With further advancements in InP technology, Keysight augmented the handheld lineup with a 50 GHz model in 2015.¹¹ The broadband linearity and noise performance of InP technology enabled lab-grade performance in handhelds, rivaling that of benchtop instruments. As a result, handhelds have become more than an installation and maintenance solution. Handheld instruments are used in wide-ranging applications in communications, aerospace and defense and even in the medical field for research into early breast cancer detection.¹²

InP HBT usage in the design, emulation and test market is not limited to the two previous applications. This technology permeates a significant percentage of high-end test and measurement solutions in the industry. This will continue to be the case in the future.

INP MATERIAL AND PRODUCTION CHALLENGES

Despite the advantages of InP technology, adoption of RF applications has not been widespread. The primary reason for this is cost. Cost is driven largely by raw material availability, difficulty procuring quality large-diameter wafers and challenges in MMIC fabrication.

Indium is produced exclusively as a byproduct of processing other metal ores, mainly sulfidic zinc. China is the leading producer, followed by South Korea, Japan and Canada.¹³ With these limited sources of production, indium is on the list of critical minerals for the U.S., Canada, Australia and the European Union.¹⁴ According to the European Chemical Society, the indium supply will be exhausted in 50 years at the current rates of use.¹⁵ Solving these raw material supply issues requires the discovery of previously unknown reserves or finding new ways to collect or recycle and then purify the element.

The availability of high-quality, large-diameter substrates also influences the price of InP RF devices, with most InP wafers being 3 or 4 inches in diameter. As early as 2002, 6-in. InP wafers were announced,¹⁶ but these larger diameter wafers still prove difficult to produce, limiting supply. In contrast, 6-in. diameter GaAs wafers are standard with some 8-in. wafers being shipped commercially.¹⁷ From a cost-per-area standpoint, InP wafers can cost 10x to 20x more than GaAs wafers by the time epitaxial layers are grown on substrates.

A third factor contributing to the cost of InP for RF applications is the difficulty of manufacturing MMICs. InP substrates are very brittle when compared to GaAs substrates and even more so when compared to silicon. In wafer processing steps, extreme care must be taken when handling and managing the layer stresses to avoid wafer fractures. With these difficulties, very few InP RF device fabs exist, especially at a commercial level. Though not exhaustive, a 2019 report from Yole lists only six fabs worldwide.⁶

INP HBT FUTURE APPLICATIONS AND CHALLENGES

Despite the challenges, numerous industries are using InP RF devices in narrow applications and continue to investigate future use cases. An appealing property of InP HBTs is their ability to enable high frequency power amplifier (PA) designs with exceptional power-added efficiency (PAE). The performance advantages of InP HBTs enable high frequency circuits operating at large current and power densities that achieve world-leading PAEs. A survey of PAE versus frequency for amplifiers operating over 100 GHz is dominated, almost exclusively, by InP HBT-based designs. These results are shown in Figure 8.¹⁸

Figure 8 Reported PA PAE results for Si CMOS, SiGe HBTs, GaAs pHEMTS, GaN HEMTs and InP HBTs. Figure from DARPA ELGAR BAA and attributed to Buckwalter, private communication, 2021.¹⁸

For over a decade, researchers contemplated using InP HBTs as GaAs PA replacements for handsets,¹⁹ but high cost and lack of availability thwarted this effort. These amplifiers operate below 7 GHz. With more applications requiring frequency bands well above 7 GHz, researchers are once again considering InP.

One example is next-generation, G-Band (110 to 300 GHz), high speed wireless communication. G-Band is advantageous because it is a mostly unused portion of spectrum. While some commercial applications and associated components operate at these frequencies, the vast majority are at lower frequencies. A second advantage is that these frequencies support extremely high data rates. With a higher carrier frequency, more bandwidth can be allocated to a communication channel, enabling higher data rates. Lastly, there are local minima in the atmospheric absorption of microwave radiation at 140 and 220 GHz as shown in Figure 9. Using these windows, a communication system can transmit signals farther.

Figure 9 Atmospheric attenuation in rain and sun.²⁰

In 2021, DARPA initiated the ELectronics for G-band ARrays (ELGAR) program targeting high data rate communications.¹⁸ This program has a goal of producing a 200 mW PA operating at 30 percent PAE. InP HBT technology is a prime candidate for this PA performance. Achieving these goals requires InP semiconductor processing advances that allow on-chip interconnect chassis to support the full native transistor performance. The ELGAR Broad Agency Announcement states that current III-V on-chip interconnects are “large and inefficient, leading to passive circuits…that are physically large and lossy.”

Even with advances in the InP HBT interconnect chassis, the challenge of integrating a MMIC into a system with minimal performance impact still exists. At 100 GHz and above, traditional chip and wire and PCB technologies do not work well due to parasitics. Even short wire bonds add inductance that adversely affects high frequency performance. To overcome this limitation, manufacturers are using flip-chip technology with solder bumps to attach the die.

Technologies that allow high frequency signal propagation to signal conditioning or processing blocks with minimal degradation extend beyond flip-chip attachment. These technologies include chip stacking and high performance interposers. While both technologies are used in low frequency applications, operation above 100 GHz has not been realized commercially. Besides maintaining signal integrity, these technologies will be important to meet integration challenges. For the ELGAR program targeting 220 GHz operation, an antenna array with associated transmit/receive electronics spaced on a λ/2 grid would require ~0.68 mm spacing.¹⁸ This constraint demands progress beyond the most advanced technologies to date.

CONCLUSION

While numerous challenges have limited the widespread adoption of InP HBT technology, its applicability in the design, emulation and test market has been a success story. The technology has enabled state-of-the-art instruments that have provided a giant leap in performance. With future wireless technologies looking to higher frequency V-, W- and G-Bands, InP HBTs are poised to make a broader commercial breakthrough. To realize the potential of InP HBTs, economic challenges need to be overcome and fabrication, packaging and integration technologies need to evolve. Perhaps in a decade or less, InP HBT usage in RF devices will expand beyond the design, emulation and test market to play a larger role in products we use every day.

References

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