At RFMD, we have upgraded our BiFET technology, replacing the JFET device that allowed integration of new DC circuits with a D-mode pHEMT. The result: more efficient products offering greater functionality.

Within the compound semiconductor industry, one of the most competitive markets is undoubtedly high-volume manufacture of GaAs-based chips. In this market, which is dominated by sales to handset manufacturers, chipmakers are only successful if they continually drive down average selling prices while developing smaller, high performance components.

Today, sales of wireless tablets and smart phones are ramping fast, and the health of the order book of a GaAs chipmaker is governed by their ability to produce devices with very high levels of integration that can be housed in tiny packages. Long-term profitability also hinges on keeping a step ahead of trends in the consumer market, to ensure that products are built into next-generation prototypes of compact, data-rich devices.

Innovation holds the key to launching products offering ever-higher levels of integration and smaller footprints. In the latter part of the last decade, many of the world’s leading GaAs chipmakers excelled in this area by combining an amplifier and a switch on the same chip. At RFMD, which is based in Greensboro, NC, we succeeded in this endeavour, combing a JFET and a HBT on a single die.

Staying at the forefront of GaAs technology demands continual improvement, and to this end we have recently developed a second generation of our BiFET technology, which we call our BiFET2 process. New products being designed around our technologically advanced BiFET2 process will essentially provide a ‘refreshed,’ or enhanced, version of our original BiFET1. Turning to the BiFET2 will equip products with greater functionality and a higher level of performance, including reduced current consumption and exceptional efficiency.

BiFET evolution

During the last decade, the backbone of the wireless handset front-end has been the pairing of a GaAs HBT power amplifier (PA) with a GaAs pHEMT, which is responsible for RF switching. Initially these chips were fed with a regulated supply, but this was subsequently replaced with either a silicon controller IC or a GaAs FET die, due to cost pressures. The multi-die silicon approach has many downsides, requiring large design teams, high assembly costs, increased wire bonding and an increased size for the overall end product. So it is of little surprise that BiFET technology, which addresses all of these concerns, is popular.

Our first-generation BiFET employed a HBT alongside a simple JFET device, enabling internal voltage regulators to be created from this device [1]. Due to the need to incorporate an RF switching capability and DC regulation requirements, we, along with our competitors, have upgraded technology to next-generation BiFETs [2-4]. Our BiFET2 technology unites our latest generation InGaP HBT and D-mode pHEMT on a single die (see Figure 1 for a comparison of our two generations of BiFET technology).

 



Figure 1. RFMD has upgraded its BiFET technology. The significant changes introduced for the second-generation technology are the replacement of the JFET with a D-mode pHEMT and additional optimization to the HBT. The impact on manufacturing that results from this new generation of BiFET is the ability to independently optimize and control the HBT and pHEMT performance

 

One of the great strengths of the BiFET is its short development time. This stems from elimination of silicon control chips: we find that wafer fabrication cycle times for silicon are up to four times longer than those for our inhouse manufacture of GaAs wafers.

The concept of technology re-use has dominated our thinking, right from our initial steps in research and development through to our final refinements of a highvolume process delivering high yields. We decided that the building blocks for our BiFET2 would be our first generation D-mode pHEMT and second generation InGaP HBT.

By leveraging existing, well-known, high-yielding device technologies, we are able to reuse the epi-capability and the fab processes that went into those original process technologies. Taking this approach enables us to increase BiFET process yields and take our end-products to new levels of performance and functionality.

 

Better BiFETs, better products

Our Cellular Products Group and Multi-Market Products Group have both designed products that exploit the benefits of the BiFET2. In the Cellular Products Group, this technology has given our designers the opportunity to integrate more functionality into the 3G product portfolio, empowering us to deliver industry-leading performance.

For example, the BiFET2 technology has underpinned our development of the RF722x product family. These pin-forpin compatible WCDMA PA modules, which are housed in compact 3 mm x 3 mm packages, cover all the major frequency bands and are optimised for chipsets using three-mode control schemes. Turning to three digital modes of operation cuts current consumption as power levels decrease and yields best-in-class performance (see Figure 2 for the block diagram for the RF722x product family).

 





Figure 2. RFMD’ s RF722X family delivers incredibly low levels of power consumption at low power levels,  thanks to the introduction of three digital modes of operation

 

In the RF722x family of 3G products, our designers isolate each mode independently, thanks to the integration of the FET devices with the HBT PA. This is possible because of the high impedance of the FET switching device, and has paved the way to improved performance at each mode: The optimum matching impedance can be set in every case without trading-off performance in other modes. The upshot is minimization of current drain in each mode of operation, culminating in longer battery life (see Figure 3 for a detailed block diagram of the RF matching and switches).

 



Figure 3. The RF722x family of 3G products isolate each mode independently

 

The Cellular Products Group has also used the BiFET2 product family to create the RF724x portfolio of WCDMA PAs, which address all major frequency bands and deliver exceptional peak efficiency and current through multi-bias control.

This family of WCDMA products that is accommodated in the industry standard 3 mm x 3 mm package delivers peak linear efficiencies of 48-51 percent. In comparison, the current industry standard is just 40 percent. The pin out for the RF724x is identical to the RF722x shown in Figure 2, but the functional block diagram only has one RF PA line-up similar to the high power path in RF722x.

One of the biggest differences between the RF724x and the RF722x series is design: the latter achieves efficiency optimisation through bias circuit control and the former realises its performance through RF switching and matching. In both cases the product family delivers a significant step up in performance over its predecessor thanks to incorporation of BiFET2 technology. When paired with an RFMD companion DC-DC converter, this BiFET2 product delivers best-in-class current consumption while seamlessly integrating into platforms where General Purpose Input/Output (GPIO) two-bit control is utilized.

The user provides the PA and DC-DC converter with Vmode 0 and Vmode 1 logic control, and the collection of devices optimises collector voltage and bias voltage to deliver unprecedented performance. The output power versus current for the RF722x and RF724x is shown in Figure 4, where the RF724x PA is paired with the RF6562 DC-DC buck converter (the reference design is referred to as RFRD6562).

 





Figure 4. The latest BiFET technology enables increased battery life. This plot shows the output power versus current for the RF722x and RF724x. The latter is paired with the RF6562 DC-DC buck converter, and the reference design is referred to as RFRD6562.

 

A useful metric for comparing products serving the WCDMA protocol is the DG09 curve, which provides an estimate of the probability density of varying output powers for WCDMA voice modulation (see Figure 5 for an example). A similar approach has been used in the past to construct a CDG curve, which was developed for CDMA based mobile operators in search of a more adequate depiction of what was really happening with a mobile phone’s output power during typical operation. GSMA has quickly adopted a similar metric, as the expansion of WCDMA modulation showed similar attributes. Designers can efficiently compare WCDMA solutions by looking at the DG09 probability density function and the swept output-power-versus-current data that is collected. The DG09 for the RF722x and the RF724x with DC-DC converter (RFRD6652) are listed in Table 1. Further improvements are possible. The DG09 current of 21.6mA that is realised with the RF6562 converter can be reduced to just 13 mA, by introducing a continuously varied DC-DC converter.

 



Figure 5. One suitable approach to comparing products serving the WCDMA protocol is the DG09 curve, which estimates the probability density of varying output powers for WCDMA voice modulation

 

In the Multi-Market Products Group, a representative BiFET2 product under development is the RF5612, a linear PA module housed in a 4 mm x 4 mm laminate package that is designed for WiMAX and LTE applications. Targets for the RF5612 include 25 dBm power at 2.5 percent error vector magnitude (EVM) for a 3.3 V supply voltage. BiFET technology is widely used in the RF5612. It provides RF and DC switching and results in a product capable of dual-mode operation. The transition between the high-power and low-power modes occurs at power levels of 8 dB. In low-power mode, an RF switch connects the output of the first stage directly to the output pin, cutting current consumption thanks to DC switches that turn off the bias to the second and third stages. The pin out and block diagram for the RF5612 PA module is shown in Figure 6, and Figure 7 compares the EVM, current and gain – all as a function of output power – for both modes of operation. The plots reveal that the overall module current drawn in low power mode is less than 100 mA compared to more 500 mA at full power. Gain steps from 36 dB to 7 dB over this range.

 





Figure 6. The RF5612 features RF and DC switching and is capable of dual-mode operation

 



Figure 7. Plots of the current (a) error vector magnitude (b) and gain (c) for the RF5612 PA module. The black, red, and green represent frequencies 2.5, 2.6, and 2.7 GHz, respectively

 



Table 1. DG09 current for RF722x vs. RF724x with DC-DC converter (RFRD6652)

Our development of the RF5612 PA module, plus that of other products detailed above, highlights our efforts to exploit BiFET technology in new products where integration, performance and small footprint are required. Cutting-edge manufacturers and developers of wireless products continually pushing the envelope for performance, size, cost, functionality and customer easeof- use welcome this endeavour.

This article was written by RFMD’s Bob Baeten, Director of Engineering, Multi Market Products Group and RFMD’s Jackie Johnson, Director of Engineering, Cellular Products Group.

© 2011 Angel Business Communications. Permission required.

REFERENCES

[1] Brian Moser, et al. RFMD gives the BiFET a new twist, Compound Semiconductor, November 2008

[2] William Peatman, et al., InGaP-Plus: Advanced GaAs BiFET Technology and Applications, CS MANTECH 2007

[3] Ravi Ramanathan, et al., Commercial Viability of a Merged HBT-FET (BiFET) Technology for GaAs Power Amplifiers, CS MANTECH 2007

[4] C. K. Lin, et al., Monolithic Integration of E/D-mode pHEMT and InGaP HBT Technology on 150-mm GaAs Wafers, CS MANTECH Conference 200.