Radio frequency (RF), usually referred to as high frequency, VHF, and UHF, with a frequency of 3 MHz-10 000 MHz, is the most active frequency band in wireless communications. In the last decade or so, wireless communication technology has made a leap forward. RF devices have rapidly replaced hybrid circuits using discrete semiconductor devices. These technologies are all challenges for designers.
RFIC (Radio Frequency Integrated Circuit) is a new type of device that has emerged with the improvement of IC technology since the mid 90s. The technical basis of RFIC mainly includes: 1) new device research with higher working frequency and smaller size; 2) dedicated high-frequency, high-speed circuit design technology; 3) dedicated testing technology; 4) high-frequency packaging technology. This article will briefly review and analyze some of the recent trends in the field from the perspective of IC technology.
CMOS appears to be slow at the beginning, and bipolar devices are often used in RF circuits. However, as the semiconductor process progresses rapidly with Moore's Law, the channel length of the MOS transistor is greatly reduced, its operating speed is greatly increased, and power consumption is also greatly reduced, and it has become a very economical platform for RFIC. For example, Intel released a CMOS Wi-Fi RFIC this year. At present, with each chip manufacturing stepping into the 90-nm era, CMOS circuits can already operate above 40 GHz, and even reach 100 GHz. This advancement can realize wireless communication chips with data rates ranging from 100Mbit/s to 1Gbit/s, serving broadband wireless communication systems and high data rate switching devices such as wireless high-speed USB2.0 interfaces.
Currently, TIebout et al. have reported RFIC samples integrating LNAs and mixers and PLLs for ISM band applications from 17.1 GHz to 17.3 GHz; TIebout also reported on the injection of locked-up frequency divider samples, whose operating frequency has been as high as 40 GHz. The advances in CMOS technology have made it possible for low-cost RFICs to develop to higher frequency bands, which can greatly reduce the cost of RF devices in the microwave band. Therefore, this technology poses a challenge to the GaAs technology that traditionally dominates the microwave frequency band.
In terms of compound semiconductors, GaAs is the current mainstream technology, but since the beginning of the 21st century, III-V nitride semiconductors such as GaN, AlN, and InN have attracted attention. The electronic saturation of these materials is high, and the operating frequency can reach sub-millimeter wave, quasi-optical wave, and light wave bands, and it is expected to be used in applications that require high power, high speed, and high temperature work. In addition, SiC is also a high-temperature and high-power semiconductor material. At present, these chemical semiconductor materials have certain difficulties in single crystal substrate preparation, processing processes, and the like. Generally, a single device is dominant, and the corresponding IC has no advantage in cost compared with the silicon-based CMOS technology. It is expected that in this year and early next year, the corresponding processing technology will achieve a major breakthrough, and will enter the stage of large-scale industrialization. SiGe material has been widely appreciated in recent years. Its appearance makes it possible to apply the energy band engineering theory widely used in compound semiconductor heterojunction devices to silicon-based devices. On the basis of silicon bipolar transistor and MOS technology, SiGe HBT (heterojunction transistor) and strained channel PMOSFET can be made by replacing the conventional Si base region with strained layer of GeSi alloy, etc., and the process cost is low. Good process compatibility. SiGeHBT is mainly due to the mismatch of the valence band edge of SiGe/Si heterojunction. The current gain of the device exhibits an exponential relationship with the SiGe/Si valence band edge difference, so that the base region can have a high doping concentration and the device noise figure. The corresponding reduction. In addition, due to the band edge effect, the current gain has a negative temperature coefficient, and has a self-inhibitory effect on the current gain. The device is relatively stable and has excellent thermal performance and power performance. Currently, the fT of the SiGe HBT exceeds 200 GHz, and the noise figure at 2 GHz is less than 0.5 dB. It can be used not only for mobile communications, but also fully meets the requirements of LAN and fiber communication. SiGe technology has implemented almost all single-function wireless communication circuits. Its best application areas are radio-frequency front-end chips and power amplifier modules for wireless communication handsets (especially 3G mobile phones). Other applications include wireless access, satellite communication, GPS positioning navigation, wired communication (Gigabit Ethernet, SONET/SDH, etc.), automotive radar, intelligent electronic toll collection systems and even military communications. The rise of SiGe will greatly change the traditional Si, GaAs market division. It is expected that SiGe RFIC will reach 1.9 billion U.S. dollars in 2005. In addition, HBT can also be made using InP materials. IBM is the main pioneer of SiGe technology. It provides SiGe chip manufacturing services for many companies such as Alcatel, Intersil and Nortel. It also cooperates with companies such as Siemens and RF MicroDevices to develop SiGe chips. Its mainstream process technology is the SiGeBiCMOS 5HP process that is industrialized on 8-inch wafers. Other companies with SiGe HBT technology include Freescale, Maxim, Lucent, ST, Infineon, Philips, and others.
Change in Integration - RF SoCSoC (System-on-a-Chip) is a hot spot in the development of the international semiconductor industry in recent years and is also the future direction of the semiconductor industry. As the IC process reaches and crosses the 90nm node, the operating frequency of a single MOS device on the chip can already rise to the microwave and millimeter wave bands. Therefore, the RF front end can be integrated with the digital baseband to make an RF SoC. This new concept product will greatly reduce the number of devices in the entire communication system, thereby reducing product costs, reducing its size and increasing functionality, while improving reliability. The promotion of this technology is expected to cause changes in the industrial chain. Currently, Agilent, IBM, STMicroelectronic, and Frees Cale are all developing RF SoC products, which are expected to be launched in the market next year. The RF SoC's process platform can be CMOS, SiGe, etc. Millimeter-wave circuits have now been implemented in CMOS technology in laboratories. If new substrate technologies such as SOI (Sion-Insulator) and SoN (Sion-Nothing) are further adopted, since these substrates have a highly resistive buried oxygen layer, it is possible to ensure low RF loss and devices. High-speed operation and low crosstalk between RF and baseband components. In addition, designers can combine nMOSFETs and SiGe HBTs through the BiCMOS process platform to utilize two high-speed performances to achieve low-voltage, low-power 30-80GHz range. Millimeter wave chip. Currently the technology has introduced samples. The commercial success of the RF SoC is the same as that of a normal SoC, depending on whether it can guarantee a short turnaround time, low cost, and good IP or design library reuse.
New design technologyLike other ICs, the success or failure of RFIC design in business lies in its design cycle and time to market. Therefore, the design and verification tools selected by the developers should ensure the design optimization and testability, reliability, and reduce or eliminate the need for tapeout verification. The design software must include Top-Down design and verification modules at all levels, and allow designers to freely exchange design data and simulation results between processes and modules, and coordinate the update of design data until The design document is finally issued; the design software should also interface with the test system in order to use the test data to modify the original design.
Currently representative design software includes ADS from Agilent, Microwave Office from Applied Wave Research, and software tools from Analog Office. They generally have a friendly design interface, a flexible and open architecture, and have different levels of design modules from synthesis to layout design, support for third party design, test software, and easy-to-use physical design tools and model extraction tools.
Among them Ansoft Company has introduced Ansoft Designer with data input and visualization function and time, frequency, mixed mode simulation. In system-level simulation, in addition to its RF and DSP component libraries, Ansoft Designer supports co-simulation of compiled and interpreted C and C++ user-defined models, and Mathworks's Matlab co-simulation. Circuit simulation solutions include analysis for nonlinear noise, transients, digital modulation, nonlinear stability, and load and source pull-ups. It also has a design synthesis function suitable for filters and transmission lines. The product includes a layout and manufacturing module and a 3D planar electromagnetic simulation engine.
However, RFIC is essentially analog. Its design often requires full use of the performance of active/passive devices. It is still plagued by inaccuracy of device models, noise, and nonlinearity. Currently, these softwares must be able to function as digital circuits. The simulation software has a very high simulation efficiency and there is a long way to go.
As the RF SoC concept is increasingly applied, designers will increasingly face the design issues of RF, analog and digital mixed-signal circuits. The simulation of digital circuit blocks in the frequency domain is meaningless, and the design of the analog section is also different from the RF section, so various design methods often do not match. Designers almost always perform digital design in the time domain and RF design in the frequency domain (in order to increase the simulation speed). Integrating two types of designs on the same chip may mean that the entire chip simulation time will be stretched to an unrealistic point. In analog circuit design, people have already tried to achieve some degree of Top-Down synthesis capability and reuse of IP, but in the RF part of the design, people still focus on analysis, and this analysis must include active With passive components, the integration of RF design seems to be far behind. Designers need a tool that can handle high-speed digital circuits, analog circuits, and RF circuits at the same time.
The RF SoC is a small system. People must observe and analyze it from the system point of view. Therefore, the design must take into account the problems of integrating digital, analog, and RF circuits onto a single substrate, including: integrated antennas and no Source device simulation and parameter extraction, VCO traction problems, substrate modeling and coupling of signals through the substrate, fast system-level simulation, etc. The above-mentioned issues regarding the RF SoC will be the focus and difficulty of the next RF IC circuit design.
With the rapid development of computer computing capabilities, the speed of electromagnetic simulation, the scale of problems that can be handled, and the accuracy of calculations have also been continuously improved. Therefore, in the future, simulation methods based on CEM (computational electromagnetics) will also increasingly penetrate the design of RF ICs. Various full-wave simulation methods (such as the moment method and finite element method) physically guarantee the physical structure of the circuit (especially the connection Collectors, planar transmission lines, discontinuities, and passive components). Their use will be the fundamental guarantee for improving the accuracy of RF ICs, and various CAD tool vendors have been working hard to integrate CEM-based methods into RFIC simulations. Users can also use Agilent Momentum, a 2.5D simulation technique based on the moment method, to generate an electromagnetic field-based accurate model of on-chip passive components and interconnect lines. You can simulate these electromagnetic wave-based models directly in the Cadence circuit schematic without having to perform the usual conversions to approximate the lumped component model, resulting in higher accuracy for wireless and high-speed wired devices. Momentum's electromagnetic modeling and verification capabilities are also a collaborative tool for existing RC extraction tools. It helps critical design networks get the required modeling accuracy, and the failure of these networks can damage the entire process. Next year or later years, CAD software that simulates the passive part of any of the various RF modules will likely emerge.
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