As the world continues to strive for higher speed connections and requires low latency and high reliability, the energy consumption of ICT continues to soar. These market demands not only bring 5g to many key applications, but also put forward restrictions on energy efficiency and performance. The 5g network performance goal puts forward a series of new requirements for basic semiconductor devices, increases the demand for highly reliable RF front-end solutions, and improves energy efficiency, greater bandwidth, higher working frequency and smaller floor area. Driven by the large-scale MIMO system, the number of semiconductor devices in base station radio has increased sharply, and mobile network operators are facing more severe pressure in reducing capital expenditure and operating expenditure. Therefore, limiting equipment cost and power consumption is very important for the installation and operation of efficient 5g network.

The RF power amplifier (PA) deployed in the modern 5g radio architecture plays an important role in meeting the obvious contradiction between higher performance and lower cost. Although LDMOS technology dominated the RF power amplifier of wireless access network in the previous cellular standards, this situation is changing with the implementation of 5g. Gallium nitride is a strong competitor because of its excellent RF characteristics and significantly lower power consumption. However, it should be noted that silicon carbide based gallium nitride, which is mainly used for the new 5g active antenna radio, is still one of the most expensive RF semiconductor technologies due to its non mainstream semiconductor technology. This limits its potential to achieve large-scale economic benefits. In contrast, silicon-based gallium nitride realized through standard semiconductor process combines two advantages: competitive performance and huge economies of scale. In this paper, we will explain how the progress of silicon-based gallium nitride makes this technology a very strong competitor of RF power amplifier in 5g radio.

5g requirements

The surge of digital social media, video calls with great bandwidth demand and heavy Internet use on mobile devices are increasing the demand for high-performance 5g wireless networks to provide sufficient coverage and quality of service. During the COVID-19, this trend intensified. Therefore, operators are promoting 5g below 6GHz as an effective way to cope with this exponential growth in data consumption. However, the push for higher data rates has had a huge impact on the global energy bill, and ICT is expected to grow to 21% of global energy consumption. one

From the perspective of RF radio, the new 5g function is transformed into more challenging RF characteristics. Higher carrier frequency up to 7GHz, instantaneous bandwidth greater than 400MHz, higher-order modulation mode, more channel number and MIMO antenna configuration are some of them. In addition, as radios become more complex, the need to keep weight and power consumption to a minimum has never been so important, both of which require higher energy efficiency to save energy and cooling equipment costs. RF power amplifier is still the key equipment in 5g MIMO radio and the last active device before wireless transmission. Up to 50% of the energy consumption of the base station is here. 3 modern semiconductor technology for RF power amplifier needs to meet some harsh conditions to meet the requirements of 5g and pave the way for the future generation.

In this case, gallium nitride has become the leading high-power RF power amplifier technology of 5g MIMO radio due to its excellent RF performance. However, the current implementation cost is too high. Compared with silicon-based technology, gallium nitride is grown on expensive III / V SiC wafers. With expensive lithography technology, the production cost is particularly high. At first, we tried to grow gallium nitride on silicon wafer, but it was not adopted by the market because of its poor performance and no cost advantage. That is changing. In this paper, we describe a new silicon-based gallium nitride technology running on an 8-inch process, which meets all technical requirements and provides commercially attractive economic benefits.

RF power amplifier technology

LDMOS – LDMOS FET (Figure 1) was introduced from the late 1960s to the early 1970s to improve the breakdown voltage of power MOSFET. 4 the performance, robustness and ease of use of transverse diffusion structures 5 and 6 exceed that of silicon bipolar transistors. LDMOS became the mainstream RF power technology in the 1990s.

In the past 30 years, LDMOS has been the standard technology of high-power transmission stage in wireless infrastructure, with excellent performance below 3GHz. Before the emergence of GaN HEMT, LDMOS has been difficult to be replaced in the wireless base station market because of the inherent cost advantage of manufacturing devices on 8-inch silicon substrate and full compatibility with standard silicon process.

poYBAGHxDo2Af57wAAC3Q8M7T3Y506. png

Fig. 1 functional cross section of LDMOS device.

pYYBAGHxDpmAKfzjAABcfPuYXJ8639. png

Fig. 2 functional cross section of GaN HEMT device.

pYYBAGHxDqOAPXd8AAEd2rTsoYk380. png

Figure 3 PSAT and PAE of various PA technologies, measured in the range of 2 to 6 GHz. eleven

poYBAGHxDq2Ac4F9AAFjPpUwNb8040. png

Fig. 4 Relationship between load pull drain efficiency and pout of encapsulated 5.8mm silicon-based gallium nitride transistor.

SiC Based gallium nitride – born in the DARPA program in the early 2000s, 7,8 this program is after the successful gallium arsenide MMIC program in the 1970s and 1980s. 9 gallium nitride RF devices (Figure 2) are developed to meet the requirements of military applications (such as radar) for high power, wide bandwidth and high frequency.

Compared with LDMOS, gallium nitride has the inherent advantages of higher critical electric field and maximum carrier density in the channel, which means higher power density, higher impedance at a given output power, and lower efficiency with the increase of frequency. The attractive properties in military applications also make gallium nitride attractive in wireless infrastructure, 10 especially the combination of high power density – usually five times that of LDMOS transistors – and low parasitic capacitance, which enables the device to support a wider modulation bandwidth.

The market trend towards higher frequencies is also conducive to gallium nitride transistors, which can maintain higher peak efficiency with the increase of power and frequency. As shown in Figure 3, the efficiency of Gan power amplifier can exceed 80% even if it exceeds 2GHz. This efficiency advantage is becoming more and more important for 5g and future communication systems.

Silicon based gallium nitride – cost has been a major factor limiting gallium nitride’s use in cost sensitive applications such as wireless infrastructure. This is especially true for applications at 2GHz and lower frequencies, because the performance gap between LDMOS and Gan is not obvious in this frequency band. In order to solve the high cost problem of SiC Based Gan, people have been pursuing the growth of Gan on Si substrate since the beginning of the 21st century. The main challenges in performance and reliability relate to the difficulty of growing high-quality Gan on Si substrate due to lattice mismatch. In the past 10 years, a large number of research and development, especially in power conversion applications, have produced many improved EPI quality, and then released many silicon-based gallium nitride products, even for industrial applications. twelve

Current situation of silicon based gallium nitride

Despite this progress, several challenges need to be overcome to prove that the performance of silicon-based gallium nitride is equivalent to that of SiC Based gallium nitride and has good reliability. Infineon has developed silicon-based gallium nitride technology for RF power, which can give full play to its potential. After years of development, silicon-based gallium nitride is ready to become the mainstream technology. The most important criteria for determining maturity – performance, thermal resistance, reliability and cost – will be discussed in the following chapters.

RF performance – one of the most important performance parameters driving the replacement of LDMOS is RF efficiency. Figure 4 shows the 2.7ghz load traction measurement results of a packaged transistor with a gate periphery of 5.8mm and a bias voltage of 28V. At the 3dB compression point (p3db) indicated by the circle, the peak drain efficiency is about 85%, the peak output power density exceeds 5.5w/mm, and the performance is equivalent to that of SiC Based Gan. The contour shows that the efficiency from deep deviation to near saturation is quite stable, which makes the device technology suitable for Doherty PA.

Thermal resistance – a fundamental difference between silicon-based gallium nitride and silicon carbide based gallium nitride is thermal resistance, which reflects the difference in thermal conductivity between silicon and silicon carbide substrate. SiC Based gallium nitride has better thermal conductivity. However, through wafer thinning and device layout, 32V biased silicon-based gallium nitride transistors and 48V silicon carbide based gallium nitride devices can achieve the same junction temperature. By extension, assuming that the failure mechanism is similar, silicon-based gallium nitride devices operating at lower voltage will achieve the same reliability as silicon carbide based gallium nitride devices.

Reliability – device failure and drift are two factors to evaluate device reliability. The mean time to failure (MTTF) is determined by the failure mechanism and depends on the device temperature (Fig. 5). At lower temperatures, the MTTF of silicon-based gallium nitride transistors is limited by electromigration. However, electromigration is independent of the Gan transistor itself and is determined by the metallization and layout of the device. MTTF caused by electromigration can be prolonged by changing the layout. Infineon silicon-based gallium nitride device adopts copper metallization commonly used in silicon process, which has high robustness to electromigration. MTTF reaches 108 hours at 150 ℃.

poYBAGHxDuuAS-UcAABPkn4ePB0373. png

Fig. 7 Relationship between pout drift of silicon-based gallium nitride and HTRB time.

poYBAGHxDvKAX1dSAABnk0ld3pg237. png

Figure 8 block diagram of single-stage Doherty PA.

When evaluating the drift of this technology, Figure 6 shows the IDq drift of the device at 25 ℃ and 100 ℃, the bias voltage is 10mA / mm, VDS = 28V. It is inferred that the IDq drift will be less than 25% after 10 years. Figure 7 shows the decay of output power over time of a 20 mm packaged transistor under high temperature reverse bias (HTRB) pressure test. The bias voltage of the device is VGS = – 15V, VDS = 100V and the temperature is 150 ℃. At 1000 hours of HTRB pressure, the output power decreases by less than 8%.

Cost – the unit area cost of SiC Based gallium nitride devices is determined by the processing cost of SiC substrate and III / V typical small wafer. In contrast, Infineon’s silicon-based gallium nitride is implemented on a standard 8-inch silicon wafer, so it is compatible with other silicon wafer production. Silicon based gallium nitride wafer production adopts modern eight inch silicon production equipment, making use of the inherent integration, performance, output and supply chain infrastructure of silicon. RF integration leading to more complex MMICs is a long-term trend, so the unit area cost of mass production of silicon wafers is still an important distinguishing factor.

Silicon based gallium nitride PA module

The key performance parameters of wireless infrastructure power amplifier module (PAM) include power increase efficiency (PAE) at rated RF output power, dynamic peak output power, and linearization ability in frequency division duplex (FDD) and time division duplex (TDD) modes.

One trend of RF power of each antenna unit in active antenna system (AAS) is to increase the nominal linear output power of PAM from 3W to 8W, and may increase to 12W or even higher. The size change of frequency and antenna array limits the size of PAM, so it should be suitable for the component spacing on RF printed circuit board (PCB) to minimize the system cost. Power Gan technology supports this compact size because it can withstand higher junction temperatures.

In order to evaluate the capability of Infineon silicon-based gallium nitride technology, a single-stage Doherty PAM was designed on a multilayer organic laminated substrate, with an average modulation linear power of 39dbm in the 3.4-3.6ghz band (Fig. 8). In Doherty’s design, the input signal is divided into two parts, which enter the “main pipe” and “peak tube” amplifiers respectively, and are combined at the output end through a 90 degree phase shifter. Measurement conditions, 28V bias voltage, single tone signal input, room temperature, the relationship between PAM gain and drain efficiency (DE) and output power was measured (Fig. 9). At the output of 39dbm, including 3dB splitter, combiner and other passive losses, 10.5db power gain is realized. The measured maximum output power is 47.5dbm.

5g NR modulation waveform with peak to average ratio of 7.5db (peak clipping and filtering) is used, and the rated RF working power is 39dbm. The first peak of De is near this point to ensure the minimum deviation between modulated de and single tone De. Single tone De is 52% to 54%. The performance of silicon-based Gan PAM is comparable to that reported by SiC Based Gan. 13-15

pYYBAGHxDw6AKm8dAAD9T2zOnHQ588. png

Fig. 9 relationship between measured gain (a) and de (b) of single-stage Doherty PA and input power.

poYBAGHxDxeAJuFPAAC4VmCT6yM380. png

Figure 10 gain and pout of Doherty PA with 3.6ghz modulation signal, performance without DPD calibration (blue) and performance after DPD calibration (red).

The dynamic peak power of PAM with modulated signal and using digital predistortion (DPD) was measured at 3.6 GHz using a spectrum analyzer (FIG. 10). The measured peak power is 47.5dbm. This figure compares the modulation am-am dependence with and without DPD, and shows that DPD produces excellent linear output characteristics. The ability of DPD to linearize PAM reflects the low nonlinearity of devices and the low memory effect of circuits and devices. It is an important feature of device technology and amplifier design that it is easy to realize linearization by using DPD engine on the market.

pYYBAGHxD0aAUxNTAACDL7vfXhc530. png

Figure 11 Doherty PA spectrum measured using DPD without long-term memory model in FDD and TDD modes.

The outdoor applications of the PAM are FDD and TDD base stations. Due to the diversity of 5g standard of 3GPP, the time chart of transmission signal may be quite complex and irregular, and single symbol transmission is possible. Thermal, charge capture and video bandwidth determine the dynamic response of PAM, which is represented by different output power and error vector along the symbol sequence in a transmission subframe. To illustrate this, FIG. 11 plots the power spectrum of the first symbol of a transmission sequence, showing the performance of DPD without long-term memory model in FDD, hybrid and TDD modes. VC refers to the clamping voltage or off stage gate bias. The following modulation signals are used for the measurement of TDD mode: 3gppd TM3 1a,1 × 20 MHz channel, 5g NR OFDM, 256-qam, 60KHZ SCS and 7.5db par.

Trends and challenges

With the increase of RF transmission power, thermal management becomes more and more important. For MIMO AAS, there are several thermal management considerations: 1) system overheating leads to degradation of component performance and long-term reliability, 2) low energy efficiency and high operation cost, and 3) passive heat dissipation of radio system.

Although discrete modules can provide better heat management through lower packaging density, they will bring bottlenecks in BOM and PCB size in larger AAS products, which requires a lot of design optimization by system integrators. Controlling chip thickness, using appropriate chip connection technology and welding PAM to PCB are the key to heat dissipation. Maintaining a nearly constant output power over a certain temperature range requires a small design margin and produces a high PAE. The power gain coefficient of Infineon’s silicon based Gan PAM is -0.02db / ℃, which is equivalent to that of SiC Based Gan and LDMOS PA.

Wider instantaneous bandwidth and the use of frequency bands above 5GHz are two other market trends, leading to more integrated PAM solutions on GaN. Infineon’s silicon-based gallium nitride technology is capable of MMIC integration, which brings great benefits. It can not only meet the output power specification, but also overcome the performance limitations caused by the parasitic effects of cascaded discrete devices, transistors and bonding lines, which usually leads to reduced bandwidth and energy efficiency.

Summary

This paper discusses the development of gallium nitride based wireless infrastructure technology to improve the cost performance of gallium nitride. After years of development, the technology has matured and can play its potential to provide the same efficiency as silicon carbide based gallium nitride at a lower cost on the basis of silicon wafer processing. Silicon based gallium nitride can meet the efficiency, linearization and power density requirements of 5g wireless communication system. We believe that this is the beginning of a long journey. The further development of the industry will push the capability of silicon-based gallium nitride to a higher frequency and higher power level, which may be extended to applications other than wireless infrastructure.

reference

1. N. Jones, “How to Stop Data Centres from Gobbling Up the World’s Electricity,” Nature 561, 2018, pp. 163–166, doi. org/10.1038/d41586-018-06610-y.

2. 3GPP, Release 16, www.3gpp. org/release-16.

3. “5G Power White Paper,” Huawei Technologies Co. Ltd., https://carrier.huawei.com/ ~/media/CNBG/Downloads/Spotlight/5g/5G-Power-White-Paper-en. pdf.

4. Y. Tarui, Y. Hayashi and T. Sekigawa, “Diffusion Selfaligned MOST; A New Approach for High Speed Device (1.Electrotechnical Lab.)” https://doi.org/10.7567/SSDM.1969.4 –1.

5. A. Wood, C. Dragon and W. Burger, “High Performance Silicon LDMOS Technology for 2 GHz RF Power Amplifier ApplicaTIons,” IEEE InternaTIonal Electron Devices MeeTIng (IEDM), 1996, pp. 87–90.

6. H. F. F. Jos, “Novel LDMOS Structure for 2 GHz High Power BasestaTIon Application,” European Microwave Conference, 1998, pp. 739–744.

7. M. Rosker, “The Wide and the Narrow: DARPA/MTO Programs for RF Applications in Wide Bandgap and Antimonide-based Semiconductors,” IEEE Compound Semiconductor Integrated Circuit Symposium, 2005, pp. 4, https://doi.org/10.1109/CSICS.2005.1531739.

8. “Wide Band Gap Semiconductors for RF Applications,” Federal Grants, https://www.federalgrants.com/Wide-Band-Gap-Semiconductors-for-RF-Applications-WBGS-RF-1240.html.

9. E. Cohen, “The MIMIC Program——A Retrospective,” Microwave Magazine, June 2012, pp. 77–88. https://doi.org/10.1109/MMM.2012.2189989.

10. B. Green, K. Moore, D. Hill, M. CdeBaca and J. Schultz, “GaN RF Device Technology and Applications, Present and Future,” IEEE Bipolar/BiCMOS Circuits and Technology Meeting (BCTM), 2013, pp. 101–106, https://ieeexplore.ieee.org/document/6798154.

11. H. Wang, T.-Y. Huang, N. S. Mannem, J.Lee, E. Garay, D. Munzer, Ed. Liu, Y. Liu, B. Lin, M. Eleraky, H. Jalili, J. Park, S. Li, F. Wang, A. S. Ahmed, C. Snyder, S. Lee, H. T. Nguyen and M. E. Duffy Smith, “Power Amplifiers Performance Survey 2000-Present,” Georgia Tech Electronics and Micro-System Lab (GEMS), https://gems.ece.gatech.edu/PA_survey.html.

12. T. Detzel, A. Charles, G. Deboy, O. Haeberlen and T. McDonald, “The Commercialization of GaN Power Devices: Value Proposition, Manufacturing, and Reliability,” Compound Semiconductor Week (CSW), 2019, pp. 1–1, https://ieeexplore.ieee.org/document/8819303.

13. S. Sakata et al., “A Fully-Integrated GaN Doherty Power Amplifier Module with a Compact Frequency-Dependent Compensation Circuit for 5G massive MIMO Base Stations,” IEEE/MTT-S International Microwave Symposium (IMS), 2020, pp. 711–714, https://ieeexplore.ieee.org/document/9223897.

14. K. Moore et al., “High Performance 150 mm RF GaN Technology with Low Memory Effects,” 2020 IEEE BiCMOS and Compound Semiconductor Integrated Circuits and Technology Symposium (BCICTS), 2020, pp. 1–4, https://ieeexplore.ieee.org/document/9392951.

15. P. Saad, R. Hou, R. Hellberg and B. Berglund, “An 80 W Power Amplifier with 50% Efficiency at 8 dB Power Back-off over 2.6-3.8 GHz,” 2019 IEEE MTT-S International Microwave Symposium (IMS), 2019, pp. 1328-1330, https://ieeexplore.ieee.org/document/8701113.

Leave a Reply

Your email address will not be published. Required fields are marked *