Leakage Current
In all wireless systems, if there is no data to transmit, the PA is disabled and ideally it consumes no power at all. However, unless a switch is placed in series with the supply voltage driving the PA (which is not attractive because of cost, size and power consumption), the power amplifier will always have a supply voltage applied to the collector (bipolar devices) or drain (FET devices).
While the PA can be ‘turned off’, in practice there is always a small amount of leakage current that flows even when the PA is disabled. This leakage current is a parasitic battery drain, and reduces standby times for mobile devices. Low leakage is often specified as a firm requirement in devices like handsets where standby time is important.
The requirement for low leakage is met with most technologies. GaAs HBT, SiGe HBT and CMOS power amplifiers can all be manufactured with low leakage currents, typically under 10 μA. The one technology that may have a problem with leakage current is PHEMT. These devices typically have leakage currents an order of magnitude larger than those manufactured with other technologies. The high leakage current seen with PHEMT PAs is intrinsic to this technology.
Technically speaking, a PHEMT gate looks like a diode, so the threshold voltage needs to be quite low (significantly less than a diode drop). As a result, with 0 V applied to the gate there can be appreciable leakage. Other technologies have insulated gates so threshold voltages are higher and leakage currents are much smaller.
The high leakage current of PHEMT devices is often cited as a reason not to use PHEMT technology for mobile devices. A PHEMT PA turned off and consuming a 100 μA leakage current would deplete a typical 1,000 mA-hr battery in 10,000 hours (417 days), and will have a minor impact on the mobile device’s standby time. While this seems to be a very small contributor, there are a large number of parasitic drains on the battery that all reduce standby time, and phone manufacturers wish to minimize each contributor.
So, for leakage current, the loser appears to be GaAs PHEMT. This is a factor in devices like mobile phones where standby time is important, but will be much less important in devices like laptops.

Front-end ICs
As Smartphones incorporating dual-band WiFi, multi-band cellular, GPS, FM and Bluetooth radios grow in popularity, it becomes increasingly difficult to fit everything into the required form factor. The RF front-end, comprising all components between the transceiver and antenna, can contribute significantly to the overall footprint. RF front-end vendors have responded, and the size of RF front-end components in communications devices has been continually shrinking.



Figure 4 Evolution of RF front-end sizes for WiFi radios.

Figure 4 shows a timeline giving an example of the degree to which integration has been applied to RF front-ends for WiFi, and one can see that integration has significantly reduced their footprint. In 2002, front-ends comprised unmatched PAs with many discrete devices, and the RF front-end had a size of about 16 x 18 mm. By 2005, front-end laminate-based modules were available incorporating discrete surface-mount components for matching, and the size had been reduced to about 8 x 7 mm. In 2007, many of these discrete matching components had been replaced by integrated passive devices, and one could now achieve the same functionality in a 4 x 4 mm module without the need for a laminate.
The next logical step in this integration process is to develop a front-end integrated circuit (FEIC), shown in the last photo in Figure 4, achieving a 3 x 3 mm form factor. FEICs offer the possibility of much greater levels of integration by integrating PAs, LNAs, switches and filters onto a single chip. Of course, the pattern of progressive integration has been repeated numerous times in the history of Silicon IC development. GaAs PHEMT and BiFET technologies are well suited for FEICs as they can be used to make excellent LNAs, PAs and switches.
As has been discussed, the SiGe BiCMOS process, at first glance, might not seem to be a great choice, since it is difficult to produce high quality, low loss switches with this technology. However, SOI switches are now available with performance rivaling GaAs switches. As a result, a SiGe BiCMOS process is also a highly suitable platform for FEIC development and one would expect significant growth in this area. In fact, the SiGe BiCMOS platform is even more compelling when considering the possible integration of battery management circuits onto the same die.
To summarize, for front-end IC development, CMOS and GaAs HBTs will not be suitable. GaAs PHEMT and BiFET processes, as well as SiGe BiCMOS processes incorporating SOI technology, are all good choices.


Power amplifiers with Serial Interfaces
Historically, PAs have been standalone, independent components. Even today, most PAs are controlled with only a single analog enable signal, often requiring precision regulators. In RF front-end modules where power amplifiers, low noise amplifiers and switches are all integrated into a single packaged device, routing the control signals from the baseband chip to the RF module can be very challenging, especially with the advent of multi-band and multi-PA MIMO technologies. For example, an 802.11a/b/g MIMO radio will require two 5 GHz PAs, two 5 GHz LNAs, two 2.4 GHz PAs, two 2.4 GHz LNAs, filters and Rx/Tx switches, each of which must be controlled separately.
A new trend that is emerging is to use a serial interface to control the PA and/or components within the RF front-end module. A serial-interface-controlled PA has the potential to revolutionize PA operation, bringing the digital interface one step closer to the antenna. In the context of complex front-end modules, the serial interface can reduce or eliminate control lines, greatly simplifying routing from the baseband chip. One could also use the serial interface to report temperature and detector voltages directly over the serial bus, thereby reducing pin-count and eliminating the need for A/D converters on the baseband chip.
Serial interfaces favor Silicon processes like CMOS and SiGe BiCMOS. Most GaAs processes lack complimentary devices (pFET or PNP transistors). As a result, it is not possible to implement significant logic or logic control like a truth table on a GaAs die. Therefore, HBT, BiFET, or PHEMT-based devices would all require an external CMOS logic die to properly implement a serial interface. Consequently, if serial interface control of PAs or RF front-ends is important, the logical choice is CMOS or SiGe BiCMOS.


Conclusion
There have been a number of important issues that have impacted the design of power amplifiers in recent years. This article has summarized several new issues, and has looked at how each affects the choice of technology for the power amplifier, particularly for PAs used with OFDM modulations. CMOS PAs are suitable for lower output powers, and require the use of digital adaptive predistortion to achieve linearity required for operation.
While GaAs HBT technology has traditionally been used for high power and high frequency power amplifiers, high performance SiGe BiCMOS power amplifiers are now competing very effectively with them. SiGe BiCMOS power amplifiers can be preferred to GaAs HBT PAs based on the availability of digital logic for serial interface control, as well as for the high levels of integration possible for front-end IC development. Consequently, GaAs HBT and GaAs PHEMT PAs will be used at progressively higher power levels and in more specialized applications. Slowly but surely, Silicon is progressing in the III-V versus Silicon battle on the power amplifier front.

Darcy Poulin holds a BS degree with honors in engineering physics from Queen’s University at Kingston, and a PhD degree in applied physics from McMaster University in Hamilton, Ontario, Canada. He brings to SiGe Semiconductor more than 15 years of experience in RF engineering and IC design. He is currently principal engineer, RF Systems and Technical Marketing, and is responsible for RF systems work, standards development, and technical marketing activities for WiFi, WiMAX and LTE.

Richard: 原来是SiGe公司的人写的，怪不得处处为Si辩护。Si大有可为，但那需要一个漫长的过程。很可能当CMOS与SiGe BiCMOS彻底解决一系列技术问题(如高衬底损耗、低线性度、低效率、低击穿电压从而低输出功率等等)的时候，GaAs技术的成本也一降再降了了。实际上据俺所知，目前GaAs HBT的GSM PA成本并不比CMOS的对手AX502/508贵多少，差别甚至可以忽略不计了。Anyway，有竞争才有进步。战斗吧！