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InGaP HBT vs. CMOS for mobile handset power amps

Posted: 15 Jun 2007 ?? ?Print Version ?Bookmark and Share

Keywords:InGaP HBT? CMOS? mobile handset? power amps?

By Aditya Gupta
Anadigics Inc.

Designers of mobile handsets and wireless networking products know that the power amplifier (PA) is a make-or-break component in terms of performance, footprint and battery life. Selecting the right type of PA is one of the key decisions that can help achieve a compelling design that delivers a competitive advantage.

Today, most PAs for mobile applications are manufactured using a GaAs-based bipolar transistor (called InGaP HBT, for indium-gallium-phosphide heterojunction bipolar transistor) and a relatively small number are manufactured using silicon (Si) CMOS. CMOS offers the tantalizing possibility of much higher levels of integration and lower cost as compared to GaAs and therefore the question of if and when GaAs will be supplanted by Si is often raised.

To answer that question, the designer needs to develop a clear understanding of the competing technologies for wireless applications and make a comparison in terms of realistic integration levels, performance and cost. While CMOS may be the dominant general-purpose semiconductor technology, it is by no means the best choice for every consumer application. For instance, GaAs PAs dominate the market for wireless networking and handsets because of their superior ability to support high-frequency and high-power applications with good efficiency. CMOS, on the other hand, dominates in Bluetooth and ZigBee applications because they typically operate at lower power levels and have less stringent performance requirements.

RF integration
CMOS is the technology of choice today for the highly integrated baseband and transceiver die for mobile radios due to its ability to integrate an enormous number of transistors as well as its relatively low cost at high volume. While baseband and transceiver ICs used to be two different die a few years ago, the trend now is to merge the two on a single CMOS chip with lower overall cost. It is natural to wonder if the PA could also be integrated on the same die to achieve true single-chip radios. There are, however, some practical considerations that make this integration unlikely.

Highly complex and highly integrated baseband and transceiver RFICs require the most advanced, smallest gate-length CMOS process in order to integrate functions in the smallest possible die. CMOS PAs, on the other hand, have relatively few transistors to integrate but they require specialized, area consuming, passive circuit elements (resistors, inductors and capacitors) that are not usually provided in the most advanced CMOS processes. Those processes are tailored in favor of digital circuitry.

CMOS PAs use inductors, capacitors and resistors that are fundamental to RF applications but are not normally part of a digital circuit. In addition, the Si substrate used in traditional CMOS is conductive, which increases RF loss and severely degrades the performance of these passive circuit elements unless special steps, not normally a part of the CMOS process flow, are taken. These steps include raising the metal transmission lines off the surface of the Si substrate by putting them on thick dielectric layers, etching a trench below the passive structures to reduce eddy currents and other innovative, though non-standard, processes and structures. These specialized processes and structures increase the cost of manufacture. Also, the large size of these structures poses a problem with integration with other components of the wireless radio.

Components such as the highly integrated transceiver are manufactured in the latest CMOS node to minimize their size and therefore the cost. It does not make economic sense to use a very expensive process to minimize the size of the transceiver circuit only to give away the benefits by adding large structures that do not require the same high-performance technology on the same die. CMOS PAs are best fabricated using a process tailored to their special needs.

Another point to consider is the suitability of transistors fabricated with the latest-generation CMOS for RF applications. As gate lengths shrink with each successive generation of CMOS technology, the transistors retain the property of behaving like switches (which is what is needed for digital applications) but they become increasingly non-ideal for processing analog signals (such as those encountered in the PA). In fact, it is often preferred to use an older version of CMOS for RF applications because transistor characteristics are more ideal.

The models used for circuit design are very different for RF circuits as compared to digital circuits. RF circuit models tend to be much more complicated, especially for power applications, because all the non-idealities of the transistor need to be accounted for including the deleterious effects of the conducting substrate. Accurate large-signal RF models for CMOS transistors are still not widely available. While this is not a fundamental problem, it does increase product development cost and the number of design spins needed to converge to an acceptable performance level.

Packaging is another area where the needs of components such as transceivers and power amplifiers differ substantially. Power amplifiers are typically packaged in laminate-based or low-temperature co-fired ceramic (LTCC)-based modules whereas transceivers are packaged in much simpler plastic BGA (PBGA) type packages. PA packages have only a few I/O ports but the module needs to accommodate low-loss RF matching circuitry and a few SMDs like capacitors and inductors. PBGA packages, on the other hand, are designed for large numbers (typically over 100) of I/O pins and have no place for external SMDs and RF circuitry. Although it might be possible to come up with a package that meets the requirements of both types of circuits, it is not a natural fit.

Instead of struggling to integrate the PA and transceiver, a more logical integration could occur in the RF section, bringing together the PA, voltage regulation, antenna switch, LNA and other components onto a single die to create a complete RF front end. This die could be packaged along with filters in a single, highly functional module. This approach simplifies handset/WLAN designs and allows the overall design to be nicely partitioned for maximum performance and lowest overall cost.

Performance challenges
While CMOS amplifiers have steadily increased in frequency and power handling capabilities, they still face challenges in satisfying the stringent performance requirements of each successive generation of wireless standards (3G, 4G, etc.). Even in applications where both CMOS and InGaP amplifiers are available, for instance, the InGaP devices tend to offer better linearity, efficiency, and harmonic performance at higher power levels. For example, Table-1 compares the high-band performance of the Si4300, a dual-band GSM CMOS PA from Silicon Labs with the performance of the AWT6166, a quad-band InGaP GSM PA from Anadigics. It is clear that the InGaP PA offers higher gain, higher efficiency, better isolation, lower harmonics and lower receive band noise than the silicon counterpart.

Figure1: DCS datasheet spec comparison between the AWT6166 and the Si4300 PAs.

Linearity is a critical concern in the current generation of amplifiers designed for both handset applications (W-CDMA, EDGE) and WLAN applications because the amplifiers are operated in a linear mode. PAs in GSM handsets, on the other hand, are run in a saturated mode. The challenge facing circuit designers is that it is difficult to achieve good efficiency and good linearity simultaneously. Linearity is needed to meet system specifications while efficiency is needed for extending talk time in mobile applications. When an amplifier is backed off from saturation, it becomes more linear and also less efficient. Typically, a CMOS PA has to back off from saturation so much farther than an InGaP PA for the same linearity that its efficiency becomes unacceptably low. This is perhaps the main reason why CMOS PAs are not widely available for 3G handsets and 801.11a/g WLAN products. A CMOS PA that meets the linearity specifications for WLAN, WCDMA and EDGE typically has poor efficiency because of that necessary design trade off.

Efficiency and harmonics
The power added efficiency (PAE) of a power amplifier is directly related to battery life and talk time, two key features for mobile handsets. As a general rule, the PAE of an InGaP PA is markedly better than that of a comparable one implemented in CMOS, often by 10 percentage points or more. A leading CMOS PA offers power-added efficiency (PAE) of 48 percent for GSM900 and 40 percent for the DCS1800 bands. An InGaP amplifier for the same application offers PAE of 55 percent and 53 percent respectively for these same bands.

At high power levels, such as when the handset is on the edge of the coverage area, this difference in efficiency can have a profound effect. The 7-13 percentage point increase in PAE translates to lower battery current drain and longer talk time, a key attribute desired by consumers. The trade-off in this design decision (InGaP or CMOS) is the cost differential between the two options. In spite of its lower expected cost, a CMOS PA remains unattractive because of its performance issues. Even with ultralow-cost handsets, phone manufacturers are unlikely to trade off talk time to marginally reduce manufacturing cost.

Meeting harmonics specifications is another challenging area for CMOS PAs. GSM applications, for instance, require typical second and third-order harmonics to be -22dBm to -25dBm. This poses a significant challenge to designers of CMOS PAs. Further bad news for CMOS is that mobile handset manufacturers are looking for PAs to go beyond these minimums and offer performance margins so that variations in their phone boards can be accommodated by the PA without adversely affecting overall yield.

For newer wireless standards such as EDGE and W-CDMA, GaAs has become the leading process technology for PAs. CMOS has found a home in a few less demanding wireless applications such as Bluetooth and ZigBee, where its performance attributes are good enough and higher integration levels can realized with the attendant cost savings.

InGaP HBT technology
Although less well known than CMOS, InGaP heterojunction bipolar transistor (HBT) technology has several characteristics that make it attractive in high-frequency applications. InGaP HBTs are formed in GaAs, which is well known in the RF industry as the substrate of choice for fabricating RF ICs. The reasons for this are simple: (1) the electron mobility of GaAs is roughly six times higher than of silicon, the substrate for CMOS; and, (2) the GaAs substrate is semi-insulating in contrast to the substrate employed in CMOS which is conductive. Higher electron mobility leads to higher-frequency operation.

GaAs ICs, for instance, are used for producing power well into the millimeter-wave bands. The only way for CMOS to improve frequency performance is to reduce gate length which leads to non-ideal transistor behavior unsuited to power amplifiers. The semi-insulating GaAs substrate permits better signal isolation and low loss passive elements (transmission lines, inductors, transformers) on the IC. This advantage is lost if the substrate is conductive. With CMOS, it is difficult to build functional microwave circuit elements, such as high-Q inductors and low-loss transmission lines due to the relatively high conductivity of the substrate. These difficulties can be partially overcome by performing several non-standard processes during IC fabrication that raise the manufacturing cost of CMOS devices and partially offset the main reason for using CMOS in the first place.

InGaP is particularly well suited for high-frequency applications requiring moderately high power outputs. Advances in InGaP processing techniques have improved yields and led to higher levels of integration, which is bringing additional functions (such as voltage regulation) onto the die. This simplifies the system design, reduces the bill of materials and saves board space. Some InGaP PAs are also offered in multi-chip packages that include CMOS control circuitry. Today, front-end WLAN modulesincorporating the PA and LNA on the receive end, as well as RF switchesare available in a compact package. One variation of InGaP, called InGaP-Plus, allows bipolar and field-effect transistors on the same InGaP die. This technology is being employed for the newer CDMA and WCDMA PAs with improvements in size and PAE.

Figure 2: Schematic of the layer structure used in InGaP-Plus. The pHEMTs are grown first on the GaAs substrate, then layers comprising the HBT are grown on top of the pHEMT.

Advanced silicon technology
SiGe has been called an evolutionary step for CMOS and a viable alternative to GaAs but, to date, it has been unable to find the proper combination of low-cost, high-yield production and performance to meet specific application requirements. OEMs considering SiGe should look for proven robustness that will be necessary to build reliable products.

For example, a concern for WLAN, CDMA, and GSM applications is ruggedness: Will the device be able to withstand the operational stresses of the application? One way to measure ruggedness is by the breakdown voltage at a given frequency. InGaP has a breakdown voltage of 14V in both the 2.4GHz and 5GHz bands. SiGe has breakdown voltages of 8V and 4V, respectively, giving InGaP the advantage in ruggedness for these applications. Silicon on insulator is another advanced silicon technology in use today. However, it is not a mainstream silicon process, so its cost advantage is compromised.

Undoubtedly, bulk CMOS and advanced silicon processes have made huge technical advances. Silicon is the technology of choice for baseband applications and it is a good fit for low-end PAs. However, for high-performance PA applications, GaAs remains the technology of choice to meet the stringent performance needs of the most advanced mobile handsets and wireless networking products.

In terms of integration road maps, if the integration path is to combine the transceiver, baseband and PA, then it will be necessary to use a silicon process. However, the industry is advancing on an integration path where the PA remains separate from the transceiver, and in different packaging, and GaAs is positioned to enable this path.

About the authors
Aditya Gupta
is VP of technology development at Anadigics Inc.

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