stability of temperature compared to BJT. The device size of FET is smaller than BJT and hence they are used in IC (integrated circuit) fabrication.

FET is classified into two categories: Junction FET (JFET) and Insulated Gate FET (IGFET) or (metal oxide semiconductor FET (MOSFET) . MOSFET divides into two types: depletion and enhancement. MOSFETs have become the most important devices for the applications of integrated circuits design and construction.

2.4.1 Junction FET

Junction FET or JFET has three terminals in its device structure. JFETs use a simple p-n junction diode with a shape modification. In JFET, the n type region is enlarged and two terminals are prepared with it. These two terminals are recognized as drain (D) and source (S) in JFET configuration while the n type region is recognized as channel. One terminal connected with p type section is recognized as gate (G) . The symbol and configuration of both the types of JFETs are revealed in figure 2.6 and 2.7.

The majority charge carriers enter the bar through the source (S). The majority carriers depart the bar through the drain (D) terminal. A heavily doped p type area (region) is formed on both sides of the n type piece. These regions are called gates (G). Usually, the two gates are joined together a form a single gate. The section squeeze in between the drain and the source is known as channel. The electron in nFET and holes in pFET shift towards the drain (from S to D) throughout this path (channel). The width of this channel changes by varying the gate voltage.

VGS = 0: Here there is no voltage (gate to source voltage) applied to FET. In this condition current does not flow and depletion region residue of a reasonable size.

VGS > 0: Now a positive voltage is applied to make JFET as functional device with drain as positive configuration. Therefore, the electrons travel from S to D according to the applied voltage through the channel. The magnitude of flowing current in channel is determined by the channel resistance according to the law of Ohm. The size of the channel is controlled by the depletion region. Thus, its resistance is superior in the middle part as compared to the other part in channel.

In forward bias configuration between the gate to the channel, the depletion region decreses and does not control the flow of current in channel. Generally, this condition is not favoured for practical applications. In reverse bias configuration between the gate and the channel, the depletion region spreads into the channel due to the expantion in depletion region by an applied voltage. Therefore, the width of functioning path in channel is decreased and the resistance in the channel becomes bigger appreciably. Vgs has an intensive consequence on the flowing current in the channel. The resistance of the channel in FET is controlled via produced electric field by Vgs, hence, the current flows. Therefore, it is called FET (field effect transistor). The channel can be fabricated from p-type or n-type materials and according to this they are called as n-channel or p-channel JFET, respectively. The semiconductor regions and polarity of applied field are reversed in p-JFET. The holes are the majority charge carriers in device.

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Figure 2.6. Symbol of (a) n-channel and, (b) p-channel JFET.

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Figure 2.7. Schematic or configuration of JFET.

JFETs have the positive as well as negative aspects during applications. The operation is depends on Vgs and gate current does not flow during a little leakage in current. This is a disadvantage in practical applications. The radiation effect on the reverse biased junction in FET is very small and needs major amount of radiation to damage the device. Therefore, JFETs are used even in high radiation atmosphere.

2.4.2 Metal Oxide Semiconductor FET (MOSFET)

MOSFETs are classified into two categories: depletion and enhancement MOSFET. These terms of MOSFET classification describe their operation mode. Each MOSFET has the different characteristics and operation.

Depletion MOSFET or IGFET

In metal oxide semiconductor FET, metal is used to make contact with appropriate surface of the three terminals of the device. The gate is made to control the accessible area for channel, the oxide for insulating layer of SiO2, and the semiconductor as base for the device, lying with diffused regions of p-type and n-type material on top. MOSFET uses an insulating layer and hence it is also defined as insulated gate FET. The symbol, structural configuration and I-V behaviour of depletion type MOSFET are given away in figure 2.8, 2.9 and 2.10, respectively.

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Figure 2.8. Symbol of (a) n-channel and (b) p-channel depletion MOSFET.

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Figure 2.9. The p-channel depletion type MOSFET.

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Figure 2.10. I-V characteristics of n-depletion MOSFET.

The stronger electric field makes a bigger depletion region and places the better control on the channel. The main element of the channel is n-type or p-type silicon. Silicon dioxide is a simply glass, which is a good insulator. A thin layer of SiO2 (silicon dioxide) can be formed on oneside of the channel surface in device and after that put a metallic gate electrode. This structure is known as insulated gate FET (IGFET), which is shown in Figure 2.9.

If VGS is zero then the channel behaves as a semiconductor resistance and current will flow according to VDS (applied voltage between drain and source terminal). So, there will be no depletion region in the absence of p-n junction. If VGS is negative, means a negative voltage is supplied to the gate, a tiny negative charge will appear on the gate terminal and this charge will repel e-s away from the gate terminal. In the case of n-type silicon channel, the majority charge carriers are free es-. The depletion area nearby the gate can be created with an applied gate voltage by repelling majority carriers away from the gate region. In such a way, the functional channel width is being restricted such as p-n junction. Here, FET is being operated by making a depletion (decreasing) area inside an existed channel and hence, it is recognized as depletion MOSFET.

Enhancement MOSFET

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Figure 2.11. Symbol of (a) n-channel, and (b) p-channel enhancement MOSFET.

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Figure 2.12. Schematic or configuration of p-channel enhancement MOSFET.

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Figure 2.13. I-V transfer character of p-channel enhancement MOSFET.

The symbol and device configuration of enhancement MOSFET are shown in figure 2.11 and 2.12, respectively. The structure of enhancement MOSFET can be made by placing two n-type materials alongside within a p-type material and then puting a gate between the n-type regions in an existed IC.

If there is no applied bias to an NPN transistor, both the n-type regions becomes isolated (or inaccessible) from each other as well as they are separated electrically. If a positive VDS and positive VGS are applied then there will be an attraction of free electrons towards the gate. The electric field becomes wider for larger positive VGS and more free electrons will be attracted. If the source (S) junction is configured in forward biase, positive VG can attract electrons to the gate terminal across this junction. As a result, these attracted electrons enhace the channel within the p-type area. This channel is made up of free electrons, which fulfil the gap between S and D. A channel can carry out (conduct) the electrical current from S to D through the device. This device is operated with a enhanced channel and therefore it is known as enhancement type MOSFET. Similarly, p-channel version can be built. Complementary MOS or CMOS IC consists this type of devices for construction and use in a pair in such a way that one device is OFF and other is ON. I-V characteristics for this type MOSFET is shown in figure 2.13.

MOSFETs applications

FETs have advantageous applications due to its low power consumption, small size, high input impedance, and high power gain.

MOSFETs are used for digital and analog functions. MOSFETs are the key element for the circuit family of complementary MOS or CMOS.

MOSFETs are used for memory function such as dynamic RAM and flash erasable programmable ROM. They are also used in imaging devices, CCD cameras, and LCDs.

MOSFET is the most efficient device to to construct the processors due to its fast switching speed, i.e. ON state to OFF state and vice versa.

2.5 Nanoscale MOSFET

Most general way to construct the nanoscale electronic circut is just shrinking the dimensions of every component of the circuits by a constant factor, known as scaling. Scaling down the MOSFET structure has worked well. When MOSFETs are constructed below 100 nm, certain factors limit the MOSFETs scalability. These factors are tunnelling, shrinkage of depeletion region, heat dissipation, and high electric field. 0.1 microns or 100 nm is known as 0.1 μ barrier. MOSFETs will have to be replaced by some other nanoscale devices after this barrier.

Generally, MOSFETs is classified into n-MOSFET or NMOS and p-MOSFET or PMOS transistor. Figure 2.14a shows the schematic representation of NMOS transistor. The working principle of nanoscale MOSFET is same as traditional MOSFET. Figure 2.14b and Figure 2.14c shows the device representation in the absence and presence of VGS, respectively. MOSFET is controls the current between D and S by changing VGS. If VGS = 0, no current or negligle current will flow across the channel and device will turned OFF. If VGS > 0, a conducting path in channel is formed by the electrons isolated from the holes. The current will flow across the channel and device will turned ON. This makes possible to use MOSFET as a switching device.

Abbildung in dieser Leseprobe nicht enthalten

Figure 2.14. (a) Schematic of n-MOSFET or NMOS, (b) if VGS = 0, and (c) if VGS > 0.

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