5G technology has led to the introduction of a significant number of new RF features that need to be implemented in mobile networks while considering stringent constraints in board space and power consumption. To meet these increasingly challenging requirements, RF designers have turned to the use of alternative materials, such as wide-bandgap (WBG) semiconductors, capable of offering significant improvements in terms of both power density and efficiency compared with traditional silicon-based RF power ICs.
The fifth-generation mobile network offers services with higher bandwidth and lower latency than previously deployed infrastructures, requiring higher-performing and more efficient power devices. While silicon still offers excellent performance at lower frequencies, WBG semiconductors like gallium nitride (GaN) and silicon carbide (SiC) power ICs are more suitable for above-6-GHz and millimeter-wave applications.
5G RF challenges
Although 5G technology delivers an increase in bandwidth, the use of portions of the spectrum with higher frequencies creates inevitable signal attenuation issues. A higher bandwidth implies a lower signal-to-noise ratio, which can be compensated for by increasing the signal level; that is, increasing the transmitter power, the number of antennas, and the number of cells. The market, however, requires solutions that offer the lowest possible form factor, cost, and power consumption. As a result, RF system designers face both the technological challenges that 5G implementation requires and the constraints imposed by commercial network operators.
Another significant challenge is related to power amplifiers (PAs). To achieve high efficiency in the spectral band, the 5G network uses the 256-quadrature amplitude-modulation (QAM) scheme with signals that offer a high peak-to-average-peak ratio (PAPR). High PAPR is critical, as it pushes the power Amplifier into the non-Linear area, resulting in distortion and interference. If an amplifier has been designed to operate efficiently and linearly at peak levels, it will typically offer low efficiency at medium-power levels.
These challenges imposed by 5G can be overcome by using advanced materials, such as WBGs, as well as specific design solutions for this type of radio application, such as Doherty amplifiers. The latter includes two amplifier circuits that handle signals with different power levels, significantly increasing both the efficiency and the linearity of the PA. Doherty amplifiers can also be combined with digital pre-distortion circuits, thus further linearizing the power device.
Power device technologies
Currently, there are three main technologies capable of achieving the high level of performance required for 5G: lateral double-diffused metal oxide semiconductors (LDMOS), gallium arsenide (GaAs), and GaN.
LDMOS, introduced in the 1970s to increase the breakdown voltage of power mosfets, immediately proved to be a superior technology than bipolar transistors, becoming in the 1990s the reference standard for RF high-power devices. Although LDMOS devices, simpler and cheaper to make with current manufacturing processes, are progressively giving way to components based ON WBG materials, it is expected that in the future, they will continue to be used mainly for the lower band deployment (frequencies up to 2 GHz). So both GaN and LDMOS will coexist in 5G systems.
To meet the requirements of low power consumption, small form factor, and better thermal management, power devices based on GaAs technologies and, more recently, GaN-on-Si and GaN-on-SiC have become increasingly popular in RF applications. These compound materials offer significant advantages over traditional silicon-based semiconductors, such as higher switching frequency, lower losses, higher power density, and better thermal management.
Thanks to its high thermal conductivity, GaN-on-SiC is mainly used for new 5G active antenna radios. However, it is one of the most expensive materials for RF applications because of its non-mainstream Semiconductor processing and is prone to defects during the fabrication. GaN-on-Si, which can be produced in 8-inch fabs, achieves higher yield (more dies per wafer) but offers lower performance than GaN-on-SiC.
RF Power ICs
Here is a sampling of RF power ICs suitable for 5G applications. They include a range of GaN and GaN-on-SiC devices.
NXP has recently announced a new family of 32T32R discrete solutions, which enables smaller and lighter 5G base stations for easy deployment in urban and suburban areas. The new series, which adds to the existing portfolio of discrete power amplifier solutions for 64T64R radios, covers all cellular frequency bands from 2.3 to 4.0 GHz and is based on NXP’s latest-generation GaN technology. By using 32 Rx/Tx antennas instead of 64, massive MIMO coverage can be increased into less dense urban and suburban areas, providing a more cost-effective solution.
The new 32T32R solutions, which contain 32 power amplifiers, deliver twice the power in the same package as its predecessor solutions, resulting in a smaller and lighter 5G solution. The pin compatibility enables network operators to scale rapidly across frequency and power levels. Providing 10-W average output power at the antenna (targeting 320-W radio units), the 32T32R device includes driver and final-stage transistors based on NXP’s highly linearizable RF GaN technology manufactured in NXP’s new GaN fab in Arizona.
A typical lineup is shown in Figure 1, where the A5G26H110N 15-W asymmetrical Doherty RF power GaN Transistor, covering the frequency range between 2,496 and 2,690 MHz, is preceded by the A5G26S008N 27-dBm RF power GaN Transistor, acting as a driver.
Figure 1: Typical lineup for NXP’s 32T32R antenna solutions (Source: NXP Semiconductors)
Qorvo has a broad range of RF connectivity solutions for infrastructure and mobile applications, including power amplifiers, switches, phase shifters, integrated modules, and other high-performance discrete RF devices. The QPA3908, for example, is a Doherty power amplifier module (PAM) targeting sub-6-GHz 5G applications. Based on GaN technology, this PAM achieves high performance in a compact footprint, enabling a reduction of size and weight of active antenna solutions, such as massive MIMO base stations and O-RAN networks.
The QPA3908 (Figure 2) supports U.S. C-band applications through an operating frequency range of 3.7–3.98 GHz, whereas the QPA3810 supports applications in European markets providing an operating frequency range of 3.4–3.8 GHz. Both modules are input-/output-matched at 50 Ω, have a drain Voltage of 48 V, achieve an excellent linearity, and require minimal external components. The device incorporates a driver PA and Doherty final stage delivering 8-W average output power. The power amplifier module, available in an 8 × 10-mm package, is completely assembled and does not require any additional tuning. This solution makes 5G network architecture easier, reducing the design time compared with a multiple discrete power amplifier solution.
Figure 2: Qorvo’s QPA3908/3810 GaN-on-SiC PAM (Source: Qorvo Inc.)
Ampleon offers a broad range of RF power devices, monolithic microwave integrated circuits (MMICs), pallets, and modules, available in both LDMOS and GaN technology. The C4H27W400AV (Figure 3) is an asymmetric Doherty power Transistor for base station and multi-carrier applications that are capable of operating at frequencies in the range of 2,300–2,700 MHz. Using innovative GaN technology, this high-efficiency device delivers 400-W output power, an excellent digital pre-distortion capability, and lower output capacitance for improved performance in Doherty applications. The C4H27W400AV comes in a DFM6 (SOT1275-1) package and is internally matched for ease of use.
Figure 3: Ampleon’s C4H27W400AV GaN power transistor (Source: Ampleon)
Due to its intrinsic high operating frequency and wider bandwidth performance, GaN is the ideal solution for covering 5G NR requirements. Additional benefits like higher efficiency and power density allow GaN-on-SiC to be effectively used in the sub-6-GHz (5G FR-1) band, where it can replace LDMOS devices. An example is the Wolfspeed GTRB266908FC, a high-power RF GaN-on-SiC high-electron–mobility transistor (HEMT), which addresses the requirements of multi-standard cellular power amplifier applications.
Operating in the 2,515- to 2,675-MHz frequency range, the new HEMT (Figure 4) offers 549 W of POUT and 69.2% efficiency at a 3-dB compression point (P3dB). Key specs include a POUT (average) of 50.1 dBm, 57.8-dBm Psat, 48% efficiency, 15-dB gain, and a higher 194-MHz IBW. With an operating voltage of 48 V, the power device has a thermally enhanced package with an earless flange. It is lead-free as well as Rohs– and Human Body Model Class 1B (per ANSI/ESDA/JEDEC JS-001)-compliant.
Figure 4: Wolfspeed’s GTRB266908FC GaN HEMT (Source: Wolfspeed Inc.)
Infineon Technologies AG
Thanks to its proprietary CoolGaN technology, Infineon offers a wide selection of GaN devices suitable for power-conversion applications in the voltage range up to 600 V. The CoolGaN family includes e-mode HEMTs that can provide the high efficiency and extremely fast switching speeds required in 5G applications.
An advantage of GaN HEMTs is their relatively temperature-independent on-resistance, which results in a figure of merit much better than similar silicon-based counterparts. GaN properties have a significant impact on both the weight and size of the power solutions, significantly reducing board space and weight. This is an important factor in 5G applications, where the space reserved for installations is very limited.
Infineon’s IGO60R070D1 is a new CoolGaN 600-V e-mode power transistor, offering fast turn-on and turn-off speed and minimum switching losses. It also enables simple half-bridge topologies with the highest efficiency. This normally off switch (Figure 5) has ultra-fast switching, no reverse-recovery charge, low gate and output charge, and reverse-conduction capability.
Figure 5: Infineon’s IGO60R070D1 CoolGaN 600-V e-mode power transistor. (Source: Infineon Technologies AG)
Microchip Technology Inc.
Microchip has recently expanded its RF power portfolio with the introduction of new MMICs and discrete devices. Covering frequencies up to 20 GHz, the new devices combine high power-added efficiency (PAE) and high linearity, meeting the challenging requirements of 5G applications. Like other Microchip GaN RF power products, the MMICs are based on GaN-on-SiC technology, providing the best combination of high power density and yield, as well as high-voltage operation and lifetime. They include GaN MMICs covering 2–18 GHz, 12–20 GHz, and 12–20 GHz at P3dB.
Among the devices recently launched by Microchip is the ICP2840 (originally released as the GMICP2731-10) GaN MMIC power amplifier, designed to meet the requirements of 5G networks, as well as Satcom and aerospace and defense applications. Built using GaN-on-SiC technology, the GMICP2731-10 delivers up to 10-W output power on a bandwidth of 3.5 GHz and with a frequency range from 27.5 to 31 GHz. Achieving a 20% PAE and high linearity, the chip features 22 dB of small-signal gain and 15 dB of return loss. The balanced topology allows the device to be well-matched to 50 Ω, while blocking Capacitors simplify design integration.
Figure 6: Block diagram of Microchip’s ICP2840 GaN MMIC power amplifier (Source: Microchip Technology Inc.)
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