Radio Frequency Transistors Principles and Practical Applications Norm Dye Helge Granberg MOTOROLA Series in Solid Slate Electronics Butterworth-Heinemann Boston London Oxford Singapore Sydney Toronto Wellington Motorola reserves the right to make changes without further notice to any products herein to improve reliability, function, or design. Motorola does not assume any lia­ bility arising out of the application or use of any product or circuit described herein; neither does it convey any license under its patent rights nor the rights of others. Motorola products are not authorized for use as components in life support devices or systems intended for surgical implant into the body or intended to support or sus­ tain life. Buyer agrees to notify Motorola of any intended end use whereupon Motorola shall determine availability and suitability of its product or products for the use intended. Motorola is a registered trademark of Motorola, Inc. Motorola, Inc. is an Equal Opportunity/Affirmative Action Employer. Copyright © 1993 by Butterworth-Heinemann, a division of Reed Publishing (USA) Inc. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or trans­ mitted, in any form or by any means, electronic, mechanical, photocopying, record­ ing, or otherwise, without prior written permission of the publisher. Recognizing the importance of preserving what has been written, it is the policy of Butterworth-Heinemann to have the books it publishes printed on acid-free paper, and we exert our best efforts to that end. Library of Congress Cataloging-in-Publication Data Dye, Norm, 1929- Radio frequency transistors : principles and practical applications / Norm Dye, Helge Granberg. p. cm. Includes bibliographical references and index. ISBN 0-7506-9059-3 : $49.95 1. Power transistors. 2. Transistor amplifiers. 3. Transistor radio transmitters. 4. Amplifiers, Radio frequency. I. Granberg, Helge, 1932- . Π. Title. TK7871.92.D96 1993 621.384' 131—dc20 92-54695 CIP Butterworth-Heinemann 80 Montvale Avenue Stoneham, MA 02180 Linacre House, Jordan Hill Oxford OX2 8DP United Kingdom 10 9 8 7 6 5 4 3 21 Printed in the United States of America Editorial, design, and production services provided by HighText Publications, Inc., San Diego, California. PREFACE This book is about radio frequency (RF) transistors. It primarily focuses on applications viewed from the perspective of a semiconductor supplier who, over the years, has been involved not only in the manufacture of RF transistors but also their use in receivers, transmitters, plasma generators, magnetic resonance imaging, etc. Since the late 1960s, Motorola Semiconductors has been at the forefront in the development of solid state transistors for use at radio frequencies. The authors have been a part of this development since 1970. Much information has been acquired during this time, and it is our intention in writing this book to make the bulk of that information available to users of RF transistors in a con­ cise manner and from a single source. This book is not theoretical. It is intended to be practical as the name implies. Some mathematics is encountered during the course of the book but it is not rigorous. Formulas are not derived; however, sufficient references are cited for the reader who wishes to delve deeper into a particular subject. This book is slanted toward power transistors and their applications because much less material is available in the literature on this subject, particularly in one location such as a book. Also, RF power is the primary experience of the authors. One chapter is devoted to low power (small signal) transistor applica­ tions in an effort to cover more completely the breadth of power levels in RF transistors. Chapters 1 through 4 talk about RF transistor fundamentals, such as what's different about RF transistors, how they are specified, how to select a transistor, and the differences in FETs and BJTs. Also covered are topics such as classes of operation, forms of modulation, biasing and operating in a pulse mode. Chapters 5 and 6 lay the groundwork for future circuit designs by discussing such general subjects as laying out circuit boards, mounting RF devices and the importance of die temperature. In Chapters 7, 8 and 9, the authors take the reader through various considera­ tions in planning an amplifier design. Among the diverse topics covered are viii Preface choice of circuit, stability, impedance matching (including computer aided design programs), and the power amplifier output. Chapters 10 through 12 focus on wideband techniques. Finally, Chapter 13 describes the many factors affecting small signal (low power) amplifier design. A variety of examples illustrate the concepts in an effort to make small signal amplifier design straight forward through a step-by- step approach. Acknowledgments The authors wish to thank the many application engineers in the RF product operation at Motorola Semiconductors for their contributions to the book. Special recognition goes to Phuong Le for his assistance in low power applications, to Dan Moline for making available his recently introduced com­ puter program for impedance matching with the aid of Smith Chart displays, to Bob Baeten for his assistance in computer aided design programs, to Walt Wright for answering many questions about microwaves and pulse power appli­ cations, and to Hank Pfizenmayer for his advice and expertise in filter design. Special thanks also go to Analog Instruments Co., Box 808, New Providence, NJ 07974, for their permission to reproduce the Smith Chart in several dia­ grams in Chapter 13. "Smith" is a registered trademark of Analog Instruments. And special thanks go to the management of the Communications Semi­ conductor Products Division within Motorola Semiconductor Sector whose encouragement and support has made writing this book possible. 1 Understanding RF Data Sheet Parameters INTRODUCTION Data sheets are often the sole source of information about the capability and characteristics of a product. This is particularly true of unique RF semicon­ ductor devices that are used by equipment designers all over the world. Because the circuit designer often cannot talk directly with the factory, he relies on the data sheet for his device information.1 And for RF devices, many of the specifi­ cations are unique in themselves. Thus it is important that the user and the man­ ufacturer of RF products speak a common language, i.e., what the semicon­ ductor manufacturer says about his RF device is understood fully by the circuit designer. In this chapter, a review is given of RF transistor and amplifier module parameters from maximum ratings to functional characteristics. The section is divided into five basic parts: D.C. specifications, power transistors, low power transistors, power modules, and linear modules. Comments are made about crit­ ical specifications, about how values are determined and what are their signifi­ cance. A brief description of the procedures used to obtain impedance data and thermal data is set forth, the importance of test circuits is elaborated, and background information is given to help understand low noise considerations and linearity requirements. D.C. SPECIFICATIONS Basically, RF transistors are characterized by two types of parameters: D.C. and functional. The "D.C." specs consist (by definition) of breakdown voltages, leakage currents, h^ (D.C. beta), and capacitances, while the functional specs cover gain, ruggedness, noise figure, Z and Z , S-parameters, distortion, etc. in out Thermal characteristics do not fall cleanly into either category since thermal resistance and power dissipation can be either D.C. or A.C. Thus, we will treat the spec of thermal resistance as a special specification and give it its own heading called "thermal characteristics." Figure 1-1 is one page of a typical RF power data sheet showing D.C. and functional specs. 2 Radio Frequency Transistors ELECTRICAL CHARACTERISTICS (T = 25°C unless otherwise noted ) c Characteristic | Symbol | Min [ Typ | Max | Unit | OFF CHARACTERISTICS Collector-Emitter Breakdown Voltage V(BR)CEO 16 - - Vdc <l = 20 mAdc. I = 0) c B Collector-Emitter Breakdown Voltage V(BR)CES 36 - - Vdc (l - 20 mAdc, V - 0) c BE Emitter-Base Breakdown Voltage V(BR)EB0 4.0 - - Vdc (l = 5.0 mAdc. \Q - 01 E Collector Cutoff Current "CES - - 10 mAdc (V - 15 Vdc. V - 0. T - 25°C) CE BE C ON CHARACTERISTICS DC Current Gain "FE 20 70 150 - (l =4.0 Adc. V = 5.0 Vdc) c CE DYNAMIC CHARACTERISTICS Output Capacitance cob - 90 125 pF IV = 12.5 Vdc. I = 0. f = 1.0 MHz) CB E FUNCTIONAL TESTS Common-Emitter Amplifier Power Gain Gpe 4.8 5.4 - dB (V - 12.5 Vdc, P * 45 W, l(Max) = 5.8 Adc. f = 470 MH2) CC out c Input Power Pin - 13 15 Watts IV = 12.5 Vdc. Put = 45 W. f « 470 MHz) CC 0 Collector Efficiency η 55 60 - % e (V = 12.5 Vdc, P 45W. l (Max) = 5.8 Adc, f = 470 MHz) CC out c Load Mismatch Stress No Degradation in Output Power (V - 16 Vdc, P - Note 1. f - 470 MHz. VSWR - 20:1. Φ * CC in All Phase Angles) Series Equivalent Input Impedance Zin - 1.4 + ]4.0 - Ohms (V - 12.5 Vdc. P - 45 W, f = 470 MHz) CC out Series Equivalent Output Impedance 1.2 + J2.8 - Ohms (V - 12.5 Vdc. P - 45 W. f - 470 MHz) Z°L* CC out Notes: 1. Pj » 150% of Drive Requirement for 45 W output @ 12.5 V. n Φ - Mismatch stress factor-the electrical criterion established to verify the device resistance to load mismatch failure. The mismatch stress test is accomplished in the standard test fixture (Figure 1) terminated in a 20:1 minimum load mismatch at all phase angles. FIGURE 1-1 Typical D.C. and functional specifications from a RF power data sheet. The references in the "Notes" above to a test fixture and "Figure 1" pertain to the data sheet from which this figure was extracted. A critical part of selecting a transistor is choosing one that has breakdown voltages compatible with the supply voltage available in an intended appli­ cation. It is important that the design engineer select a transistor on the one hand that has breakdown voltages which will NOT be exceeded by the DC and RF voltages that appear across the various junctions of the transistor and on the other hand has breakdown voltages that permit the "gain at frequency" objectives to be met by the transistor. Mobile radios normally operate from a 12 volt source and portable radios use a lower voltage, typically 6 to 9 volts. Avionics applications are commonly 28 volt supplies, while base station and other ground applications such as medical electronics generally take advantage of the superior performance characteristics of high voltage devices and operate with 24 to 50 volt supplies. In making a transistor, breakdown voltages are largely determined by material resistivity and junction depths (see Figure 1-2). It is for these reasons that breakdown voltages are intimately entwined with functional performance characteristics. Most product portfolios in the RF power transistor industry have families of transistors designed for use at specified sup­ ply voltages such as 7.5 volts, 12.5 volts, 28 volts, and 50 volts. Understanding RF Data Sheet Parameters 3 10 I I I I I I I ι ι J L 10™ 1015 10™ 3 DOPING DENSITY C (cnr ) B FIGURE 1-2 The effect of curvature and resistivity on breakdown voltage. Leakage currents (defined as reverse biased junction currents that occur prior to avalanche breakdown) are likely to be more varied in their specification and also more informative. Many transistors do not have leakage currents specified because they can result in excessive (and frequently unnecessary) wafer/die yield losses. Leakage currents arise as a result of material defects, mask imper­ fections, and/or undesired impurities that enter during wafer processing. Some sources of leakage currents are potential reliability problems; most are not. Leakage currents can be material related such as stacking faults and disloca­ tions or they can be "pipes" created by mask defects and/or processing inade­ quacies. These sources result in leakage currents that are constant with time and if initially acceptable for a particular application will remain so. They do not pose long term reliability problems. On the other hand, leakage currents created by channels induced by mobile ionic contaminants in the oxide (primarily sodium) tend to change with time and can lead to increases in leakage current that render the device useless for a specific application. Distinguishing between sources of leakage current can be difficult, which is one reason devices for application in military environments require HTRB (high temperature reverse bias) and burn-in testing. However, even for commercial applications particularly where battery drain is critical or where bias considerations dictate limitations, it is essential that a leakage current limit be included in any complete device specification. D.C. parameters such as h^ and C (output capacitance) need little comment. ob Typically, for RF devices, h^ is relatively unimportant for unbiased power transistors because the functional parameter of gain at the desired frequency of operation is specified. Note, though, that D.C. beta is related to A.C. beta (see Figure 1-3). Functional gain will track D.C. beta particularly at lower RF fre- 4 Radio Frequency Transistors 40 POWER GAIN 2 5 100 20 50 100 200 500 1000 FREQUENCY, MEGACYCLES fT mfax FIGURE 1-3 Relationship between transistor beta and operating frequency. quencies. An h^ specification is needed for transistors that require bias, which includes most small signal devices that are normally operated in a linear (Class A) mode (see Chapter 4). Generally RF device manufacturers do not like to have tight limits placed on h^. Primarily, the reasons that justify this position are: a) Lack of correlation with RF performance b) Difficulty in control in wafer processing c) Other device manufacturing constraints dictated by functional performance specs which preclude tight limits for h^. A good rule of thumb for h^ is to set a maximum-to-minimum ratio of not less than 3 and not more than 4, with the minimum hpg value determined by an acceptable margin in functional gain. Output capacitance is an excellent measure of comparison of device size (base area) provided the majority of output capacitance is created by the base- collector junction and not parasitic capacitance arising from bond pads and other top metal of the die. Remember that junction capacitance will vary with voltage (see Figure 1-4) while parasitic capacitance will not vary. Also, in com­ paring devices, one should note the voltage at which a given capacitance is specified. No industry standard exists. The preferred voltage at Motorola is the transistor V rating, i.e., 12.5 volts for 12.5 volt transistors and 28 volts for cc 28 volt transistors, etc. MAXIMUM RATINGS AND THERMAL CHARACTERISTICS Maximum ratings (shown for a typical RF power transistor in Figure 1-5) tend to be the most frequently misunderstood group of device specifications. Ratings for maximum junction voltages are straight forward and simply reflect the mini­ mum values set forth in the D.C. specs for breakdown voltages. If the device in question meets the specified minimum breakdown voltages, then voltages less than the minimum will not cause junctions to reach reverse bias breakdown with the potentially destructive current levels that can result. Understanding RF Data Sheet Parameters 5 2.0 S 1.6 1 f = 1.0 1y lHz ο 1-4 I-* 1.0 ?c c 0.8 S 0.6 LU C3 •8 0.2 ο 0 2.0 4.0 6.0 8.0 10 12 14 16 VCB. COLLECTOR-BASE VOLTAGE (Vdc) FIGURE 1-4 Relationship between junction capacitance vs. voltage for MRF. MOTOROLA •• SEMICONDUCTOR TECHNICAL DATA MRF650 The RF Line NPN Silicon RF Power Transistor 50 WATTS. 512 MHz ... designed for 12.5 Volt UHF large-signal amplifier applications in industrial and RF POWER TRANSISTOR commercial FM equipment operating to 520 MHz. NPN SILICON • Guaranteed 440. 470, 512 MHz 12.5 Volt Characteristics Output Power = 50 Watts Minimum Gain - 5.2 dB (u 440, 470 MHz Efficiency - 55% (a 440. 470 MHz IRL - 10 dB • Characterized with Series Equivalent Large-Signal Impedance Parameters from 400 to 520 MHz • Built-in Matching Network for Broadband Operation • Triple Ion Implanted for More Consistent Characteristics • Implanted Emitter Ballast Resistors • Silicon Nitride Passivated • 100% Tested for Load Mismatch Stress at all Phase Angles with 20:1 VSWR u< 15.5 Vdc, 2.0 dB Overdrive MAXIMUM RATINGS Rating Symbol Value Unit Collector-Emitter Voltage VCEO 16.5 Vdc Collector-Emitter Voltage VCES 38 Vdc Emitter-Base Voltage VEBO 4.0 Vdc Collector-Current — Continuous IC 12 Adc Total Device Dissipation (<» TQ - 25'C PD 135 Watts Derate above 25'C 0.77 W'-C T Storage Temperature Range stg -65 to -150 X THERMAL CHARACTERISTICS Characteristic Symbol Max Unit Thermal Resistance, Junction to Case R«JC 1.3 FIGURE 1-5 Maximum power ratings of a typical RF power transistor, the Motorola MRF650. The value of BV^q is sometimes misunderstood. Its value can approach or even equal the supply voltage rating of the transistor. The question naturally arises as to how such a low voltage can be used in practical applications. First, BV is the breakdown voltage of the collector-base junction plus the forward CEO drop across the base-emitter junction with the base open, and it is never en- 6 Radio Frequency Transistors countered in amplifiers where the base is at or near the potential of the emitter. That is, most amplifiers have the base shorted or they use a low value of resis­ tance such that the breakdown value of interest approaches BV . Second, CES BV involves the current gain of the transistor and increases as frequency CE0 increases. Thus the value of BV at RF frequencies is always greater than the CE0 value at D.C. The maximum rating for power dissipation (P ) is closely associated with d thermal resistance (9 ). Actually maximum P is in reality a fictitious JC d number—a kind of figure of merit—because it is based on the assumption that case temperature is maintained at 25°C. However, providing everyone arrives at the value in a similar manner, the rating of maximum P is a useful tool with d which to compare devices. The rating begins with a determination of thermal resistance—die to case. Knowing G and assuming a maximum die temperature, one can easily deter­ JC mine maximum P (based on the previously stated case temperature of 25°C). d Measuring 0 is normally done by monitoring case temperature (T ) of the JC c device while it operates at or near rated output power (P ) in an RF circuit. The 0 die temperature (Tj) is measured simultaneously using an infra-red microscope (see Figure 1-6) which has a spot size resolution as small as 1 mil in diameter. Normally, several readings are taken over the surface of the die and an average value is used to specify Tj. It is true that temperatures over a die will vary typically 10-20°C. A poorly designed die (improper ballasting) could result in hot spot (worst case) tempera­ tures that vary 40-50°C. Likewise, poor die bonds (see Figure 1-7) can result FIGURE 1-6 Measurement of die temperature using an infra-red microscope.