Chapter 1 Introduction The advent of the wireless telecommunications infrastructure has been responsible for one of the social revolutions in the last century1. In the beginning, the benefits of being able to communicate unfettered by landlines was taken advantage of in the business environment. As the networks grew in size and accessibility with concomitant reductions in cost, the benefits expanded to the consumer market2. In more recent years, the plethora of services available in the market have often led to wireless technology supplanting landlines demonstrating that a paradigm shift has occurred. Today, in many under-developed nations, it is often more economical to deploy a wireless network than to invest in the traditional telephone infrastructure. The semiconductor devices discussed in this book are core components in wireless systems. They serve the purpose of boosting the strength of the RF signals to a level essential for consumers to access the telephone network over long distances. Although specific products are optimized for the requirements of individual networks, the operating principles for all these transistors is the same irrespective of the modulation schemes used in the network. The goal of this book is to provide the reader with an introduction to the different styles of silicon power Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) that have been developed over the last five years to serve this market. Although the silicon bipolar RF transistors were predominantly used until 1995, the silicon RF MOSFETs have now completely displaced them in network base-stations. In order to elucidate the operating principles and characteristics of these devices, the results of extensive numerical simulations of the device structures are reported in this book. These simulations provide guidelines for understanding the design of modern RF high power transistors. The results of these simulations have been validated in numerous products that are available in the market.
2 SILICON RF POWER MOSFETs Links to the datasheets of some representative products have been included in the appendix for reference. Although the focus of this book is on the operating principles of silicon RF power MOSFETs, a description of the application environment is beneficial to the understanding of the design requirements for these devices. For this reason, the rest of this chapter provides some background on the evolution of the mobile communications network. After the advent of the first generation mobile cellular telecommunication systems in the 1980s, the networks have evolved from an analog to a digital framework to enable an increase in voice traffic as well for adding services such as Internet access. The implementation of the third generation system is currently underway with more advances anticipated into the P e r f 0 r rn a n 0 1 G Analog 2G Digital 3G MultiMedia W-CDMA TACS, C-NET2 L Fig. 1.1 Evolution of Wireless Technology. A roadmap for the evolution of cellular mobile communication systems is illustrated in Fig. 1.1 to provide a perspective of the time frame over which the transitions are occurring. Some background on this evolution is provided in subsequent sections. 1.1 First Generation Mobile Communication Networks The first generation (1G) of mobile cellular systems was introduced in the 1980s and grew in significance in the early 1990s. The concept of
Introduction 3 dividing the network coverage area into small segments, referred to as cells, provided a modular approach to enhancing telecommunication service while reusing the available frequency spectrum. This method enabled rapid growth in system capacity using analog technology for providing communication by voice. Many competing modulation schemes were deployed in different parts of the world. The most prevalent standards that evolved are shown in Fig. 1.2 below. System AMPS Deployment Argentina, Australia, Bangladesh, Brazil, Brunei, Burma, Cambodia, Canada, China, Indonesia, Malaysia, Mexico, Mongolia, New Zealand, Pakistan, Philippines, Russia Singapore, South Korea, Sri Lanka, Taiwan United States, Vietnam C-NETZ Germany, Portugal, South Africa Austria, Belgium, Cambodia, Denmark, Finland, France, Germany, Hungary, Iceland, Indonesia, Italy, Malaysia, Netherlands, Norway, Poland, Romania, Russia, Spain, Sweden, Thailand, Turkey, Ukraine Cambodia, Cyprus, Denmark, Finland, France, Greenland, Netherlands, Norway, Serbia, Sweden, Switzerland, Thailand Argentina, Bahrain, China, Ireland, Italy, Japan, Kuwait, Malaysia, Philippines, Singapore, Spain, Sri Lanka, United Arab Emirates, United Kingdom Fig. 1.2 Deployment of 1G Wireless Networks. The Advanced Mobile Phone System (AMPS) is a U.S. standard that uses the 800 MHz radio frequency band. In addition to North America, it is utilized in the Far Eastern and South American countries listed in the figure. The C-Netz standard is used primarily in West Germany. The Nordic Mobile Telephone (NMT) standard was initially developed for Scandinavian countries and then adopted in central and southern Europe. The older NMT-450 system is based on the 450 MHz radio frequency band while the more recent NMT-900 system utilizes the 900 MHz band. The Total Access Communication system (TACS) is a
4 SILICON RF POWER MOSFETs standard developed in the United Kingdom that has been adopted by some countries in Southern Europe and the Middle East. It uses the 900 MHz radio frequency band. 1.2 Second Generation Mobile Communication Networks The second generation (2G) mobile cellular systems were based upon maintaining the cellular signal distribution concept but migrating from an analog to one of several digital modulation schemes to enable an increase in voice traffic. This allowed an increase in network capacity within the same radio frequency spectrum licensed by the networks. Four main standards used globally are the Global System for Mobile (GSM) communications, Digital Advanced Mobile Phone Service (D-AMPS), the Code-Division-Multiple Access (CDMA), and the Personal Digital Cellular (PDC) schemes. The most widely used system in the world today is GSM using the 900 MHz radio frequency band. The GSM network utilizes the Time-Division Multiple Access (TDMA) scheme to enable increases in the number of channels. In TDMA, the frequency carrier is divided into short time slots. The Digital Advanced Mobile Phone Service (D-AMPS) was developed in the United States to be backwards compatible with the analog AMPS system. It is also based on the Time-Division Multiple Access (TDMA) scheme to enable increases in the number of channels. The Personal Digital Cellular (PDC) scheme was originally developed in Japan. It operates in the 800 MHz and the 1500 MHz bands. These second generation or 2G networks have been recently upgraded to allow transfer of data. In the High-speed Circuit-Switched Data (HSCSD) technique, the mobile terminal can use up to four time slots for data connection with each time slot providing either 9.6 Kbps or 14.4 Kbps data rates. This provides a relatively limited data transfer capability as a temporary solution to migration to other techniques. In the General Packet Radio Services (GPRS) method the data rates can be increases up to 115 Kbps making it suitable for e-mail and Web-surfing but not for real-time applications. Although implementation of GPRS requires network investments, it represents an important step towards the migration of the GSM networks to third-generation networks. Another improvement can be achieved with the Enhanced Data rate for Global Evolution (EDGE) scheme which uses phase shift keying to enhance GSM data rates by up to three times. Although the
Introduction 5 EDGE implementation can be done by software upgrades at base stations, the RF amplifiers need to now handle non-constant envelope modulation with a relatively high peak-to-average ratio. When EDGE is used with GPRS, the combination called enhanced GPRS can provide a maximum data rate of 384 Kbps by using eight time slots. The original Code-Division-Multiple Access (CDMA2000) standard, also referred to as the IS-95 standard, provides 9.6-14.4 Kbps data rates. This system can be upgraded to the IS-95B standard to obtain 64 Kbps data rates followed by the IS-95C standard, also referred to as the CDMA2000 IXRTT, to enable 144 Kbps data rates. The Japanese Personal Digital Cellular (PDC) system has also undergone upgrades to provide data services. NTT DoCoMo introduced a packet data network (PDC-P) called i-mode that allows consumers to access Internet services over the wireless network. Over 10 million subscribers were signed up within 18 months of launching this service for Web surfing and wireless email. This provided a much needed validation of the premise that data services which complement voice traffic can attract customers. 1.3 Third Generation Mobile Communication Networks The third generation (3G) mobile cellular system is based up on the Universal Mobile Telecommunications System (UMTS) platform. The radio frequency spectrum allocated to UMTS lies between 1900 and 2200 MHz. Wideband CDMA (WCDMA) is one of the more popular proposals for the development of 3G networks. Using a bandwidth of 5 MHz, data rates of 144 and 384 Kbps are anticipated. The UMTSIWCDMA scheme employs spectrally efficient, non-constant envelope digital modulation techniques. Clipping the signal during peak envelop excursions can lead to spectral re-growth that can violate regulatory requirements on the Adjacent Channel Leakage Power Ratio (ACLR). To prevent this problem, high linearity RF power amplifiers are an essential component in WCDMA base stations for 3G networks. 1.4 Migration Path to Third Generation Networks The benefits of migrating from the first generation analog transmission networks to the second and third generation networks are summarized in
6 SILICON RF POWER MOSFETs Fig. 1.3. The digital transmission capability provided by the 2G and 3G networks enables augmentation of voice signals with data that can be used for video display on handsets. This provides the ability to obtain Internet access, which puts vast resources at the fingertips of customers. This evolution has concurrently provided global roaming capability. Fig. 1.3 Evolution of Wireless Networks. The improvements in data transmission capability when migrating from the second generation to the third generation mobile communication networks are provided in Fig. 1.4. The migration from the GSM networks in Africa, Asia, Europe, South America, and the USA to the third generation W-CDMA network occurs via the HSCSD, GPRS, and EDGE upgrades, which provide progressive increases in the data transmission rates given in the figure. The migration of TDMA networks to W-CDMA in the USA occurs via the EDGE upgrades, while the cdma2000 networks in Asia and the USA require implementation of the IS-95 B and cdma2000 lxrtt capability to migrate to the lxev standard. Meanwhile in Japan, the PDC networks will migrate to the lxev and the W-CDMA standards as well, as shown in the figure. When fully implemented, the third generation networks are expected to provide data rates of 144 Kbps at full mobility, data rates of 384 Kbps at limited mobility, and data rates of 2 Mbps at fixed locations.
Introduction ------------------,r----------------------------------------------.-------------------. Second Generation j i I Migration Path j! Third i i Generation i ; :I :I EDGE, W-CDMA 1-1 psh 3@4 Kbps I j TDMA,, 14.4 Kbps j! 384 Kbps i: W-CDMA I Y Antenna e r Transceiver Fig. 1.4 Enhanced Data Rates in Wireless Networks. Multi-Carrier Power Amplifier 1 Low Noise Amplifier 1 Linearization Circuit SplittersICombiners Power Amplifier RF Receiver ADC Radio DAC Radio RF Modulation DSP Baseband 1 1.5 Base Station Market Fig. 1.5 Block Diagram of a Typical Base Station. The deployment of wireless networks that serve an increase in voice traffic with the capability for data transmission at high rates requires investments in new infrastructure. Of particular relevance to this book is
8 SILICON RF POWER MOSFETs the upgrading of the base station architecture to multi-carrier power amplifiers with improved linearity. To gain a proper perspective on the importance of the power amplifier within the base station architecture, a block diagram of the base station footprint is shown in Fig. 1.5. The base station serves as an interface between the antenna for broadcasting the cellular signals and the fixed line telecommunications backbone. The upper portion of the diagram contains the RF circuitry. For a typical cellular base station, the RF section costs about $ 65,000 with the power amplifier comprising $ 40,000 of the cost. Within the power amplifier, the MCPA and linearization circuitry each constitute one-third of the cost today. Since the power amplifier is recognized to be the most expensive portion of the capital expenses in a base station, technology that can reduce the complexity and cost of these components is of great interest to the wireless industry5. As discussed later in the book, the most commonly used method for reducing the distortion produced in today's LD-MOSFET based multi-carrier power amplifiers is the feed-forward technique. The super-linear or (SL) MOSFETs described in this book have been demonstrated to allow the design of multi-carrier power amplifiers that can operate without the feed-forward distortion correction circuitry. The elimination of the linearization circuit from the base station platform would not only reduce cost but also reduce the space and power requirements because of the higher efficiency for RF power transmission h t 3000 2500 C al $ 2000 E 8 1500 C 0 4 1000 C 8 500.- g o m Memory El Power and Control Power Amplifier Components Radio Components Baseband DSP Fig. 1.6 Base Station Semiconductor Content.
Introduction 9 The growth in the semiconductor content within base stations is illustrated in Fig. 1.6, including the radio frequency, base band processing DSP chips, and the power management segments. The power amplifier component is a significant share of the capital outlay for new deployments. After a drop in revenue during the last two years, it is anticipated that the demand for semiconductors will grow as a result of the deployment of the third generation technology5. The annual revenue for the power amplifier components is about $800 Million. 1.6 Summary The cellular wireless network industry is undergoing a revolutionary advance from analog to digital technology. The push to increase the voice traffic and incorporate data transmission has provided the impetus for upgrading the infrastructure. Although the migration path differs depending upon the modulation protocol used in each system and country, all cellular networks require build-out of base stations with multi-carrier power amplifiers replacing the single-carrier amplifiers that were satisfactory for the analog systems. The implementation of the third generation networks based on W-CDMA requires enhancements in the linearity and efficiency of the power amplification path. The currently used feed-forward architecture required to suppress the distortion products created with LD-MOSFET based power amplifiers is one limitation to achieving this goal. A new super-linear power MOSFET technology has been developed that allows multi-camer power amplifier designs without the need for the feed forward linearization circuitry. The physics of operation of these transistors is described in this book after first reviewing the principles of operation of the currently used LD- MOSFETs. Many novel super-linear power MOSFETs are introduced in this book to demonstrate that the super-linear physics can be achieved using a variety of structural designs. When describing these structures and their characteristics, the same format has been carefully adhered to for each chapter to provide the reader with a unified treatment that allows ease of comparison between the structures.
10 SILICON RF POWER MOSFETs References 1 L. Harte, S. Kellog, R. Dreher, and T. Schaffnit, "The Comprehensive Guide to Wireless Technologies", APDG Publishing, 2000. 2 S. Baker, N. Gross, and I. M. Kunii, "The Wireless Internet", Business Week, May 29,2000. M. J. Riezenman, "Communications", IEEE Spectrum, January 1998. 4 J. Korhonen, "Introduction to 3G Mobile Communications", Artech House, 2001. 5 S. Lavey and A. M. Leibovitch, "World-wide Base Transceiver Station Semiconductor Forecast, 2002-2006, IDC Report #28297, November 2002.