The following is excerpted from Chapter 7 from a new edition of the book, RF Circuit Design, 2e by Christopher Bowick. (If you order a copy of this book before March 30, 2008 you can receive additional 20% off. Visit www.newnespress.com or call 1-800-545-2522 and use code 91603. )
The most popular semiconductor materials used in the manufacture of RF components—especially power amplifiers—are silicon (Si) and gallium arsenide (GaAs). Silicon devices are typically much cheaper to manufacture than gallium arsenide compounds. Unfortunately, silicon-based RF devices usually don't work as well as GaAs for most high-frequency or for high-power applications. Two exceptions include silicon-based lateral double-diffused MOSFET (LDMOS), a version of power MOSFETs, which are used as high-powered amplifiers (100W and up) in wireless base station applications.
Silicon germanium is another silicon-based device that can exceed the performance
of GaAs devices, though only in low-power, high-frequency applications, such as in the front-end design of mobile phones (see Figure 7-13). SiGe power amplifiers provide better linear performance and better power efficiency over GaAs. Better efficiency, combined with the lower cost of silicon manufacturing, has made SiGe more popular in recent years.
7-13. RF Power (W) vs. Frequency (Hz) for RF Power Semiconductors.
In addition to silicon germanium (SiGe), several relatively newer semiconductor compounds are gaining acceptance by RF engineers (see Table 7-1). The first is indium phosphide (InP), which provides exceptional low noise performance at very high frequencies, especially in the millimeter wave range (>40 GHz). Also, InP power amplifiers work well at higher frequencies,
though are more expensive to make than SiGe.
Table 7-1. Newer Semiconductor Compounds.
Gallium Nitride (GaN) compounds hold great promise for high frequency, high power amplifiers (100W and up) for wireless transmitters. When combined with RF receivers in mobile phones, GaN amplifiers could enable the direct assessing of communication satellites. The advantage of GaN devices is its high power density, which is many times that of GaAs or InP. The main
disadvantage of GaN (as with all gallium-based compounds) is one of high manufacturing expense.
Monolithic Microwave Integrated Circuits (MMIC)
An integrated circuit (IC) results when you add more than one device to a semiconductor substrate: for example, transistors, diodes and other electronic components. If the device operates at microwave frequencies (1 GHz to 300 GHz) and performs such functions as microwave mixing, power and low noise amplification, and switching, then the device is called a monolithic microwave IC, or MMIC (pronounced mimic). MMICs are most often made from GaAs, InP or SiGe (see Figure 7-14).
7-14. GaAs MMICs are used in defense, space, and selected commercial markets. (Courtesy of M/A COM.
Like other mass-produced IC devices, MMICs enjoy the benefit of low cost in high volume and small chip size (from around 1mm2 to 10 mm2). The main disadvantage of a MMIC is that they can have worse performance on certain parameters than the same devices made out of separate components.
For example, if low noise is a critical performance requirement in a microwave low noise amplifier, then it may be best to use a discrete component amplifier or build one out of transistors, rather than use a multifunction MMIC. This follows from the fact that, since MMICs are integrated into a single semiconductor device, the separate parts of a MMIC cannot be easily tuned as with discrete components distributed on a PCB. Once the MMIC circuit has been designed, its performance characteristics are set.
Filters and MMICs
Technologies currently used to fabricate front-end RF and IF filters are diverse, although the general trend for all design approaches is to produce the smallest possible filter with the highest performance and power-handling capability. RF/microwave filters have been constructed with many materials and structures, including slabline, combline, and waveguide filters, in addition to filters based on different types of resonators, such as ceramic resonators, crystal resonators, dielectric resonators, film-bulk-acoustic resonators (FBARs), surface-acoustic wave
(SAW) resonators, and even the exotic yttrium-iron-garnet (YIG) resonators.
Monolithic IC filters fabricated on a chip with other semiconductor devices borrow from the traditional use of passive inductors (Ls) and capacitors (Cs) to form the resonant circuits at the basis of an RF/microwave filter. Because the values of on-chip Ls and Cs are limited, and the fact that the size of these passive elements is determined by the wavelength or center frequency of the filter, the performance of IC-based filters is extremely limited, typically prompting designers to specify FBAR, SAW, or other types of filters in RF front ends.
Similarly, active monolithic filters based on operational amplifiers (op-amps) that combine resistor (R) and capacitor (C) elements can achieve high rejection for typically IF signals, although they are limited in frequency range for other RF front-end applications.
Printed with permission from Newnes, a division of Elsevier. Copyright 2008. "RF Circuit Design, 2e" by Christopher Bowick. For more information about this title and
other similar books, please visit www.newnespress.com.