The IGBT Device: Physics, Design and Applications of the Insulated Gate Bipolar Transistor

By B. Jayant Baliga

The IGBT Device: Physics, Design and Applications of the Insulated Gate Bipolar Transistor, Second Edition provides the essential information needed by applications engineers to design new products using the device in sectors including consumer, industrial, lighting, transportation, medical and renewable energy. The IGBT device has proven to be a highly important Power Semiconductor, providing the basis for adjustable speed motor drives (used in air conditioning and refrigeration and railway locomotives), electronic ignition systems for gasoline powered motor vehicles and energy-saving compact fluorescent light bulbs.

The book presents recent applications in plasma displays (flat-screen TVs) and electric power transmission systems, alternative energy systems and energy storage, but it is also used in all renewable energy generation systems, including solar and wind power. This book is the first available on the applications of the IGBT. It will unlock IGBT for a new generation of engineering applications, making it essential reading for a wide audience of electrical and design engineers, as well as an important publication for semiconductor specialists.

• Presents essential design information for applications engineers utilizing IGBTs in the consumer, industrial, lighting, transportation, medical and renewable energy sectors
• Teaches the methodology for the design of IGBT chips, including edge terminations, cell topologies, gate layouts, and integrated current sensors
• Covers applications of the IGBT, a device manufactured around the world by more than a dozen companies with sales exceeding $5 Billion
• Written by the inventor of the device, this is the first book to highlight the key role of the IGBT in enabling electric vehicles and renewable energy systems with global impacts on climate change

• Language: English
• Publisher: Elsevier Science
• Release date: Nov 25, 2022
• ISBN: 9780323917148

B. Jayant Baliga

Professor Baliga obtained his Bachelor of Technology degree in 1969 from the Indian Institute of Technology, Madras, India. He was the recipient of the Philips India Medal and the Special Merit Medal (as Valedictorian) at I.I.T, Madras. He obtained his Masters and Ph.D. degrees from Rensselaer Polytechnic Institute, Troy, NY, in 1971 and 1974, respectively. His thesis work involved Gallium Arsenide diffusion mechanisms and pioneering work on the growth of InAs and GaInAs layers using Organometallic CVD techniques. At R.P.I., he was the recipient of the IBM Fellowship in 1972 and the Allen B. Dumont Prize in 1974. From 1974 to 1988, Dr. Baliga performed research and directed a group of 40 scientists at the General Electric Research and Development Center in Schenectady, NY, in the area of Power Semiconductor Devices and High Voltage Integrated Circuits. During this time, he pioneered the concept of combining MOS and Bipolar physics to create a new family of discrete devices. He is the inventor of the IGBT which is now in production by many International Semiconductor companies. For his work, Scientific American Magazine named him one of the ‘Eight heroes of the semiconductor revolution’ in their 1997 special issue commemorating the Solid-State Century. Dr. Baliga is also the originator of the concept of merging Schottky and p-n junction physics to create a new family of JBS power rectifiers that are commercially available from various companies. In August 1988, Dr. Baliga joined the faculty of the Department of Electrical and Computer Engineering at North Carolina State University, Raleigh, North Carolina, as a Full Professor. At NCSU, in 1991 he established an international center called the Power Semiconductor Research Center (PSRC) for research in the area of power semiconductor devices and high voltage integrated circuits, and has served as its Founding Director. In 1997, in recognition of his contributions to NCSU, he was given the highest university faculty rank of Distinguished University Professor of Electrical Engineering. In 2008, Professor Baliga was a key member of an NCSU team - partnered with four other universities - that was successful in being granted an Engineering Research Center from the National Science Foundation for the development of micro-grids that allow integration of renewable energy sources. In 2010, Dr. Baliga was inducted into the Engineering Design Magazine’s “Engineering Hall of Fame” for his invention, development, and commercialization of the Insulated Gate Bipolar Transistor (IGBT), joining well known luminaries (e.g. Edison, Tesla, and Marconi) in the electrical engineering field. The award announcement states: “While working at General Electric in the late 1970s, Baliga conceived the idea of a functional integration of MOS technology and bipolar physics that directly led to the IGBT’s development… it remains undeniable that Baliga’s vision and leadership played a critical role in moving the IGBT from a paper-based concept to a viable product with many practical applications.” Professor Baliga has received numerous awards in recognition for his contributions to semiconductor devices. These include two IR 100 awards (1983, 1984), the Dushman and Coolidge Awards at GE (1983), and being selected among the 100 Brightest Young Scientists in America by Science Digest Magazine (1984), and, on October 21, 2011, President Obama personally presented Dr. B. Jayant Baliga with the National Medal of Technology and Innovation, the highest form of recognition given by the United States Government to an Engineer, in a ceremony at the White House. Dr. Baliga’s award citation reads: For development and commercialization of the Insulated Gate Bipolar Transistor and other power semiconductor devices that are extensively used in transportation, lighting, medicine, defense, and renewable energy generation systems.

Book preview

Preface to the Second Edition

It has been 5 years since The IGBT Device book was published. It was well received and won the prestigious PROSE Award as the best book in engineering and technology published in 2015. Semiconductor technology makes rapid advances due to demands from applications. This tenet is certainly applicable to the case of insulated gate bipolar transistor (IGBT) devices. This has motivated the preparation of a second edition of this book. Modern publication methods also make color illustrations affordable. Color illustrations have therefore been extensively used in the new edition to enhance the reader experience and convey the information more effectively. My intention was to add new published technical information to all sections of the book to make it up to date. It was also important to revise the written materials on the device’s social impact because this has grown enormously during the last 5 years.

Before preparation of the second edition, the publisher asked some reviewers for suggestions on the proposal. The reviewers suggested the addition of new IGBT device concepts, such as reverse conducting IGBTs and fine-mesh IGBTs, which were already planned. Many new IGBT structural concepts have therefore been added to the book. The reviewers also suggested adding more information on device fabrication. This has been done in the new edition. The information on packaging and gate drivers has been expanded based on reviewer suggestions. Many new references were added to each chapter when preparing the second edition. This brings the book up to date. A reviewer suggested that more in-depth analysis be provided for all the power circuits described in the book. This suggestion was not practical due to limitations on the size of the book and not appropriate because the book was intended to focus on the IGBT rather than on power electronics.

During the past 5 years, important new technology trends have emerged. The first is a growing role of energy storage to complement various applications, such as renewable energy generation and data centers. Consequently, a new chapter on energy storage has been added to the book. Another new trend was the rapid acceleration of renewable energy deployment. Chapter 15 on renewable energy was therefore expanded with more recent information. In addition, the replacement of gasoline power automobiles with electric vehicles has become a globally recognized imperative to reduce greenhouse gas emissions. The role of the IGBT in enabling electric vehicles was already extensively discussed in the original book. This information has been updated in the new edition. More importantly, the future environmental benefits of these developments have been discussed in the chapter on social impact.

One of the most important changes during the past years is the growth of the impact of IGBT in many sectors of the economy. The 1990–2015 time frame saw a growth in IGBT-enabled technologies from a small scale at the beginning of the 1990s to a major impact by 2015. This high impact has continued during the past 5 years, making the already significant cumulative effect much larger in magnitude. The cost savings enjoyed by consumers has grown from $15 trillion to $33.7 trillion while the reduction of carbon dioxide emissions has increased from 100 trillion pounds to 180 trillion pounds. This impact will continue to grow in the future.

Silicon carbide- and gallium arsenide-based power devices finally became commercialized in 2015 after a 30-year period of investment in materials and device development. In principle, these devices could displace the silicon IGBT in various applications due to superior on-state and switching losses. However, it is now recognized that this displacement will occur in the near term only in niche areas due to the high cost of this new technology and the lack of confidence in its reliability on the part of end users. Consequently, the strong position of the silicon IGBT in all sectors of the economy is unlikely to change for a significant length of time. This makes the new edition of this book valuable to all engineers, technologists, economists, and environmentalists for the foreseeable future.

B. Jayant Baliga

Preface to the First Edition

In 1977, I submitted a patent disclosure while working for the General Electric (GE) Company on vertical metal oxide semiconductor (MOS)-gated thyristors that contain the basic insulated gate bipolar transistor (IGBT) structure. After a V-groove process for making the structure was developed, the fabrication of the devices began in November 1978 with completion in July 1979. In addition to the latched-up thyristor mode of operation, my measurements clearly showed the IGBT mode of operation. In response to the need for an improved power switch for adjustable speed drives for heat pump applications by GE, I prepared a patent disclosure in September 1980 that described all the characteristics for the IGBT that we now take for granted. It was immediately apparent that this new device would have a widespread impact on the company’s products in the small appliance, large appliance, medical, factory automation, and lighting business units. Due to its impact across the company, my proposal received the attention of Chairman Jack Welch, who supported its commercialization. I was fortunately able to deliver a 600 V, 10 A IGBT within a year by fabricating the device in the existing power MOSFET production facility with my chip and process design, including suppression of the latch-up of the parasitic thyristor. I simultaneously developed an electron irradiation lifetime control process with a unique annealing step that healed the damage created by electron irradiation to the gate oxide. This allowed production of IGBTs optimized for a wide range of switching frequencies and applications. The availability of these IGBTs spurred power electronics designers to rapidly apply them to a large variety of products at GE. GE eventually announced the commercially available IGBT product in 1983. This prompted worldwide interest in manufacturing the device, leading to products from other companies after 1985.

A few years ago, the new head of the Department of Electrical and Computer Engineering at North Carolina State University suggested that I prepare a report about the impact of my work on the IGBT to post on our website. The outcome of my effort was a 140-page document with more than 300 references titled The IGBT Compendium: Applications and Social Impact. GE recognized the impact of the IGBT on most of the company’s product divisions immediately after my invention of the device. I was personally involved with the design of IGBTs suitable for GE’s adjustable speed drives for Trane and Carrier air conditioners (heat pumps), for GE’s early efforts on creating more efficient lighting products, and a variety of small and large appliance controls. However, preparing the report on the IGBT after a time span of 30 years was a voyage of discovery. It was apparent that the IGBT had now penetrated literally every sector of the economy and enhanced the comfort, convenience, and health for billions of people around the globe.

Improving the efficiency of power management and delivery is in the very nature of power electronics. It is well recognized that power semiconductor devices play a dominant role in achieving this outcome. However, the impact of the improvements in efficiency on power savings had not been quantified using a rigorous methodology. Without this metric, it was also not possible to evaluate the environmental consequences of this technology. Because two-thirds of the electricity in the world is used to run motors, I decided to quantify the power savings derived from IGBT-based adjustable speed drives for motors. In addition, since one-fifth of the electricity in the world is used for lighting, I decided to quantify the impact of compact fluorescent lamps (CFLs) because IGBTs are used in the electronic ballast. The third sector of the economy that has benefitted from IGBTs is the transportation sector. It became quickly apparent that electronic ignition systems, enabled in the late 1980s with the availability of the IGBT to control the spark plugs of internal combustion engines in cars and trucks, had enhanced fuel efficiency. With the huge quantities of gasoline consumed around the world, it became important to quantify the impact of this innovation. With just these three applications of the IGBT, I determined that society had saved more than 100,000 terawatt hours in electricity consumption (equivalent to not building 11,000 coal-fired power plants) and more than 1.8 trillion gallons of gasoline. This not only saved worldwide consumers more than $32 trillion but also reduced carbon dioxide emissions by more than 150 trillion pounds from 1990 to 2020.

In 2012, I was encouraged by colleagues to consider writing a book on the IGBT with the above report as a foundation. My reaction was a proposal for creating a comprehensive book on the IGBT that first includes the device operation, chip design, fabrication technology, packaging, and gate drive circuits. It then provides an extensive discussion of its applications in all sectors of the economy with elaboration of the circuit topologies used in each case and the optimized IGBT device structures developed by the power semiconductor industry for each application. I was very pleased that the editors at Elsevier found my proposal compelling. The reaction from reviewers of my IGBT book proposal was also very positive, with the suggestion that I include a discussion of how I invented, developed, and successfully commercialized the IGBT in the early 1980s.

That book (the first edition) was the result of 2 years of effort to create a single resource for the reader regarding the operation and design of the IGBT as well as its social impact. It was immensely gratifying that the first edition of the book was recognized with the Professional and Scholarly Excellence Award as the best book published in the electronics area in 2015. This second edition has been prepared after a period of 7 years to capture the advancements in IGBT technology since the first edition was published. In addition, many papers have been published during these years on IGBT applications in all sectors of the economy; these applications are included in the new edition to bring it up to date. Most importantly, a new chapter on energy storage has been added in the second edition. This technology is of great importance in renewable energy generation and for server farms used in cloud computing and data storage. It is worth pointing out that many of the figures have been redrawn in color in the second edition to make the book more appealing and to make it easier to assimilate the information. I hope this second edition will be received as favorably as the first.

Chapter 1 provides a high-level perspective of the applications of the IGBT and its power ratings. It includes a discussion of the history behind the conception of the device and its commercialization. Chapter 2 describes various IGBT structures that have evolved over the years. The very first IGBT I developed at GE in 1981 was a 600 V symmetric blocking device, followed soon after with the 600 V asymmetric blocking devices. The power semiconductor manufacturers focused their attention on the asymmetric structure for motor drive applications over the next 30 years. More recently, there has been interest in the symmetric blocking IGBT for current source inverters and matrix converters. The first IGBTs made use of the planar gate structure, but significant improvement in the trade-off curve between on-state voltage drop and switching losses was later achieved using trench gate devices. The transparent emitter IGBT structure had an important role in scaling the voltage ratings of the IGBT to allow application to traction drives.

Chapter 3 provides a description of the physics of operation of the IGBT structure to allow its design using analytical models. The symmetric, asymmetric, and transparent emitter structures are systematically analyzed in terms of the blocking characteristics, the on-state voltage drop, and the power loss trade-off curve. Silicon carbide IGBTs are included here for completeness, although no commercial devices are as yet available.

The excellent ruggedness of the IGBT with a wide safe operating area has been one of its prime features from an application standpoint. Chapter 4 provides analytical models for designing the safe operating area of the IGBT. It includes device cell innovations that have been responsible for preventing latch-up of the internal parasitic thyristor, which was considered a show stopper when I originally proposed the IGBT.

Chapter 5 provides a practical description of the layout of the active area for the IGBT chip and its edge termination. Techniques for overcurrent, overvoltage, and overtemperature protection are described here. Lifetime control processes that enable adjustment of the switching speed of the IGBT without damaging its gate oxide are described as well.

Chapter 6 describes packaging technology for both discrete IGBTs and for IGBTs packaged in modules. The power module designs range from low- to high-power levels. In Chapter 7, various gate drive circuits are provided that enable controlling the reverse recovery of the fly-back diodes and switching losses in the IGBT itself. Chapter 8 provides models employed for simulation of IGBTs in power circuits.

In Chapters 9–19, the applications of IGBTs in various sectors of the economy are reviewed. These chapters demonstrate the breadth of the impact of this singular innovation on society. In each chapter, the circuit topology, such as hard switching versus resonant switching, and the IGBT specifications that ensure efficient operation in these circuits, are described. The optimization of the IGBT structure by the device manufacturers to reduce power losses in each case is then provided.

In the transportation sector discussed in Chapter 9, at the individual consumer level, the IGBT is essential for operating the internal combustion engine in gasoline-powered automobiles, and for driving the electric motors in electric vehicles and hybrid electric vehicles. The IGBT is essential for mass transit systems, ranging from electric buses and trams to high-speed rail networks around the world. With the growth in ratings for the IGBTs, they have even penetrated propulsion systems for large ships and enabled the all-electric aircraft.

The discussion of the industrial sector in Chapter 10 includes adjustable speed drives for motor control, factory automation systems, robotics, welding, induction heating, milling and drilling, mining, and paper, textile, and metal mills. Chapter 11 on the lighting sector provides extensive coverage of various circuits used for this high-volume application. In addition, the use of IGBTs in strobe lights for the flash in cameras and for powering xenon arc lamps in automobiles and movie projectors is described.

Chapter 12 deals with the consumer section where the IGBT has been utilized for a large variety of appliances. Among large appliances commonly used in our homes are the air conditioner, the refrigerator, the washing machine, the microwave oven, the induction cooktop, and the dishwasher. Among small tabletop appliances that are essential conveniences for food preparation in kitchens are portable induction cooktops, rice cookers, blenders, mixers, juice makers, and mixers. In addition, the IGBT is an essential component in older generation television sets with cathode ray tubes and in modern plasma TV sets.

Society has greatly benefitted from IGBTs in the medical sector as well for improving medical diagnosis and life-saving events during cardiac arrest. Chapter 13 describes the use of IGBTs in the power supply for x-ray machines, CT scanners, MRI scanners, and ultrasound machines to produce high-quality images for medical diagnosis and treatment of physical trauma. The automatic external (portable) defibrillators could not have been deployed in low-cost, lightweight, laptop-size units without the availability of the IGBT. The creation of this device saves more than 100,000 lives in the United States each year and many more around the world.

The IGBT was at first reluctantly adopted in the United States by the defense sector, as described in Chapter 14, but now occupies an essential role in equipment deployed by all the armed forces. The navy utilizes it in power distribution systems in warships, aircraft carriers, and nuclear submarines. The Army is developing electric vehicles that depend on IGBTs for their inverters and the Air Force makes use of IGBTs to replace hydraulic systems with more reliable, lightweight electrical actuators.

The mitigation of global warming due to increased carbon in the atmosphere from fossil fuel (carbon and natural gas)-powered electricity generating plants requires increased deployment of solar and wind power generation capacity. These renewable resources all utilize the IGBT in the inverter for delivering well-regulated AC power to the transmission grid. Chapter 15 describes the power electronics technology not only for these renewable resources but also for hydroelectric power, wave power, tidal power, and geothermal power.

Chapter 16 describes the penetration of the IGBT into the power transmission sector. This has occurred more recently after the power ratings of IGBT modules were enhanced by semiconductor suppliers to handle megawatt power levels. IGBT-based static VAR compensators and static synchronous compensators (STATCOM) have been employed for AC power transmission grids. The HVDC-lite concept using IGBTs is gaining dominance in new power transmission deployments.

The IGBT has even benefitted the financial sector of the economy, as discussed in Chapter 17. With the advent of computer-based high-speed transactions in the banking, credit card, and investment sectors, any interruption of power can lead to a loss of millions of dollars in revenue each hour. IGBT-based uninterruptable power supplies have become essential equipment in protecting data centers against not only power interruption but voltage sags, voltage swells, and other power quality issues.

The addition of energy storage capability has been gaining increasing importance. It is necessary to supplement energy generation from renewable resources due to their intermittent nature. It is essential for the success of cloud computing and information storage to ensure reliable operation, an expectation of millions of subscribers to these modern conveniences. The role of the IGBT in energy storage deployment is described in Chapter 18.

Chapter 19 has been written to capture the myriad of other IGBT applications that do not fit into the sectors of the economy discussed in earlier chapters. These applications include smart homes; printing and copying machines; airport security machines; particle accelerators, including the CERN Large Hadron collider used for the discovery of the Higgs boson; food and water sterilization; water desalination; roller coasters; and the NASA space shuttle and the International Space Station.

The social impact of the IGBT is covered in Chapter 20. Here, three case studies are described: adjustable speed motor drives, CFL lamp ballasts, and electronic ignition systems. These three applications enabled by IGBTs have tremendously improved efficiency.

My intention is to create a book on the IGBT that provides not only a comprehensive description of its operation and design but also its breadth of application across all sectors of the economy as well as to quantify its social impact. This book should be of interest to all power semiconductor and power electronics engineers. In addition, it should be of interest to social scientists who are interested in the impact of technology on society.

Jayant Baliga

Chapter 1: Introduction

Abstract

The pervasive use of the insulated-gate bipolar transistor (IGBT) in all sectors of the economy has made it an essential element for improving the comfort, convenience, and quality of life for billions of people around the world. After a discussion of the applications spectrum for power devices, this chapter describes the basic structure and operating principle of the IGBT. The circumstances and efforts undertaken to take the device from concept to a commercialized product are described to provide a historical perspective. The rapid growth in power ratings for the IGBT over the last 40 years was achieved by increasing the blocking voltage to 6.5 kV and making multichip modules that can handle hundreds of amperes. Recently, the extension of the concept to silicon carbide material has allowed extending the blocking voltage to the 20 kV level.

Keywords

Applications spectrum; Asymmetric; Commercialization history; IGBT structure; Ratings growth; Symmetric

Today, the insulated-gate bipolar transistor (IGBT) is pervasively used in power electronic systems and their applications to improve the comfort and quality of life for billions of people around the world. The impact of the IGBT on society can be measured by asking the question: What would happen if IGBTs were removed from all the applications they serve today? The answer is quite revealing:

1.Our gasoline-powered cars would stop running because the electronic ignition systems would no longer function.

2.Our hybrid and electric cars would stop running because the inverters used to deliver power from the batteries to the motors would no longer function.

3.Our electric mass transit systems would come to a standstill because the inverters used to deliver power from the electric grid to the motors would no longer function.

4.Our air-conditioning systems in homes and offices would stop working because the inverters used to deliver power from the utility company to the heat pumps and compressors would no longer function.

5.Our refrigerators and vending machines would no longer function, making the delivery and storage of perishable products impossible.

6.Our factories would come to a grinding halt because the controllers used to run the robots would cease to function.

7.Our new low-energy compact fluorescent bulbs would stop functioning, significantly increasing power consumption for lighting due to reverting back to incandescent bulbs.

8.Our portable defibrillators recently deployed in emergency vehicles, airplanes, and office buildings would no longer be operational, putting more than 100,000 people at risk of death from cardiac failure every year.

9.Our new solar- and wind-based renewable energy resources would not be able to deliver power to the grid because the inverters would stop functioning.

10.Our uninterruptible power supplies would no longer work, jeopardizing financial transactions conducted by banks and investment firms.

11.Our modern power transmission systems would become less efficient, resulting in greater carbon emissions from fossil fuels.

In other words, the quality of life in our society would be greatly impaired if the IGBT was no longer available. The IGBT has become an embedded technology that enriches the lives of billions of people around the globe by providing comfort in the home, food preservation, industrial manufacturing, transportation, and even medical assistance.

In September 2005, when celebrating their 30th anniversary of covering trends in power semiconductor technology, Power Electronics Technology magazine published a review article [1] with a milestone chart. In this chart, the first significant event highlighted is the invention of the bipolar transistor by Brattain, Bardeen, and Shockley in 1947, for which they received the Nobel Prize in 1956. The next important milestone is the invention of the integrated circuit by Jack Kilby, who received a Nobel Prize in 2000. The conception of the integrated circuit is also credited to Robert Noyce, who received the National Medal of Technology and Innovation in 1987. During the 1950s, power thyristors were also commercially introduced for high-power applications. Major manufacturers for these bipolar devices were General Electric (GE) and Westinghouse Corporation. According to the milestone chart, the next major innovation in power devices was the introduction of the power metal-oxide semiconductor field-effect transistor (MOSFET) by Siliconix in 1975 and International Rectifier in 1978. At this time, the power semiconductor industry was bifurcated into two tracks, with one group of companies producing bipolar power devices and a separate group of companies producing power MOSFET devices. At that time, the manufacturing of these devices was considered to be incompatible because the MOS devices required know-how in the control of semiconductor surface properties while the bipolar devices relied on control of minority carriers within the bulk regions of semiconductors.

The milestone chart states that I invented the IGBT in 1979–80. This was accomplished by proposing the functional integration of MOS and bipolar physics within the same monolithic structure. In December 2010, I was inducted into the Electronic Design Engineering Hall of Fame for the invention, development, and commercialization of the IGBT. The award announcement states [2]: "While working at General Electric in the late 1970s, Baliga conceived the idea of a functional integration of MOS technology and bipolar physics that directly led to the IGBT’s development … it remains undeniable that Baliga’s vision and leadership played a critical role in moving the IGBT from a paper-based concept to a viable product with many practical applications." On October 21, 2011, President Obama presented the National Medal of Technology and Innovation to me at the White House in recognition of my development and commercialization of the IGBT and other power semiconductor devices. This recognition put the spotlight on IGBTs and acknowledged the impact of power electronics on our society. In 2015, I was awarded the Global Energy Prize due to the huge impact of the IGBT on the efficient generation, management, and distribution of electrical energy. This was followed by my induction into the National Inventors Hall of Fame as the sole inventor of the IGBT in 2016. This was a fitting recognition of the IGBT, as it was listed among just 500 inventions out of 10 million US patents that had been issued at that time.

1.1: IGBT Applications Spectrum

IGBTs are required for applications that operate over a broad spectrum of current and voltage levels, as shown in Fig. 1.1. Their characteristics are ideal for applications with operating voltages above 200 V. Typical examples are lamp ballasts, consumer appliances that utilize motors, and electric vehicle drives. Other examples are high-power motor control in steel mills and for traction (electric trains). They are now being utilized even in power transmission and distribution systems. The on-resistance of conventional silicon power MOSFET structures is too large to serve these applications. Consequently, these applications utilize silicon IGBTs today. Silicon carbide (SiC) IGBTs offer very promising characteristics for applications that require blocking voltages of above 10–15 kV for use in smart grid applications [3].

Figure 1.1

Figure 1.1 Application spectrum for insulated gate bipolar transistors (IGBTs).

It is worth noting that the current ratings for IGBTs increase with increasing voltage rating for these applications, with the exception of the smart grid. In the case of silicon IGBTs, this issue is tackled by resorting to multichip press-pack modules. The smart grid application is unique in requiring very high-voltage devices with low current ratings. These applications can be served by using SiC-based IGBTs despite the lower chip manufacturing yield and higher cost of SiC wafers. These SiC IGBTs can operate at higher frequencies than their silicon counterparts, resulting in a smaller size for the magnetic elements used in the power circuits.

1.2: Basic IGBT Device Structures

As illustrated in Fig. 1.2, there are two basic IGBT structures, namely symmetric blocking and asymmetric blocking devices. The symmetric blocking structure allows supporting a high voltage in the first and third quadrant, that is, the device has high forward and reverse blocking capability. This feature is required for any power devices used in high-voltage AC power applications. In contrast, the asymmetric blocking structure can support a high voltage only in the forward blocking mode. This structure is optimized for applications that utilize a high-voltage DC power bus. The presence of the N-buffer layer in the asymmetric structure allows reducing the thickness of the N-drift region, which improves the on-state voltage drop and switching time.

Figure 1.2

Figure 1.2 Basic insulated gate bipolar transistor device structures.

1.3: IGBT Development and Commercialization History

The first semiconductor power devices were bipolar transistors that evolved out of the invention of the junction transistor in 1947. A thick, lightly doped drift region was added in the power bipolar transistor, as illustrated in Fig. 1.3, to support high voltages. The high blocking voltage capability also required a relatively large base width that degraded the current gain. High-level injection effects produced further reduction of the current gain [4]. The large base drive current required for the bipolar transistor restricted its voltage rating to below 600 V.

Figure 1.3

Figure 1.3 Bipolar power devices.

High-power applications that needed devices capable of supporting more than 1000 V and controlling large currents were served in the 1970s by the development of thyristor or four-layer semiconductor structures. The gate turnoff thyristor (GTO) shown in Fig. 1.3 became popular for motor drives in traction applications for street cars and electric trains. The thyristor regenerative action within these four-layer switches allowed manufacturing high-current devices with low on-state voltage drop. However, the GTO required very large gate drive currents to achieve unity gain turnoff. The complex gate drive and snubber circuits for GTOs increased power losses while adding cost and size to the system.

During the 1970s, a concerted effort was made to create power MOSFETs after the successful development of complementary metal oxide semiconductor (CMOS) technology. The double-diffused metal oxide semiconductor (DMOS) structure shown in Fig. 1.4 was the most commercially successful power device. The MOS gate structure for the device made it a voltage-controlled device with essentially no current flow in the gate circuit during the steady-state on-mode and blocking mode. The gate drive current required for switching the devices was also modest due to the relatively low operating frequencies of power circuits at that time. Although the power MOSFET displaced the power bipolar transistor due to its superior gate drive, switching speed, and ruggedness for lower voltage (< 200 V) applications, a severe increase in its resistance became an impediment to scaling up its blocking voltage.

Figure 1.4

Figure 1.4 Power MOSFET structure.

In the late 1970s, it was clear that a high-performance power device technology was badly needed for medium- and high-power applications. In early 1977, I began to explore the use of MOS-gated structures to control a four-layer semiconductor device. My analysis indicated that, unlike the conventional thyristor structure, the gate drive current requirement for the thyristor could be decoupled from the [dV/dt] and the [dI/dt] capability by using an MOS gate structure. On July 26, 1977, I submitted a patent disclosure at GE on MOS gating of thyristors [5]. This disclosure describes a vertical four-layer structure with a V-groove region in which the MOS gate structure is prepared. The structure described in my patent disclosure is the V-MOS cross-section shown in Fig. 1.5, which is also the IGBT structure. The fabrication of this device structure required setting up and optimizing a potassium hydroxide (KOH)-based silicon etching process to form the (truncated) V-groove region. Once this process was established, the fabrication of the MOS-gated thyristors commenced under my supervision on November 9, 1978 [6], with Margaret Lazeri as my lead process technician.

Figure 1.5

Figure 1.5 V-groove insulated gate bipolar transistor/thyristor structure.

The processing was successfully completed, leading to the first experimental results obtained on July 30, 1979. These results clearly showed the IGBT mode with MOS gate-bias-controlled current saturation as well as the latch-up of the four-layer thyristor at larger current levels [7]. I discovered that some of the fabricated devices exhibited the expected enhancement mode operation, leading to a snapback in the output characteristics due to latch-up of the thyristor structure. However, other devices displayed depletion mode characteristics, demonstrating for the first time that current can flow in a four-layer vertical semiconductor device without latch-up of the thyristor. This represents the first observation of the IGBT mode of operation in a vertical semiconductor device. It demonstrated that current flow through the four-layer structure could be turned off by the application of a negative gate bias to the MOS gate electrode-definitively demonstrating nonlatch-up current transport in a vertical four-layer semiconductor device for the first time. Consequently, the conception and experimental demonstration of the IGBT at GE can be directly attributed to my patent disclosure on July 26, 1977.

After an internal GE review cycle, on August 28, 1979, the above results were submitted for publication to Electronics Letters. The results were accepted and appeared in print in this journal on September 27, 1979 [8]. The article contains photographs of the output characteristics of the devices. In particular, Fig. 1.2 in the article shows IGBT-like characteristics, namely a diode-line behavior in the on-state and MOS gate-controlled current saturation capability, with no snapback due to latch-up of the four-layer semiconductor structure. The paper states: "The structure can be seen to be similar to that of V-groove (MOSFET) devices but with the drain region of the (MOSFET) being replaced by a p-type anode region."

In early September 1980, Tom Brock, the vice president of a new GE product division set up to create high-efficiency adjustable-speed motor drives for air conditioning, visited the GE Research Center and described his frustrations with using the available Darlington bipolar transistors for this application. He challenged my group to provide a better device technology. I submitted a patent disclosure on September 29, 1980, in response to this challenge [9]. In my disclosure, I projected the following characteristics for the proposed device based on my analytical modeling of the IGBT structure: (1) both forward and reverse blocking capability; (2) forward drop similar to a P-i-N rectifier; (3) turn on and turn off using a small gate voltage with low gate current; (4) very high turn off gain; (5) high dV/dt and dI/dt capability; (6) operation at elevated temperatures; and (7) tolerance to radiation. I named the device the gate enhanced rectifier (GERECT) to emphasize its P-i-N rectifier-like on-state characteristics. I projected these characteristics based on my previous work in the 1970s on the field-controlled thyristor. In retrospect, this prescient description has withstood the test of time and encapsulated the performance attributes for the IGBT that are now taken for granted. The GE patent application for the IGBT included innovative structures that did not contain an N+ emitter region to avoid formation of a parasitic thyristor. Due to the desire on the part of GE to obtain the broadest possible claims, the patent prosecution took nearly 10 years, with the patent issued in November 1990. Meanwhile, I was granted many other patents filed by GE on structural enhancements for the IGBT.

My GERECT proposal in September 1980 was met with skepticism by colleagues at GE. They first pointed out that previous efforts at MOS gating of four-layer structures showed latch-up of the thyristors at low current levels [8,10]. They also pointed out that my proposed IGBT structure consisted of an n-channel MOSFET driving a wide-base P-N-P bipolar transistor. Prevailing wisdom based on decades of work on power bipolar transistors recommended using a narrow base N-P-N structure to get good current gain. Based on this, my critics said that my proposed device could be expected to operate at a low on-state current density (below 20 A/cm²) with a high on-state voltage drop, as observed in recent publications [10]. My projections of an on-state current density of 100–200 A/cm² for my proposed device based on the P-i-N diode model that I had proposed were considered unrealistic.

However, in October 1980, numerical simulations of my proposed IGBT structures by Mike Adler confirmed the results of my analytical modeling of its capability, leading to the opportunity to describe my idea to Brock. During this presentation, I made it clear that this innovation could impact other GE divisions, such as lighting, small appliances, and major appliances. Brock was very impressed by the potential for this device and proceeded to brief GE Chairman Jack Welch about the impact of this idea on multiple businesses within the company, prompting his visit to Schenectady to meet with me in November 1980. My presentation was very favorably received by Welch, giving me strong support within GE to commercialize and apply this innovation to GE businesses. However, Welch wanted to maximize the impact of this innovation within GE products and placed an embargo on releasing any information about the IGBT for several years. This delayed external recognition of my early work on the IGBT.

Because the potential impact of the IGBT to GE businesses was recognized by Welch, the preparation and filing of the patent application for my disclosure RD-13,112 was undertaken at the corporate level rather than by the GE Research Center. The patent application was filed on December 2, 1980, and issued only in 1990 after vigorous prosecution at the patent office [11]. With this strategy, GE obtained patent protection for the basic IGBT concept with broad claims until 2009. It is worth pointing out that this patent includes an IGBT structure without the parasitic thyristor where I proposed a tunneling current from the emitter contact metal to the inversion channel. The technology for making such devices did not exist in 1980 but has now been demonstrated for MOSFETs in integrated circuit (IC) applications. Although my patent for the basic IGBT concept was not issued until 1990, a dozen derivative patents were also filed by GE, and many IGBT patents were issued to me in the 1980s [12].

The prevalent practice in the GE Research Center was to first build an innovation in Schenectady and then transfer it into production after it had been proven. This procedure typically required at least 3 years. A major hurdle with this approach was the need to set up a manufacturing capability for the product after the research and development effort. In general, it is very expensive to create a new manufacturing line and the initial product yields can be poor due to wide variability in device characteristics. To avoid these problems, as the acknowledged inventor and principal developer of the device at GE [13], I decided it would be prudent to engineer my IGBT structure to allow its fabrication in an existing power MOSFET product line in California. In 1981, I flew to California to describe my IGBT idea to Nathan Zommer, who was responsible for the power MOSFET product line. After assimilating the power MOSFET production process, I procured the unique IGBT starting material; designed a process flow for the IGBT that included one extra step—a deep P+ region not included in the power MOSFET process at that time; then designed the cell topology and a multiple floating field ring edge termination. After creating a mask set for the fabrication of IGBTs with layout assistance from Peter Gray, I supervised its production by Zommer in California. The very first IGBT wafers out of the manufacturing line that I tested produced functional chips with a blocking voltage of 600 V at a high yield. The total time between my conception of the IGBT and my successfully producing functional devices on a production line was less than 10 months. This remarkable feat provided a large supply of devices that I could tailor with different switching speeds by using an electron irradiation process that I had already proposed and proven using power MOSFETs. These devices were supplied to numerous engineers at GE working on novel lamps, small appliances such as steam irons and toasters, major appliances such as refrigerators and washing machines, and of course for the variable-speed motor drives.

In parallel with the development of the IGBT, my group at GE undertook the development of the high-voltage integrated circuit (HVIC) that could drive the IGBT in an H-bridge topology. The combination of the IGBT and HVIC produced a unique product with an order of magnitude improvement in size, weight, and manufacturability. The first commercial Smart Switch 5 hp. motor drive product announcement was made on October 3, 1983, fulfilling the promise made to Tom Brock in 1981.

Toward the end of 1982, the GE Semiconductor Products Division decided to break the embargo and announce the IGBT as a product. This decision allowed my publication of the first IGBT paper at the IEEE Electron Devices Meeting in December 1982 [14]. The IGBT product (D94FQ4, R4 with 18 A, 400 V, and 500 V rating) was announced in June 1983, resulting in Electronic Products declaring it Product of the Year. The IGBT product was introduced by me and a GE Marketing Manager, Marvin Smith, in a trade publication, Electronic Design News, in September 1983 [15] and reported at the IEEE Industrial Applications Society Meeting in October 1983 [16]. I also promoted the IGBT in Europe at the Drives/Motors/Controls Conference held in Harrogate, United Kingdom [17]. In December 1983, I described the unique flexibility of trading-off on-state and switching losses in IGBTs by using electron irradiation [18]. This paper demonstrated that the IGBT could be optimized for a wide variety of applications. During this period at the end of 1983, GE requested my presentation of the work on IGBTs to numerous visiting delegations from outside organizations. This included a visit from a Japanese delegation with corporate representatives from many companies, including the Fuji Electric Company. My presentation to the Japanese delegation of the very promising features of the IGBT and its proven widespread utilization within GE applications prompted vigorous activity in Japan to commercialize the device [19]. During subsequent years, a systematic effort was made at the GE Research Center under my supervision to characterize the high-temperature operating capability of the IGBT [20], develop complementary (p-channel) devices [21], and scale up the voltage rating [22]. The demonstration of the excellent performance characteristics of the IGBT during these formative years played an important role in generating worldwide interest in the commercialization of the device and subsequent exploitation of the IGBT in myriad applications.

It is worth mentioning that there were many skeptics regarding the viability of the IGBT as a power device when I first proposed the idea at GE. They argued that the device would not be useful due to destructive latch-up of the parasitic thyristor within the IGBT structure. I was able to overcome this problem by the addition of a deep P+ region [23], as shown in Fig. 1.2. The skeptics then argued that even if the structure was functional, it would have very little utility because of its slow switching speed. The known lifetime control processes of heavy metal diffusion and particle bombardment had been reported to severely degrade the gate structure of CMOS devices and power MOSFETs. Fortunately, I was able to discover an annealing process following electron irradiation that removed the damage in the gate oxide of the IGBT structure while leaving di-vacancy-induced traps within the bulk to reduce the switching time [24]. This allowed tailoring the characteristics of the IGBT after complete fabrication of the devices to optimize their performance in applications with operating frequencies from 50 to 10,000 Hz. Without these breakthroughs, the barriers to commercialization of the IGBT would not have been overcome, thus vindicating the skeptics, and the IGBT may have never been developed into a commercial product.

After the IGBT concept was proven to exhibit excellent electrical characteristics, it has sometimes been denigrated as simply a power MOSFET with the drain replaced with a P+ region. Although an accurate literal description of the structure, this viewpoint does not take into account the perceived shortcomings of such a structure when first proposed. It was understood from the beginning that the IGBT operates as a P-N-P bipolar transistor with base drive current provided via the integrated MOSFET structure. The prevailing wisdom in the early 1980s from the development of power bipolar transistors was that P-N-P transistors are far inferior to N-P-N transistors due to the much greater mobility for electrons in silicon. The prevailing design philosophy for these power bipolar transistors was to make the base region as narrow as possible to obtain a larger current gain and to support the high voltage in the collector region. In stark contrast to the best practices at that time, the proposed IGBT structure contained a wide-base P-N-P transistor in which the voltage was supported in the base region. This led skeptics to conclude that the IGBT structure would work at lower on-state current density than the existing power bipolar transistors and power MOSFETs. They also concluded that the required base drive current could not be supplied sufficiently by the integrated MOSFET structure because of the large perceived series resistance of its drift region. Based on these arguments, it was postulated that devices based on on-state current flow using a thyristor with MOS gate-controlled turnoff capability were preferable to the IGBT [25,26] because these devices would operate at a high on-state current density. To make matters worse, the power device funding agencies in the United States, namely the Department of Defense and the Electric Power Research Institute, decided to only support the development of the MOS-controlled thyristor (MCT) [27] in the 1980s. Even after officials invested tens of millions of dollars in the MCT over a span of more than a decade, the MCT has been completely eclipsed by the IGBT. This occurred because my demonstration of the ease of manufacturing IGBTs, suppressing the parasitic thyristor latch-up, and tailoring its switching speed for numerous applications encouraged the worldwide power semiconductor industry to embrace the technology, driving up the ratings of IGBT products while driving down their cost. Once the IGBT was adopted in an expanding range of applications, it became impossible to displace the device, as alternatives failed to deliver all the features of this device.

Once the IGBT was announced as an excellent product by GE in 1983, there has been an effort by other researchers to take credit for the IGBT, according to Wikipedia [28]. The Wikipedia article states that the IGBT mode was first proposed by Yamagami in a Japanese patent JP 47-021739B issued to Mitsubishi Electric Company in 1972. One of the figures in the patent illustrates a four-layer semiconductor structure with an MOS gate electrode, which is the basis for a single claim that describes this structure but not its electrical characteristics. The structure shown in the patent application is unlike all manufactured IGBTs today. It is also obvious that the existence of the IGBT mode was not recognized by Yamagami, and that he certainly did not appreciate or champion its utility because his employer Mitsubishi failed to commercialize the IGBT until 1987 [29], well after I had demonstrated excellent device performance and commercialized the IGBT at GE in 1983.

A lateral, high-voltage, MOS-gated triac structure was reported by Plummer and Scharf [10] in February 1980 with the work supported by a patent application [30]. This device was created by merging two lateral double-diffused metal-oxide semiconductor field-effect transistors (DMOSFETs) with a single gate electrode. In the paper, it was demonstrated that the structure has a DMOSFET mode where the current flows along the surface through both channels, a bipolar mode "with a wide-base p-n-p transistor operating in parallel with the surface devices, and an on-state mode with the thyristor operating with regenerative action. The paper also describes a lateral insulated gate SCR structure in which the thyristor function is still directly controlled by the MOS gate. The electrical characteristics provided in the publication for this structure show a snapback to the on-state, indicating that the device proposed by the authors operates with regenerative action in the on-state. The authors also state that this concept is directly adaptable to discrete-power devices and illustrates a vertical equivalent" structure in the paper. Many years later, Plummer added claim language to cover the IGBT mode of operation by using a reexamination certificate filed in October 1994 [31] after the IGBT became established as a product. To my knowledge, neither Scharf nor Plummer pursued the commercialization of the ideas in their paper. The electrical characteristics demonstrated for MOS-gated SCRs in their work indicate very high on-state voltage drop before latch-up of the thyristor at relatively low current levels. These results discouraged any effort to pursue development of an IGBT structure by the worldwide power semiconductor industry because no products were announced outside of GE until 1985.

On March 25, 1980, Becke and Wheatley from Radio Corporation of America (RCA) filed a patent application titled "A vertical power MOSFET with an anode region," which was issued on December 14, 1982 [32]. It is remarkable that this patent was allowed without citing and taking into account close prior art [6] published in the literature 6 months before the filing date of March 25, 1980, which not only discussed the structure but even contained a sentence with the name of the patent! The single independent claim in this patent states: "…and no thyristor action occurs under any device operating conditions. It is worth pointing out that all commercial IGBT devices made since the 1980s undergo thyristor action under an appropriate rise in temperature and collector current level. Consequently, the claims in this patent do not describe the IGBTs that have been manufactured and used in applications over the past 30 years. According to Wheatley [33], he would provide the direction while Becke would handle the time-consuming yet essential ‘dogwork’." However, Becke passed away soon after the filing of this patent and the development of the conductivity modulated field effect transistor (COMFET) at RCA Laboratories was reported by Russel et al., in 1983 [34] with device characteristics resembling the IGBT. It is interesting that the paper states that the four-layer structure of the COMFET device may latch-up at high anode current levels, contradicting the claims of the RCA patent cited in the paper. To my knowledge, RCA did not follow through with a product release or work on applications for the device. The acquisition of RCA by GE in 1986 made this eventually