Authors

Ashley Jun, Rafael Perez Martinez, Srabanti Chowdhury

Abstract

5G will play an important role to keep up with the exploding demand of wireless networks at an even better quality (rate and efficiency) that is seen today. Currently, there are too many devices on the bandwidth of frequencies operating on 4G and its prior generations. As such, we are running out of spectral capacity. This comes from the past two technological waves that began starting in the 1980s. The answer does not come from minimizing device usage and cutting back on how we are using the bandwidth of frequencies already being used because there is no means of returning to a time without the technological advances we have today. Instead, the approach that 5G took was to move to higher frequencies (e.g., the mm-Wave spectrum), opening a new realm of applications in imaging, sensing, radar, communication, etc. This enabled the third wireless revolution where innovation is no longer inhibited by the lack of wireless networks and the capacities they can hold. With the great possibilities opened by 5G, the question lies of why this hasn’t been implemented if it is the solution many wireless network companies are shifting toward, how it is implemented, and what are the challenges of implementation. 
With the dominant usage of silicon and conventional III-V materials in the devices that have enabled the wireless networks up until today, experts say that we are on the brink of hitting an end to the capacity of the current semiconductor technology. This is when new semiconductor materials are being researched extensively to solve the needed expansion. In this work, the capacities of materials such as GaN, GaAs, InP, SiGe, Gallium oxide, etc., will be compared to present semiconductor technology to understand if these technologies are enough for 5G applications and beyond. The challenges of this becoming an accepted shift in the materials used in wireless networks surrounds the reality that the semiconductor industry has been used for decades with billions of dollars of investment to get us where we are today. For these other materials, there are misconceptions of 5G and unknowns of the possibilities because we haven’t seen the same kind of investment into these new technologies to where we experienced two major technological waves. The combination of understanding the foundations of 5G and comparisons of semiconductor technology provides insight of answers of how the future of wireless networks will work and if the technologies investigated today are enough for our issues with not enough spectral capacity and beyond.

Background

Ever since 1980, there have been four generations of cellular systems, and 5G is projected as the newest addition to open a new avenue for technological advances. 1G was for voice communications consisting of a basic analog system that was known as the analog FM cellular system. 2G moved into digital technology with modulations with code divisions of multiple access, which improved spectral efficiency. In 2001, 3G jump-started the mainstream of technology with high-speed Internet access as well as improved capabilities of both video and audio streaming. 3G brought upon technologies and regulations that were used worldwide such as Wideband Code Division Multiple Access(W-CDMA) and High-Speed Packet Access (HSPA) which included mobile telephone protocols: High-Speed Downlink Packet Access (HSDPA) and HighSpeed Uplink Packet Access (HSUPA). Examples of the expansion can be seen with over 350 communication service providers over multiple frequency bands. However, 4G vastly increased mobile communications technology. 4G LTE enabled a scalable transmission bandwidth up to 20 MHz from radio access technology, which has offered a fully 4G-capable mobile broadband platform. One key technology that allowed for the high data rates was Multiple-Input Multiple-Output (MIMO). MIMO accommodated high spectrum efficiency for multi-stream transmission as well as other improvements of link quality and adaptive radiation patterns for the signal gain and interference mitigation of beamforming. However, with both LTE and HSPA working at optimal load balances, it isn’t enough for the demand of wireless networks and greater efficiency, which comes with 5G and its expansion into greater frequencies.

But, a larger range of frequencies (i.e., mm-Wave spectrum) is not the only technological advance to support the shift to 5G in hopes of answering the issue of lacking spectral capacity. 5G consists of five large components each of which has its challenges that call upon further research and understanding: mm-Wave spectrum, massive MIMO, beamforming, full-duplex (FD), and small cells. Mm-Wave spectrum has to do with the expanding frequencies (e.g., 28 GHz – 39 GHz) that will allow for more space for current and further technological advances that could allow for virtual reality, augmented reality, mixed reality, self-driving cars, and other improvements in sensing, imaging, radar, and communication. Massive MIMO simply increases the number and capacity of antenna ports by 22 folds or more of what 4G MIMO could hold to handle all cellular traffic. The drawback of such expansion is that MIMO broadcast information in all directions, so there will be interference. Beamforming combats that issue of interference by sending focused streams of data to narrow in on the signal’s intended destination. This allows for increased efficiency by taking larger amounts of incoming and outgoing data streams at the same time. Full-duplex helps further increase efficiency by having the incoming and outgoing data streams operate simultaneously on the same wave frequency known as reciprocity. Rather than having to operate on different frequencies, silicon transistors create high-speed switches to decrease transition time between incoming and outgoing data streams. Small cells address the issue with mm-Wave spectrum because at those high of frequencies the signals have a difficult time penetrating through surfaces such as buildings, trees, and people. This would greatly reduce the connection of wireless networks between a device and a large cell tower. The solution of small towers is due to the sheer increase in numbers when compared to the large cell towers because having a greater number of small towers means they can be located in various places that are closer to the devices. An example of this is expressed with what the experience would be like when receiving and sending data streams to the small towers when walking through a town. In a certain range, the device would communicate with a certain tower but when out of range of one, the device would connect to another thereby maintaining a connection despite the difficulties of interference of mm-Wave spectrum.

Methods and Materials

To advance wireless networks with 5G and generations beyond that, semiconductor technology and its advancement parallel the success because one must adapt with the other. One large component of this is the move to higher frequencies with 5G to make smaller transistors, but this increase of speed would lead to a breakdown. Various semiconductor technologies are thus compared to find what qualities are best suited for the projected future. The semiconductor that is vastly used in applications today is silicon, but research is now presenting how it may not stand on its own for the generations beyond 5G. Silicon is regarded as a slow semiconductor because of its small carrier mobility for electrons and holes, which limits its maximum velocity to 1×10^7 cm/s below normal conditions. Silicon also has a lower breakdown voltage in comparison to other materials such as GaN with a much larger breakdown voltage. In comparison, GaN has a faster switch speed, low resistance, and produces much lower waste heat than Silicon, which better suits it for high-power RF electronics (i.e., PA applications). However, there hasn’t been nearly as much investment in materials like GaN, so there hasn’t been the same dominance in semiconductor technology as Silicon. With those years of understanding and experience, silicon has been found to works very well for several reasons: yield many low cost integrated circuits per wafer, large size, great thermal properties to dissipate heat, in high dynamic range can controllably dope n- and p- impurities, abundant and easily purified, can grow an extremely high-quality dielectric of SiO2 on it, and high mechanical strength for handling and fabrication, make very low-resistance ohmic contacts. Many say that we are reaching the ending limit of silicon’s potential, which is where the comparisons of GaN and other materials help determine the next steps of investment for wireless networks in 5G and beyond. 

The usage of semiconductor technologies plays a large role in designing transistors, which is how performance in terms of the frequency range, power capacity, and noise characteristics are compared. For RF bipolar junction transistors (BJTs), as one of the oldest and most popular active RF devices in use today, they have a good operating performance and low cost. These transistors are mostly made using silicon and have shown to be useful for amplifiers up to the range of 2–10 GHz as well as up to 20 GHz in oscillators. Bipolar transistors are optimal for oscillators with low-phase noise because of their very low 1/ f -noise characteristics. In comparison to field-effect transistors at frequencies below about 2–4 GHz, bipolar junction transistors are preferred for their higher gain, lower costs, and biasing potential with a single power supply. However, bipolar junction transistors are subject to shot and thermal noise effects, which lessens the quality of their noise figure against the noise figure of FETs. FETs or field-effect transistors, which come in various forms including the MESFET (metal-semiconductor FET), the MOSFET (metal oxide semiconductor FET), the HEMT (high electron mobility transistor), and the PHEMT (pseudomorphic HEMT), operating at high frequencies have shown useful for low noise amplifiers, LNAs, and power amplifiers, PAs. One of its most common transistors is GaAs MESFETs because of its microwave and millimeter-wave applications at usable frequencies up to 60 GHz or more (more obtained with GaAs HEMTs). The applications for LNAs comes from having a lower noise figure in comparison to other active devices. PAs are very useful for high power RF and microwave amplifiers in recently developed semiconductor technologies such as GaN HEMTs as well as CMOS FETs for their RF integrated circuits at low costs and low power requirements for the high levels of integration. These PAs’ qualities are what is projected to be useful in commercial wireless applications in the expansions seen for 5G and beyond. Another transistor noted for being made of compound semiconductor technologies is a heterojunction bipolar transistor (HBT), which operates similarly to BJTs. They include semiconductor technologies such as GaAs, indium phosphide (InP), or silicon-germanium (SiGe), often in conjunction with thin layers of other materials (e.g., aluminum). This compound usage allows for high-frequency performance exceeding 100 GHz, which is the direction of generations beyond 5G in commercial wireless applications. It is understanding the transistors and qualities of the semiconductor technologies that enable the structure of our applications into greater frequencies and technological purposes.

Results

The results of this research came out of the investigation of how 5G and beyond would be implemented: the steps, the source of demand, the applications, the challenges, and the comparisons of different semiconductor technologies.

5G Implementation Step:	Small Cells	Full Duplex
Image:
Description:	Increasing the number of cell towers because the mm-Wave frequencies of 5G cannot penetrate through objects like trees, buildings, and people as readily as the current frequencies.	Maximizing the efficiency of the incoming and outgoing data to reduce interference and drag. The data has a directed path.

5G Implementation Step:	Massive MIMO and Beamforming
Image:
Description:	Expanding the number and capacity of the cell towers by 22 folds or more of what is used for 4G LTE to store and send out a greater amount of data. The data is sent out in a narrower direction towards the intended target as a means of reducing interference by the previous methods of sending out data in a 360-degree manner.

The Evolution of G established and continued from the need for expansion. 1G presented mobile communication for the first time. 2G brought upon the first digital standards. 3G allowed access to the internet, which was dramatically increased in 4G and 4G LTE for streaming. 5G is now calling upon new possibilities in areas such as communication, sensing, radar, and conjoining realities (ie. augmented and virtual).

The demand for 5G with the expansion of bandwidth frequency because our current exponential growth of devices and technological advances are crowding the current bandwidth frequencies. To diminish the crowding effect that is projected to get worse if we continue to be confined to what is currently used then, opening new usable frequencies into the mm-Wave regime is where 5G is applicable.

Application of 5G	Image(s)	Enables
Gigabit Communication		Self-driving cars by increasing the speed of communication between vehicles.
Gigabit Communication		Receiving information without the limitations of distance to cell towers or the inability of the signals from penetrating through obstacles.
Sensing		Vision and detection without the impedances of weather on clarity, which increases safety.
Imaging		Radiometry and Remote Sensing as well as imagers and body scanners to depict a more clear image by utilizing mm-Wave frequencies to bring out the smaller details.

Challenges with Semiconductor Technologies:	Image(s)	Description
Attenuation vs. Frequencies		Attenuation helps determine which specific frequency to choose. In the figure, the atmospheric attenuation is shown as a function of frequency. Several relevant atmospheric transmission windows are presented as examples of where we see going from 5G and beyond: at 5G band which is 28-39 GHz, 75-110 GHz (W-Band), 125-165 GHz (D-Band), and higher. At lower frequencies, attenuation is less of an impacting factor on performance such as 4G and below. However at higher frequencies, attenuation is chosen at the local minimum so that we can reduce attenuation.  to communicate with the small cells and have clear communication and too much attenuation prevents that clear communication. For these reasons, frequency bands that we see in the figure display area where the attenuation is low for that section.
Power vs. Frequencies		There is a tradeoff of frequency and power. We see that Silicon is a prime example of this because it works in the two extremes. Silicon works at higher frequencies but doesn’t generate a lot of power or a lot of power but a low frequency. Therefore, this tradeoff of power and frequency impacts the decision of what semiconductor technology is used. Another example presented in the figures is GaN where there is less of a trade-off and that’s why it is more applicable to the purposes of 5G and beyond.
PA Design		The three things that you need in a power amplifier are high-power (which GaN is good at), High efficiency (which requires high-gain), and small die area (to reduce cost, also integration-friendly).A good metric to go about characterizing a power amplifier is PAE or the power-added efficiency. The Power Added Efficiency of an amplifier is the ratio of produced signal power (the difference between input and output power) and the DC input power for the amplifier.Mathematically, it can be represented as the product of the drain efficiency times (1 – the inverse of the Gain) times the power combining efficiency. To maximize PAE from a technology point of view, you want a platform that has the highest gain you can get at a given frequency, which GaN provides. For that reason, Power amplifiers for transmitters utilize GaN, InP HBT, and SiGe HBT, which low noise amplifiers for receivers utilize InP HEMT, and InP MOS-HEMT.
Power vs. Frequencies (Efficiency)		The figure shows power in dBm as a function of frequency for five different semiconductor technologies: CMOS/SiGe, GaAs, GaN, InP.  This is to give an idea of how different technologies compare against each other. As you can see, GaN can produce up to 35 dBm at a very high frequency as reported by HRL.

GaN is a semiconductor technology whose qualities make it appealing for the PAs in 5G, which is mirrored in the projections of the GaN Market from 2017 to 2022. There is a projection of $1.1 Billion revenue by 2022, and we see a greater application into commercial sectors along with the largest areas of the market: military and cellular infrastructure.

Conclusion

When researching the various semiconductor technologies and transistors for the potential outcomes of performance and capacity, it was notable that the generations we see from here on out with 5G and beyond are to be carriers exceeding 100 GHz into bandwidths of 5G, W-band, and D-band. This is then what will drive transistor development into greater possibilities. If you are looking into transceiver design (the designing of both the transmitter and receiver), it is optimal to have a fmax that is twice or higher of a semiconductor technology’s operating frequency. The PAs for transmitters: GaN, InP HBT, and SiGe HBT are recognized because they have the high f maxes, which should be enough to cover the frequencies we are looking to go into. Following this, we see that LNAs for receivers: InP HEMT, InP MOS-HEMT are also optimal for the carriers exceeding 100 GHz. When looking for the semiconductor technologies that will present that best performance, in comparison to the others, no one semiconductor can accomplish the highest qualities of a multitude of semiconductor technologies. Silicon has been the forerunner in the evolution of G through 4G LTE, but it isn’t compatible enough with the carriers exceeding into higher mm-wave frequencies beyond 100 GHz. The direction is to now utilize a multitude of semiconductor technologies to offset one’s lacking qualities with another’s highest qualities in performance in the designing of transreceivers for 5G and beyond.

Comparisons of semiconductor technologies applicable to different generations beyond 6G for their qualities in transistor development based on their fmaxes. 

Future Directions

From the progress made by enabling integrated, compact, and efficient chip-scale THz technology, there is a bridging effect on the THz gap between our advances in technology and the application of our technologies. We have seen the result over a vast array of areas including solid-state and photonic devices, two-dimensional (2D) materials, heterogeneous integration and miniaturized THz technology demonstrated with quantum-cascade lasers, microbolometers, nanowires, novel plasmonic nanostructures, metamaterials, and ultrafast photoconductive semiconductor materials. Recently efforts have been dedicated to maximizing the capabilities of solid-state semiconductor technology (III–V and silicon-based) under conditions of room temperature and manufacturing at low costs to increase the economies of scale. From these understandings, the paradigm has shifted in thinking there is one master THz device and semiconductor technology to working with a multitude of semiconductor technologies for new system-level properties that can enable a moderately efficient system, which is versatile and programmable. The programmability allows for possibilities regarding electronic reconfigurability of the wavefront and polarization of the emitted THz fields, enabling applications in communication, radar and imaging, or dynamic spectral control of the radiated fields for spectroscopy and hyperspectral imaging. However for these possibilities, versatility that is not found in many current non-integrated THz platforms is required. With the dominance of silicon-based integrated technology, the qualities of a platform with massive integration and complex phased arrays allow for imaging and communication systems with output power to reach the 100 μW range at 1 THz. But, there is a greater potential of performance as seen with the expansion into other semiconductor technologies for the PAs and LNAs beyond 5G. An example of what these future devices project is multifunctional electromagnetic surfaces to allow subwavelength control to do away with many of the tradeoffs that came with the partitioned block-by-block design approach. Strives towards capabilities such as beamforming for applications in sensing, imaging, and communication are clear functionalities of integrated circuits, but there is much to be done before a fully functional THz system is implemented. THz systems require transporting THz signals across longer distances between integrated circuits, multiplexing, and demultiplexing across a network of nodes or even free-space modulation, and so on. This comes from electrically actuated external beam-steering devices, which multi-input–multi-output (MIMO) antenna systems play a crucial role in the future of THz wireless systems. In the small cells step for implementing 5G’s expansion into mm-wave frequencies, the MIMO arrays are needing to be compact, while allowing for the drastic increases of capability and numbers. Questions are surrounding its implementation and optimization for the execution of such systems of a large number of antennas in a small footprint. The other step of beamforming is also under the question of achieving its spatial and spectral multiplexing because these are all aspects that aren’t familiar to RF systems in the past. We are reaching higher frequencies while also needing to be compact, efficient, high-performance sensing devices, operable at low power and deployable at large scale, and apply integrated circuit technology. The challenges faced to enter this new expansion of frequencies for applications in the real-world come from the demand for efficient and widely

reconfigurable chip-scale systems to apply to properties of the spectrum, radiation patterns, and polarization.

Acknowledgements

I would like to acknowledge Professor Tsachy Weissman of Stanford University’s Electrical Engineering Department and head of the Stanford Compression Forum for his guidance and help with the project. I also want to thank my mentor Rafael Perez Martinez for assisting me through the understanding and research of this project by maintaining a line of open communication whenever I needed help. Additionally, I would like to thank Professor Srabanti Chowdhury for all of her advice throughout the project as well as inviting me into the WBG Lab.

References

[1] Angelov, et al., “On the large-signal modeling of AlGaN/GaN HEMTs and SiC MESFETs,” 2005 Gallium Arsenide and Other Semiconductor Application Symposium.

[2] B. Romanczyk et al., “W-band N-polar GaN MISHEMTs with high power and record 27.8% efficiency at 94 GHz,” 2016 IEEE International Electron Devices Meeting (IEDM), San Francisco, CA, 2016, pp. 3.5.1-3.5.4, doi: 10.1109/IEDM.2016.7838339.

[3] J. Chéron, M. Campovecchio, R. Quéré, D. Schwantuschke, R. Quay, O. Ambacher, “ High-gain over 30% PAE power amplifier MMICs in 100 nm GaN technology at Ka-band frequencies,” 2015 10t European Microwave Integrated Circuits Conference (EuMIC).

[4] K. Nakatani, Y. Yamaguchi, Y. Komatsuzaki, S. Sakata, S. Shinjo, and K. Yamanaka, “A Ka-Band High Efficiency Doherty Power Amplifier MMIC using GaN-HEMT for 5G Application,” 2018 IEEE MTT-S International Microwave Workshop Series on 5G Hardware and System Technologies.

[5] M. J. W. Rodwell, “50 – 500 GHz Wireless: Transistors, ICs, and System Design,” GeMiC 2014; German Microwave Conference, Aachen, Germany, 2014, pp. 1-4.

[6] R. Guo, et al “A 26 GHz Doherty power amplifier and a fully integrated 2×2 PA in 0.15μm GaN HEMT process for heterogeneous integration and 5G ,” IEEE IWS, 2018.

[7] R. Ma, K. H. Teo, S. Shinjo, K. Yamanaka, and P. M. Asbeck, “A GaN PA for 4G LTE-Advanced and 5G,” IEEE Microwave Magazine, vol. 18, issue 7, pp. 77–85, 2017.

[8] Sengupta, K., Nagatsuma, T. & Mittleman, D.M. Terahertz integrated electronic and hybrid electronic–photonic systems. Nat Electron 1, 622–635 (2018). https://doi.org/10.1038/s41928-018-0173-2

[9] Vu, Kien & Bennis, Mehdi & Debbah, mérouane & Latva-aho, Matti. (2019). Joint Path Selection and Rate Allocation Framework for 5G Self-Backhauled mmWave Networks. IEEE Transactions on Wireless Communications. 18. 2431 – 2445. 10.1109/TWC.2019.2904275.

[10] Y. Yamaguchi, Koji Yamanaka, and Toshiyuki Oishi, “A Scalable Large-Signal Distributed Model for mm-Wave GaN HEMTs”, 2-17 IEEE TWHM.