Green Energy E-Book
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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
Like most industries around the world, the energy industry has also made, and continues to make, a long march toward "green" energy. The science has come a long way since the 1970s, and renewable energy and other green technologies are becoming more and more common, replacing fossil fuels. It is, however, still a struggle, both in terms of energy sources keeping up with demand, and the development of useful technologies in this area. To maintain the supply for electrical energy, researchers, engineers and other professionals in industry are continuously exploring new eco-friendly energy technologies and power electronics, such as solar, wind, tidal, wave, bioenergy, and fuel cells. These technologies have changed the concepts of thermal, hydro and nuclear energy resources by the adaption of power electronics advancement and revolutionary development in lower manufacturing cost for semiconductors with long time reliability. The latest developments in renewable resources have proved their potential to boost the economy of any country. Green energy technology has not only proved the concept of clean energy but also reduces the dependencies on fossil fuel for electricity generation through smart power electronics integration. Also, endless resources have more potential to cope with the requirements of smart building and smart city concepts. A valuable reference for engineers, scientists, chemists, and students, this volume is applicable to many different fields, across many different industries, at all levels. It is a must-have for any library.
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Table of Contents
Cover
Title Page
Copyright
Preface
1 Fabrication and Manufacturing Process of Solar Cell: Part I
1.1 Introduction
1.2 Fabrication Technology of Diode
1.3 Energy Production by Equivalent Cell Circuitry
1.4 Conclusion
References
2 Fabrication and Manufacturing Process of Solar Cell: Part II
2.1 Introduction
2.2 Silicon Solar Cell Technologies
2.3 Homojunction Silicon Solar Cells
2.4 Solar Si-Heterojunction Cell
2.5 Si Thin-Film PV Cells
2.6 Perovskite Solar Cells
2.7 Future Possibility and Difficulties
2.8 Conclusions
References
3 Fabrication and Manufacturing Process of Perovskite Solar Cell
3.1 Introduction
3.2 Architectures of Perovskite Solar Cells
3.3 Working Principle of Perovskite Solar Cell
3.4 Components of Perovskite Solar Cell
3.5 Fabrication of Perovskite Films
3.6 Manufacturing Techniques of Perovskite Solar Cells
3.7 Encapsulation
3.8 Conclusions
References
4 Parameter Estimation of Solar Cells: A State-of-the-Art Review with Metaheuristic Approaches and Future Recommendations
4.1 Introduction
4.2 Related Works
4.3 Problem Formulation
4.4 Salient Simulations and Discussions for Future Work
4.5 Conclusions
References
5 Power Electronics and Solar Panel: Solar Panel Design and Implementation
5.1 Chapter Overview
5.2 Challenges in Solar Power
5.3 Solar PV Cell Design and Implementation
5.4 MPPT Scheme for PV Panels
5.5 Way for Utilization of PV Schemes
5.6 Future Trends
5.7 Conclusion
References
6 An Effective Li-Ion Battery State of Health Estimation Based on Event-Driven Processing
6.1 Introduction
6.2 Background and Literature Review
6.3 The Proposed Approach
6.4 Experimental Results and Discussion
6.5 Conclusion
Acknowledgement
References
7 Effective Power Quality Disturbances Identification Based on Event-Driven Processing and Machine Learning
7.1 Introduction
7.2 Background and Literature Review
7.3 Proposed Solution
7.4 Results
7.5 Discussion
7.6 Conclusion
Acknowledgement
References
8 Sr2SnO4 Ruddlesden Popper Oxide: Future Material for Renewable Energy Applications
8.1 Introduction
8.2 Experimental Work
8.3 Experimental Results
8.4 Conclusions
Acknowledgement
References
9 A Universal Approach to Solar Photovoltaic Panel Modeling
9.1 Introduction
9.2 PV Panel Modeling: A Brief Overview
9.3 Proposed Model
9.4 Current Model
9.5 Voltage Model
9.6 Simulation Results
9.7 Conclusion
Acknowledgement
References
10 Stepped DC Link Converters for Solar Power Applications
10.1 Introduction
10.2 Power Converters for Solar Power Applications
References
11 A Harris Hawks Optimization (HHO)–Based Parameter Assessment for Modified Two-Diode Model of Solar Cells
11.1 Introduction
11.2 Problem Formulation
11.3 Proposed Methodology of Work
11.4 Simulation Results
11.5 Conclusions
References
12 A Large-Gain Continuous Input-Current DC-DC Converter Applicable for Solar Energy Systems
12.1 Introduction
12.2 Proposed Configuration
12.3 Steady-State Analysis
12.4 Component Design
12.5 Real Gain Relation
12.6 Comparative Analysis
12.7 Simulation Outcomes
12.8 Conclusions
References
13 Stability Issues in Microgrids: A Review
13.1 Introduction
13.2 Stability Issues
13.3 Analysis Techniques
13.4 Microgrid Control System
13.5 Conclusion
References
14 Theoretical Analysis of Torque Ripple Reduction in the SPMSM Drives Using PWM Control-Based Variable Switching Frequency
14.1 Introduction
14.2 Prediction of Current and Torque Ripples
14.3 Variable Switching Frequency PWM (VSFPWM) Method for Torque Ripple Control
14.4 Conclusion
References
Appendix: Simulation Model Circuits
Main Model
Speed & Current Loop Controllers
VSFPWM for Torque Ripple Control
15 Energy-Efficient System for Smart Cities
15.1 Introduction
15.2 Factors Promoting Energy-Efficient System
References
16 Assessment of Economic and Environmental Impacts of Energy Conservation Strategies in a University Campus
16.1 Introduction
16.2 Materials and Methods
16.3 Electricity Consumption Pattern in Covenant University
16.4 Conclusion
References
17 A Solar Energy–Based Multi-Level Inverter Structure with Enhanced Output-Voltage Quality and Increased Levels per Components
17.1 Introduction
17.2 Proposed Basic Topology
17.3 Proposed Extended Structure
17.4 Efficiency and Losses Analysis in Suggested Structure
17.5 Comparison Results
17.6 Nearest Level Technique
17.7 Simulation Results
17.8 Conclusions
References
18 Operations of Doubly Fed Induction Generators Applied in Green Energy Systems
18.1 Introduction
18.2 Doubly Fed Induction Generators (DFIG) Systems Operated by Wind Turbines
18.3 Control Scheme of Direct Current Controller
18.4 Simulation Studies of Direct Current Control of DFIG System
18.5 Characteristics of DFIG at Transient and After Transient Situation
18.6 Pulsation of DFIG Parameters with DCC Control Technique
18.7 Effects of 5th and 7th Harmonics of IS and VGRID
18.8 Load Contribution of DFIG in Grid with DCC Control Technique
18.9 Speed Control Scheme of Generators
18.10 DFIG Control Scheme
18.11 General Description About PI Controller Design
18.12 GSC Controller
18.13 Characteristics of DFIG with Wind Speed Variations
18.14 Conclusion
References
19 A Developed Large Boosting Factor DC-DC Converter Feasible for Photovoltaic Applications
19.1 Introduction
19.2 Suggested Topology
19.3 Steady State Analyses
19.4 Design Consideration
19.5 Comparison
19.6 Simulation
19.7 Conclusion
References
20 Photovoltaic-Based Switched-Capacitor Multi-Level Inverters with Self-Voltage Balancing and Step-Up Capabilities
20.1 Introduction
20.2 Suggested First (13-Level) Basic Configuration
20.3 Suggested Second Basic Configuration
20.4 Modulation Method
20.5 Design Consideration of Capacitors
20.6 Efficiency and Losses Analysis
20.7 Simulation Results
20.8 Comparative Analysis
20.9 Conclusions
References
Index
Also of Interest
End User License Agreement
List of Tables
Chapter 3
Table 3.1 Photovoltaic parameters achieved for the constructed PSCs using differ...
Chapter 4
Table 4.1 Summary of metaheuristic algorithms applied to parameter estimation of...
Table 4.2 Datasheet of several PV modules as per [42].
Table 4.3 Range of parameters to be estimated.
Table 4.4 Comparative study of model parameters and error function of SDM using ...
Table 4.5 Comparison of model parameters and error function of DDM using differe...
Table 4.6 Comparison of model parameters and error function of TDM using differe...
Chapter 5
Table 5.1 International standards for PV systems.
Table 5.2 Power converter interconnected PV system [2].
Table 5.3 Solar power present research challenges; opportunities and emerging tr...
Table 5.4 A basic comparison of PV cell materials.
Table 5.5 Parameters of the single PV panel Synenergy SEPL 100.
Table 5.6 A PV generation system based classification and range [2].
Table 5.7 Comparison of the MPPT algorithms.
Chapter 6
Table 6.1 Compression Gain Over the Conventional Method.
Table 6.2 Processing Gain Over the Conventional Method.
Chapter 7
Table 7.1 Summary of the Compression Gains.
Table 7.2 Classification accuracies for the four–class PQ Signals recognition (k...
Table 7.3 Classification accuracies for the four–class PQ Signals recognition (N...
Chapter 8
Table 8.1 Tolerance factor of different crystal structure.
Table 8.2 Lattice parameters, density, and refinement parameters determined from...
Table 8.3 Lattice parameters, density, and refinement parameters determined from...
Chapter 11
Table 11.1 Details of some popular PV modules under standard test conditions [24...
Table 11.2 Parameter ranges.
Table 11.3 Comparison of parameters and fitness function for MDDM of Kyocera KC2...
Table 11.4 Comparative assessment of parameters and fitness function for MDDM of...
Table 11.5 Comparative evaluation of parameters and error function for MDDM of C...
Table 11.6 Statistical measures of the error function.
Table 11.7 p-values of Kruskal-Wallis test for different PV-models.
Table 11.8 Wilcoxon rank-sum test results for different PV models.
Table 11.9 Corrected p-values for Wilcoxon test adding Holm-Bonferroni correctio...
Chapter 12
Table 12.1 Status of switches and diodes in different operational modes.
Table 12.2 Comparison among some similar structure with proposed converter.
Table 12.3 Parameter values used in simulations.
Chapter 14
Table 14.1 Ripple current slope for the three-phases A, B, C with different volt...
Chapter 16
Table 16.1 Daylighting control for Cafeteria 1: dimming up to 30%.
Table 16.2 Economic Analysis of Lighting Fixtures Replacement with LED bulbs.
Table 16.3 Environmental Analysis of Lighting Fixtures Replacement with LED bulb...
Table 16.4 Economic and Environmental Analysis of Solar Panels Installation.
Chapter 17
Table 17.1 Switching Pattern of Suggested Symmetric Basic Structure (P1).
Table 17.2 Standing Voltage on Switches of Symmetric Basic Topology (P1).
Table 17.3 Switching Plot of Proposed 2nd Basic Structure (P2).
Table 17.4 Standing voltage on semiconductors of P2.
Table 17.5 Switching Scheme of Proposed Basic Structure with Trinary DC Sources ...
Table 17.6 Standing Voltage and NSV on switches of Suggested Basic Unit with Tri...
Table 17.7 Comparison results.
Table 17.8 Parameters of the simulation.
Chapter 18
Table 18.1 Pulsation of DFIG parameters with DCC technique.
Table 18.2 Harmonic distortion of iS and VGRID with DCC technique.
Table 18.3 Load contribution by DFIG with DCC.
Table 18.4 Load contribution by grid.
Chapter 19
Table 19.1 The voltage/current stresses of semiconductors.
Table 19.2 Comparison table in terms gain, number of components and modes of the...
Table 19.3 NSV and ANSV of the suggested converter and other converters.
Table 19.4 Values of elements in simulation.
Chapter 20
Table 20.1 Switching modes and charge/discharge states of capacitors in proposed...
Table 20.2 Voltage Stress (VS) and Normalized Voltage Stress (NVS) of semiconduc...
Table 20.3 Description of suggested first extended configuration.
Table 20.4 Switching modes and charge/discharge states of capacitors for second ...
Table 20.5 Voltage Stress (VS) and Normalized Voltage Stress (NVS) of semiconduc...
Table 20.6 Information of second suggested extended configuration.
Table 20.7 Longest Discharging Interval (LDI) of capacitors.
Table 20.8 Parameter values used in efficiency and loss analysis.
Table 20.9 Voltage Stress (VS) on semiconductors of suggested first basic config...
Table 20.10 Voltage Stress (VS) on semiconductors of suggested 17-level basic co...
Table 20.11 General comparison results.
Table 20.12 Comparison results of the suggested configurations with 13-Level and...
Guide
Cover
Table of Contents
Title Page
Copyright
Preface
Begin Reading
Index
Also of Interest
End User License Agreement
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Scrivener Publishing
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Publishers at Scrivener
Martin Scrivener ([email protected])
Phillip Carmical ([email protected])
Green Energy
Solar Energy, Photovoltaics, and Smart Cities
Edited by
Suman Lata Tripathi and
Sanjeevikumar Padmanaban
This edition first published 2021 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA
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Library of Congress Cataloging-in-Publication Data
ISBN 978-1-119-76076-4
Cover image: Background - Suriya Siritam | Dreamstime.com, Graphic - Alberto Masnovo | Dreamstime.com
Cover design by Kris Hackerott
Set in size of 11pt and Minion Pro by Manila Typesetting Company, Makati, Philippines
Printed in the USA
10 9 8 7 6 5 4 3 2 1
Preface
Green energy technology has not only given a concept of clean energy but also reduces dependency on fossil fuel for electricity generation through smart power electronics integration. Also, endless resources have more potential to cope with the requirements of smart building and smart city concept. The power electronics in a smart and intelligent approach can lead to highly efficient energy systems supporting the increasing demand for eco-friendly energy systems. The objective of this edition is to provide a broad view of the fundamental concepts and development process of green energy technology in a concise way for fast and easy understanding. This book provides information regarding almost all aspects to make it highly beneficial for all students, researchers and teachers of this field. Fundamental principles of green energy systems with AI and machine learning techniques are discussed herein in a clear and detailed manner with an explanatory diagram wherever necessary. The technological trends and latest developments based on green energy systems and applications are the major focus of the book. All the chapters are illustrated in simple language which will facilitate readability of the chapters.
Chapter Organization
This book is organized into 20 chapters.
Chapter 1 discusses the crystalline silicon solar cell (c-Si)–based technology that has been recognized as the environment-friendly viable solution to replace traditional energy sources for power generation. The chapter mainly elaborates the three basic c-Si solar cell configurations that are monofacial, bifacial and back-contacted solar cell configurations.
Chapter 2 is mainly focused on p-type mono-Si-PERC and n-type c-Si based TOPCon solar cell with enhanced efficiency. The use of HJ-TF cells, the Cu2O, InGaN, CuInS2, and InP fabrics are also explored for solar cell developments.
Chapter 3 summarizes the recent advancement made in the fabrication and manufacturing process for commercialization of PSC in the photovoltaic (PV) market and also various possible techniques used to improve their physical properties, and overcome hurdles and challenges while fabricating perovskite films.
Chapter 4 presents a short yet comprehensive survey on the metaheuristic approaches to the parameter assessment of solar photovoltaic systems modelled by different diode models.
Chapter 5 describes the review and challenge in solar PV cell design and implementation. Furthermore, it outlines the existing PV systems, the structure of different PV panels, MPPT, and the solid-state converter topologies.
Chapter 6 compares the developed method with its traditional counterpart, and the results of the experiment show that the new model performs better in terms of computational efficiency, compression gain, and SOH estimation accuracy.
Chapter 7 presents a detailed discussion on power quality (PQ) disturbances that cause rigorous issues in smart grids and industries. This mainly covers event-driven processing, analysis and machine learning for successful and efficient detection of PQ disturbances.
Chapter 8 is intended to provide information about the structural, optical, dielectric and conductivity with the help of compositional modification at Sr-site of Sr2SnO4 by homovalent (Ba2 +) and hetrovalent (La3 +).
Chapter 9 focuses on the detailed modelling of a solar photovoltaic (PV) panel. Here, a single-diode four-parameter model is also described that can be used for all panels in general.
Chapter 10 explores a stepped DC link converter for solar power that is focused for configurations of BCMLI, CDCLHBI and BCDCLHBI and synthesizing seven-level AC power output.
Chapter 11 derived a new modified double-diode model equation and on the basis of that formed a new objective function for MDDM. The HHO algorithm was used to estimate all the eight parameters of the system for different types of commercially available PV modules.
Chapter 12 describes a switched-inductor switched-capacitor-based large-gain DC-DC converter with low voltage stress on its switches/diodes.
Chapter 13 envisages presenting the stability concerns and issues associated with microgrids along with a state-of-the-art review of the techniques employed for improving stability of microgrids working in either islanded or grid-connected mode.
Chapter 14 deals with three-phase current ripples, and also the torque ripple of a surface-mounted permanent magnet synchronous motor (SPMSM) was completely analysed and minimized based on the variable switching frequency PWM (VSFPWM) method.
Chapter 15 discuss the implementation of Internet of Things (IoT) in the Smart Home, which plays a major role in making the things automatic and also saves a lot of energy.
Chapter 16 deals with the assessment of the economic and environmental impacts of energy conservation strategies in a university campus.
Chapter 17 suggests a novel double-source basic unit for cascaded MLIs that can produce seven output voltage steps with Trinary magnitude of DC supplies. The suggested structure can operate as symmetric or asymmetric MLI.
Chapter 18 presents the designing procedure of conventional and the vector control, Proportional Integral (PI) controller and analyzes the performance characteristics of DFIG connected to the Grid Systems.
Chapter 19 describes the combination of conventional SEPIC and quadratic boost converters with a boosting stage, which increases the boosting factor of converter. The continuous input-current as well as large boosting capability make the proposed topology applicable for Photovoltaic (PV) applications.
Chapter 20 proposes two novel basic configurations for switched-capacitor-based 13- and 17-level inverters, with high step-up capability and self-voltage balancing of capacitors.
1Fabrication and Manufacturing Process of Solar Cell: Part I
S. Dwivedi
S.S. Jain Subodh P.G. (Autonomous) College, Jaipur, India
Abstract
Crystalline silicon solar cell (c-Si) based technology has been recognized as the only environment-friendly viable solution to replace traditional energy sources for power generation. It is a cost-effective, renewable and long-term sustainable energy source. The Si-based technology has a market growth of almost 20-30% and is projected to attain an energy share of ~100 giga watt (GW) per year in the current fiscal year, 2020. There have been constant efforts in reducing manufacturing cost of solar panel technology, which is about three-four times higher in comparison to traditional carbon-based fuels. In the manufacturing domain, fabrication of three basic c-Si solar cell configurations can be utilized, which are differentiated in the manner of generation of electron-hole (E-H) pairs on exposure to sunlight. The generation of electricity by impinging light on a semiconductor material requires production of electrons and holes such that electrons in the valence band become free and jump to the conduction band by absorbing energy. Thus, jumping of highly energetic electrons to different material generates an electromotive force (EMF) converting light energy into electrical signals. This is known as the photovoltaic (PV) effect.
This chapter is an effort to outline fabrication processes and manufacturing methodologies for commercial production of large area PV modules as an alternative green source of energy.
Keywords: Solar cell, photovoltaics, p-n junction, photovoltaic panels, crystalline silicon solar cell, renewable energy, physics of solar cell, fabrication of solar cell
1.1 Introduction
There has always been a surge to discover newer sources of energy which can be effective alternatives for the orthodox sources of energy, such as, petrol, kerosene, wind energy, thermal power generators [1,2]. In this quest, the sun is a natural huge source of renewable green energy. It is noteworthy that the terrestrial soil is exposed to an enormous amount of solar energy as large as about ten thousand times of all the energy used around the globe. The terrestrial hemisphere facing the sun receives power in excess of 50,000 terawatt (TW) in each instance, which makes reception of an enormous amount of energy possible [3]. Photovoltaics (PV) technology is a technology that relies on this infinite source of sunlight and possesses inherent qualities of highly reduced service costs since the sun provides free energy, reliability, noiseless, minimum maintenance costs and readily installation features [4, 5].
As a matter of fact, thermonuclear fusion reactions happen non-stop at a temperature of millions of degrees to generate huge energy in the form of electromagnetic radiation of sunlight [5,6]. The outer layer of the earth’s atmosphere receives partial energy of the total energy from the sun with a solar constant or an average irradiance of approximately 1367 Wm-2 with a variation of ±3% [8]. This value of solar constant is dependent on the earth-to-sun distance and on the solar activity. The solar constant is defined as the intensity of solar electromagnetic radiation impinging on a unit surface area and is expressed in units of kWm-2 and is equal to the integral of the power of the individual frequencies in the spectrum of solar radiation. The geometry of the sun-to-earth distance is displayed in Figure 1.1 given below.
Solar irradiation is the integral of solar irradiance over a particular period of time depicted by kWhm-2 and the radiation falling on the surface of the earth is actually diffuse radiation [8]. Diffuse radiation is that part of light radiation striking the surface from whole of the sky, while other radiations are the part reflected from the ground, and by surrounding atmosphere. Different types of radiation received by a solar panel [9] are displayed in Figure 1.2 as shown below.
1.1.1 Introduction to Si-Based Fabrication Technology
Photovoltaics technology is a green method of energy production which is based on fabrication and manufacturing of solar cells on platform of Si wafers [9]. In this regard, it is mandatory to know about the Si wafers. So the silicon and its geometry as an integral component of the solar cell technology will be discussed first.
Figure 1.1The above schematic shows the sun-earth geometry portraying distance between the two celestial objects, diameters and the value of solar constant.
Figure 1.2 The above figure portrays different radiations occurring from the sun which consists of direct, diffuse and reflected radiations.
1.1.2 Introduction to Si Wafer
Silicon is a member of group 14 in the periodic table and is tetravalent metalloid, semiconductor and brittle crystalline solid [10-12]. In 1906, a silicon radio crystal detector was developed as the first silicon-based semiconductor device by Greenleaf Whittier Pickard [13]. Russell Ohl discovered the nonlinear semiconductor devices, p-n junction, and photovoltaic effect in the metalloid Si in 1940 [14]. In 1941, during the Second World War, radar microwave detectors were invented by developing techniques for production of high quality germanium (Ge) and Si crystals [15]. William Shockley proposed a field-effect amplifier based on Ge and Si in 1947, but could not demonstrate the prototype practically [16]. John Bardeen and Walter Brattain built the first working device, point-contact transistor, in 1947 under the direction of Shockley only [17]. The first Si-based junction transistor was fabricated by the physical chemist Morris Tanenbaum in 1954 at Bell Labs [18]. At Bell Labs in 1954, Carl Frosch and Lincoln Derick found out by accident that it is possible to grow silicon-di-oxide (SiO2) on Si wafers [19]. Later on, in 1958, they discovered that this as-grown SiO2 could be used to mask Si surfaces during diffusion processes [19].
Si atom has fourteen electrons with electronic configuration 2,8,4 [1s2, 2s2, 2p6, 3s2, 3p2] specifying that the number of valence electrons is 4 [10,11]. These valence electrons occupy the 3s orbital and two 3p orbitals. In order to complete its octet and attain the stable noble gas configuration of Argon (Ar), it can combine with other elements to form SiX4 derivatives by forming sp3 hybrid orbitals. In this case, the central Si atom taking part in the bonding with other element shares an electron pair with each of the four atoms of the bonding element.
Si and Ge crystallize in a diamond-type cubic lattice structure which has the space-lattice of face-centered cubic (fcc) [20, 21]. The atomic positions in the diamond-type cubic lattice projected on a cubic platform are shown in Figure 1.3. In a space-lattice of fcc-type, two identical atoms at 000 and form the primitive basis and are associated with each point of the fcc lattice. In the above picturesque, fractions are the heights over and above the base in units of a cube edge. In that case, points at lie on the fcc lattice, while those at and lie on the similar fcc lattice but are displaced along the line of body diagonal by a magnitude of ¼ of its length. It is a known fact that the unit cube of fcc lattice consists of 4 lattice points. As a result, diamond-type cubic lattice contains 2 × 4 = 8 atoms. The diamond-type fcc lattice in Si displays tetrahedral bonding characteristics [20, 21].
Figure 1.3Schematic to show atomic positions in diamond-type cubic lattice.
Si is tetravalent and can be made p-type by adding dopants of boron (B), aluminium (Al) & gallium (Ga), and addition of antimony (Sb), phosphorous (P) & arsenic (As) generates n-type semiconductor material [10,11,20,21]. B and Ga possess only three valence electrons and when they are mixed into the Si lattice, deficiency of an electron is created which is termed as positively charged “vacancy” or “hole”. Holes take part in conduction accepting an electron from the neighbour and transitioning over the atoms. For making Si an n-type semiconductor, Sb, P and As are added into the Si lattice in small quantities each having five valence electrons, which creates an extra electron into the lattice. The availability of these free electrons as a whole in the material creates a net flow of negatively charged carriers to constitute current. Thus, addition of small amounts of either of two types of foreign atoms changes Si crystal into a medium-type of conductor, which is a semiconductor. Joining of two types of semiconducting materials constitutes a device entailed as nonlinear semiconductor diode [10,11,20-22]. Figure 1.4 shows a similar picture to portray the doping of two types of foreign atoms in Si lattice.
1.1.3 Introduction to Diode Physics
When p-type and n-type junctions are combined to form p-n junctions, they possess a characteristic called rectification [23-26]. Rectification is a property to allow flow of current easily in one direction only [28,29]. In the case of p-type material, the Fermi level (EF) is near the valence band edge and is close to conduction band edge in n-type material as shown in Figure 1.5. In p-type configuration, holes are the majority carriers, while electrons are minority carriers. Just the opposite happens in case of n-type materials in which electrons are majority carriers and holes are minority carriers. Upon joining, large carrier concentration gradients happen at the junction to cause carrier diffusion. Majority holes from the p-type are transported by diffusion into the n-type semiconductor, while majority electrons from n-type semiconductor are diffused towards the p-type. Holes continue to leave the side of p-type while electrons keep on moving from the side of n-type semiconductor till a saturation point is reached. In this exercise of charge carrier transportation, a minor concentration of negative acceptor ions
Figure 1.4 Figure showing the doping of two types of foreign atoms of B (p-type) and P (n-type) in Si to form semiconductor material with better conductivities.
Figure 1.5 Two semiconductor blocks of p-type and n-type before the formation of junction and also showing position of Fermi level (EF) in the corresponding dopes semiconductor material.
and positive donor ions at the semiconductor junction remains unreacted. The holes possess high mobility whereas acceptor atoms are permanently fixed in the semiconductor lattice. Similar explanation follows in case of electrons leaving the n-type semiconductor. A saturation point is attained after transportation of both types of charge carriers to oppositely doped semiconductor blocks. The result of large concentration gradient is that a part of the free electrons coming from donor impurity atoms migrates across the semiconductor junction filling up holes in the p-type semiconductor material to produce negative ions. On moving from n-type to p-type, positively charged donor ions (ND) are left behind on the side of n-type. Similarly, holes coming from acceptor foreign atoms are transported across the junction in an opposite direction having large number of free electrons. The transport mechanism of holes and electrons across the p-n junction is called diffusion. As a result, a space charge region is formed in the region combining p-type and n-type semiconductor blocks. On the side of p-type block, a negative space charge region is formed, while a positive space charge region is formed on the side of n-type semiconductor. The constitution of this space charge region in the junction develops an electric field that is directed from the holes towards the negative charge. The width of the p- and n-type layers is dependent on the degree of heavy doping of each layer with acceptor impurity atoms (NA) and donor impurity atoms (ND), respectively. Figure 1.6(a) below shows the space charge region formed between the joining of two semiconductor blocks of p-type and n-type. The electric field will be directed from positive charge towards the negative charge. Figure 1.6(b) shows the energy band diagram of a semiconducting p-n junction in thermal equilibrium. It needs to be pointed out that the flow of charge carriers can be due to both drift and diffusion. It is apparent from Figure 1.6(b) that the hole drift current flows from right to left, while hole diffusion current flows from left to right. The electron drift current flows from right to the left, while electron diffusion current flows from left to right. Thus, the free charge carriers (electrons and holes) produce current in two ways under the application of an electric field in a semiconductor, i.e. by drift and diffusion. The passage of charge carriers under the effect of an externally applied electric field generates a net current called as the drift current. In case of spatial variation of concentrations of charge carriers in the semiconductor, charge carriers have the tendency to move from regions of high concentration to regions of low concentration called as the diffusion current. The spatial variation in charge carrier concentration is called as the concentration gradient. Figure 1.7 shows the current-voltage characteristics of a typical p-n junction diode. When the junction is forward-biased (+ve terminal of the battery connected to p-type having positive vacancies as majority carriers), current (I) increases rapidly as a function of voltage (V). In case of application of reverse-biasing (-ve terminal of the battery connected to p-type having positive vacancies as majority carriers and +ve terminal of the battery connected to n-type having electrons majority carriers), zero current flows initially virtually. A schematic of the two biasing regimes, reverse Figure 1.8(a) and forward Figure 1.8(b). Only a small amount of current flows on increasing the reverse potential through the battery terminals. At a critical value of the reverse bias, the current suddenly increases which is called as the junction breakdown. The diode response is achieved at relatively lower voltages (~1 V) in forward-biasing case as shown in Figure 1.8(b). In reverse-bias, the breakdown voltage or reverse critical voltage generally varies from few volts to larger voltages. This is typically dependent on the amount of doping of foreign atoms to form two types of semiconductor blocks or layers and different device parameters [10].
Figure 1.6 Space charge region formed in between the joining region of p-type and n-type semiconductor blocks is shown in (a). The energy band diagram of a p-n semiconductor junction in thermal equilibrium is shown in (b).
Figure 1.7 Current-voltage (I-V) characteristics of a semiconductor p-n junction.
Figure 1.8 The two biasing regimes of a diode, (a) reverse (b) forward, are shown in the above schematic. In reverse bias, the diode acts as an open switch, while it acts as a closed switch in case of forward bias.
In the reverse bias mode, the diode device acts as an open switch such that the positive terminal of the source will attract free electrons from n-type and negative terminals will attract holes from the p-type. As a result, concentration of ions in both the regions will increase enhancing the width of the depletion region. In any case, minority carriers will enter the depletion region and cross to other sides of the junction causing a small amount of current called as reverse saturation current (IS). The term “saturation” here means that there will not be any enhancement in the current on increasing the reverse bias potential. As can be seen from Figure 1.7, current change happens very quickly in small voltages initially reaching the saturation current and dependency of the current on further changes in voltages is lost. At a certain higher critical reverse voltage, usually after tens of voltages, a huge current is caused in the opposite direction. On increasing the reverse voltage, it creates an electric field impacting greater force on the electrons to move faster and an enhancement in kinetic energy (K.E.) of electrons follows. This higher K.E. is transported to valence shell of electrons of stable atoms by highly mobile electrons causing them to leave the atom and form the stream of reverse current flow. The critical voltage at which this rapid change happens is called the Zener voltage.
In forward biasing mode, an electric field forces free electrons in n-type block and holes in p-type block towards the depletion region. In this biasing, holes and free electrons recombine with ions in the depletion region to reduce the width of the depletion region. On increasing the forward voltage further, depletion region becomes thinner and a larger number of majority carriers are able to pass through the barrier. It needs to be pointed out that no net current flows in the diode in absence of an externally applied electric field.
1.1.3.1 Equilibrium Fermi Energy (EF)
In the state of thermal equilibrium, the individual hole and electron streams passing through the barrier are ideally zero. The state of thermal equilibrium can be defined as the steady-state condition at a given temperature when no externally applied field is present. In this case, the net current density due to both drift and diffusion currents should be zero for both holes and electrons. Thus, net current density for holes is given as [10,11,23,24],
where, is the Einstein relation. Also,
The expression for hole concentration,
Differentiating equation (1.3) with respect to x in the equilibrium condition,
From equation (1.2) with the help of equation (1.4),
Similarly, net current density for electrons is given as follows,
Hence,
It is apparent from equations (1.6) and (1.10) that the Fermi level (EF) is not dependent on x and remains uniform in whole of the semiconductor sample for zero net hole and electron densities. This is also apparent from the band diagram as shown in Figure 1.6(b). A typical space charge distribution happens at the barrier due to uniform EF in the steady state. Considering the 1D p-n junction when all donor and acceptor atoms are ionized, Poisson’s equation for electrostatic potential ψ and unique space charge distribution is given as follows [10,23,24],
The above situation is well represented in Figure 1.9(a)
