Fundamentals of Power Semiconductor Devices

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Fundamentals of Power Semiconductor Devices provides an in-depth treatment of the physics of operation of power semiconductor devices that are commonly used by the power electronics industry. Analytical models for explaining the operation of all power semiconductor devices are shown. The treatment h
B Jayant Baliga Fundamentals of power Semiconductor devices S ringer B Jayant Baliga Power Semiconductor Research Center North Carolina State University 1010 Main Campus Drive Raleigh, NC 27695-7924 USA ISBN978-0-387-47313-0 e-ISBN978-0-387-47314-7 Library of Congress Control Number: 2008923040 o 2008 Springer Science Business Media, LLC All rights reserved. This work may not be translated or copied in whole or in part without the written permission of the publisher(Springer Science+ Business Media, LLC, 233 Spring Street, New York, NY 10013, USA), except for brief excerpts in connection with reviews or scholarly analysis. Use in connection with any form of information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed is forbidden The use in this publication of trade names, trademarks, service marks and similar terms, even if they are not dentified as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights Printed on acid-free paper springer. com Dedication The author would like to dedicate this book to his wife, Pratima, for her unwavering support throughout his career devoted to the enhancement of the performance and understanding of power semiconductor devices Preface oday the semiconductor business exceeds $200 billion with about 10% of the revenue derived from power semiconductor devices and smart power integrated circuits. Power semiconductor devices are recognized as a key component for all power electronic systems. It is estimated that at least 50% of the electricity used in the world is controlled by power devices. With the widespread use of electronics in the consumer, industrial, medical, and transportation sectors, power devices have a major impact on the economy because they determine the cost and efficiency of systems. After the initial replacement of vacuum tubes by solid-state devices in the 1950s, semiconductor power devices have taken a dominant role with silicon serving as the base material. These developments have been referred to as the Second electronic revolution Bipolar power devices, such as bipolar transistors and thyristors, were first developed in the 1950s. Because of the many advantages of semiconductor devices compared with vacuum tubes, there was a constant demand for increasing the power ratings of these devices. Their power rating and switching frequency increased with advancements in the understanding of the operating physics, the availability of larger diameter. high resistivity silicon wafers. and the introduction of more advanced lithography capability. during the next 20 years, the technology for the bipolar devices reached a high degree of maturity by the 1970s bipolar power transistors with current handling capability of hundreds of amperes and voltage blocking capability of over 500 v became available. More remarkably echnology was developed capable of manufacturing an individual power thyristor from an entire 4-inch diameter silicon wafer with voltage rating over 5,000 V My involvement with power semiconductor devices began in 1974 when I was hired by the General Electric Company at their corporate research and development center to start a new group to work on this technology. At that time I had just completed my Ph. D. degree at Rensselaer Polytechnic Institute by v111 FUNDAMENTALS OF POWER SEMICONDUCTOR DEVICES performing research on a novel method for the growth of epitaxial layers of compound semiconductors. Although I wanted to explore this approach after joining the semiconductor industry, I was unable to secure a position at any of the major research laboratories due to a lack of interest in this unproven growth technology. Ironically, the OMCVd epitaxial growth process that I pioneered with Professor Ghandhi has now become the most commonly used method for the growth of high quality compound semiconductor layers for applications such as lasers. LEDs and microwave transistors My first assignment at ge was to develop improved processes for the fabrication of high voltage thyristors used in their power distribution business Since the thyristors were used for high voltage DC transmission and electric locomotive drives, the emphasis was on increasing the voltage rating and current handling capability. The ability to use neutron transmutation doping to produce high resistivity n-type silicon with improved uniformity across large diameter wafers became of interest at this time. I was fortunate in making some of the critical contributions to annealing the damage caused to the silicon lattice during neutron irradiation making this process commercially viable. This enabled increasing the blocking voltage of thyristors to over 5,000 v while being able to handle over 2,000 A of current in a single device Meanwhile, bipolar power transistors were being developed with the goal of increasing the switching frequency in medium power systems. Unfortunately, the current gain of bipolar transistors was found to be low when it was designed for high voltage operation at high current density The popular solution to this problem, using the Darlington configuration, had the disadvantage of increasing the on-state voltage drop resulting in an increase in the power dissipation. In addition to the large control currents required for bipolar transistors, they suffered from poor safe-operating-area due to second breakdown failure modes. These issues produced a cumbersome design with snubber networks that raised the cost and degraded the efficiency of the power control system In the 1970s, the power MOSFET product was first introduced by nternational Rectifier Corporation. Although initially hailed as a replacement for all bipolar power devices due to its high input impedance and fast switching speed, the power MOSFET has successfully cornered the market for low voltage (100 V) and high switching speed(>100 kHz) applications but failed to make serious inroads in the high voltage arena. This is because the on-state resistance of power MOSFETs increases very rapidly with increase in the breakdown voltage The resulting high conduction losses, even when using larger more expensive die, degrade the overall system efficiency In recognition of these issues, I proposed two new thrusts in 1979 for the power device field. The first was based upon the merging of mos and bipolar device physics to create a new category of power devices. My most successful innovation among MOS-bipolar devices has been the insulated gate bipolar transistor (IGBT). Soon after commercial introduction in the early 1980S, the IGBT was adopted for all medium power electronic applications. Today Preface manufactured by more than a dozen companies around the world for consumer industrial, medical, and other applications that benefit society. The triumph of the IGBT is associated with its huge power gain, high input impedance, wide safe operating area, and a switching speed that can be tailored for applications depending upon their operating frequency The second approach that I suggested in 1979 for enhancing the performance of power devices was to replace silicon with wide bandgap semiconductors. The basis for this approach was an equation that I derived relating the on-resistance of the drift region in unipolar power devices to the basic properties of the semiconductor material. This equation has since been referred to as baliga's figure of merit (BFOM). In addition to the expected reduction in the on-state resistance with higher carrier mobility the equation predicts a reduction in on-resistance as the inverse of the cube of the breakdown electric field strength of the semiconductor material The first attempt to develop wide-bandgap-semiconductor-based power devices was undertaken at the General Electric Corporate Research and Development Center. Schenectady NY, under my direction The goal was to leverage a 13-fold reduction in specific on-resistance for the drift region predicted by the boom for gallium arsenide. a team of ten scientists was assembled to tackle the difficult problems of the growth of high resistivity epitaxial layers, the fabrication of low resistivity ohmic contacts, low leakage Schottky y contacts, and the passivation of the GaAs surface. This led to an enhanced understanding of the breakdown strength' for GaAs and the successful fabrication of high performance Schottky rectifiers and MESFETs. Experimental verification of the basic thesis of the analysis represented by BFOM was therefore demonstrated during this period. Commercial GaAs-based Schottky rectifier products were subsequently introduced in the market by several companies In the later half of the 1980s, the technology for the growth of silicon carbide was developed at North Carolina State University (NCSU) with the culmination of commercial availability of wafers from CREE Research Corporation. Although data on the impact ionization coefficients of Sic were not available, early reports on the breakdown voltage of diodes enabled estimation of the breakdown electric field strength. Using these numbers in the BFOM predicted an impressive 100-200-fold reduction in the specific on-resistance of the drift region for SiC-based unipolar devices. In 1988, I joined NCSU and subsequently founded the Power Semiconductor Research Center(PSrC)-an industrial consortium- with the objective of exploring ideas to enhance power device performance. Within the first year of the inception of the program, SiC Schottky barrier rectifiers with breakdown voltage of 400v were successfully fabricated with on-state voltage drop of about 1 v and no reverse recovery transients 10 By improving the edge termination of these diodes, the breakdown voltage was found to increase to 1,000 V. With the availability of epitaxial Sic material with lower doping concentrations, Sic Schottky rectifiers with breakdown voltages over 2.5 kv have been fabricated at PSRC. These results have motivated many other FUNDAMENTALS OF POWER SEMICONDUCTOR DEVICES groups around the world to develop Sic-based power rectifiers. In this regard, it has been my privilege to assist in the establishment of national programs to fund research on silicon carbide technology in the United States, Japan, and Switzerland-Sweden. Meanwhile, accurate measurements of the impact ionization coefficients for 6H-SiC and 4H-Sic in defect-free regions were performed at PSRC using an electron beam excitation method. 2 USing these coefficients, a BFOM of over 1,000 is predicted for Sic, providing even greater motivation to develop power devices from this material Although the fabrication of high performance high voltage Schottky rectifiers has been relatively straightforward, the development of a suitable silicon carbide MOsFet structure has been problematic. The existing silicon power D MOSFET and U-MOSFET structures do not directly translate to suitable structures in silicon carbide. The interface between SiC and silicon dioxide, as a gate dielectric, needed extensive investigation due to the large density of traps that prevent the formation of high conductivity inversion layers. Even after overcoming this hurdle, the much higher electric field in the silicon dioxide when compared with silicon devices, resulting from the much larger electric field in the underlying SiC, leads to reliability problems. Fortunately, a structural innovation called the ACCUFET, to overcome both of these problems, was proposed and demonstrated at PSRC.In this structure a buried p region is used to shield the gate region from the high electric field within the SiC drift region. This concept is applicable to devices that utilize either accumulation channels or inversion channels, Devices with low specific on-resistance have been demonstrated at PSrC using both 6H SiC and 4H-SiC with epitaxial material capable of supporting over 5,000 V. This device structure has been subsequently emulated by several groups around the orld The availability of power semiconductor devices with high input impedance has encouraged the development of integrated control circuits. In general, the integration of the control circuit is preferred over the discrete counterpart due to reduced manufacturing costs at high volumes and improved reliability from a reduction of the interconnects. Since the complexity of including additional circuitry to an IC is relatively small, the incorporation of protective features such as over-temperature, over-current, and over-voltage has become cost effective. In addition, the chips can contain encode/decode Cmos circuitry to interface with a central microprocessor or computer in the system for control and diagnostic purposes. This technology is commonly referred to as Smart Power Technology 15 The advent of smart power technology portends a Second Electronic Revolution. In contrast to the integrated circuits for information processing, this technology enables efficient control of power and energy. These technologies can therefore be regarded as complementary, similar to the brain and muscles in the human body. Smart power technology is having an enormous impact on society The widespread use of power semiconductor devices in consumer, industrial, transportation, and medical applications brings greater mobility and comfort to Preface billions of people around the world. Our ability to improve the efficiency for the control of electric power results in the conservation of fossil fuels, which in turn provides reduction of environmental pollution Due to these developments, it is anticipated that there will be an increasing need for technologists trained in the discipline of designing and manufacturing power semiconductor devices. This textbook provides the knowledge in a tutorial format suitable for self-study or in a graduate/senior level university course 17 In comparison with my previous textbooks ,( which have gone out of print), this book provides a more detailed description of the operating physics of power devices. Analytical expressions have been rigorously derived using the fundamental semiconductor Poissons, continuity, and conduction equations. The electrical characteristics of all the power devices discussed in this book can be computed using these analytical solutions as shown by typical examples provided in each section. Due to increasing interest in the utilization of wide bandgap semiconductors for power devices, the book includes the analysis of silicon carbide structures. To corroborate the validity of the analytical formulations, I have included the results of two-dimensional numerical simulations using medici in each section of the book. The simulation results are also used to elucidate further the physics and point out two-dimensional effects whenever relevant Chap. 1, a broad introduction to potential applications for power devices is provided. The electrical characteristics for ideal power rectifiers and transistors are then defined and compared with those for typical devices. Chapter 2 provides the transport properties of silicon and silicon carbide that have relevance to the analysis and performance of power device structures. Chapter 3 discusses breakdown voltage, which is the most unique distinguishing characteristic for power devices, together with edge termination structures. This analysis is pertinent to all the device structures discussed in subsequent chapters of the book Chapter 4 provides a detailed analysis of the Schottky rectifier structure On-state current flow via thermionic emission is described followed by the impact of image force barrier lowering on the reverse leakage current. These phenomena influence the selection of the barrier height to optimize the power losses as described in the chapter. The influence of the tunneling current component is also included in this chapter due to its importance for silicon carbide Schottky rectifiers Chapter 5 describes the physics of operation of high voltage P-i-N rectifiers. The theory for both low-level and high-level injection conditions during on-state current flow is developed in detail. The impact of this on the reverse recovery phenomenon during turn-off is then analyzed. The influence of end region recombination, carrier-carrier scattering, and auger recombination are included in the analysis In Chap 6, an extensive discussion of the operating principles and design considerations is provided for the power metal-oxide-semiconductor field effe transistor(MOSFET) structure. The influence of the parasitic bipolar transistor on the blocking voltage is described together with methods for its suppression. The

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