Manufacturing Bits: Jan. 7

Beyond 5G chips; RF SOI PAs; boron nitride RF switches.


Beyond 5G chips
At the recent IEEE International Electron Devices Meeting (IEDM), NTT and the Tokyo Institute of Technology presented a paper on a technology that could enable high-speed wireless devices beyond the 5G standard.

Researchers have devised a 300GHz wireless transceiver (TRx) that supports a data rate of more than 100Gb/s. The device is based on a technology called indium phosphide (InP), which is a binary semiconductor compound based on indium and phosphorus. In effect, researchers have developed an InP high electron mobility transistor (InP-HEMT).

5G, a fifth-generation mobile wireless network, is ramping up in the market at sub-6GHz bands. The industry is also working on mmWave technologies for 5G. 5G is faster than 4G, delivering up to 20Gb/s peak data rates, according to Qualcomm. 5G supports an 100x increase in traffic capacity with lower latencies.

In R&D, the industry is also working on “beyond 5G” technologies like 6G. This in turn will require mmWave and terahertz (THz) transceivers that support a data rate of 100Gb/s.

“The main advantage of mmWave and THzwave for implementing high-speed wireless communication systems is their large bandwidths,” said Hiroshi Hamada from NTT in the IEDM paper. “Frequencies above 275GHz are gaining attention because they have not been allocated, and there is a potential to implement extremely high data rate wireless systems by using their wide frequency ranges. The 275-320GHz range (hereafter, 300GHz band) is suitable for wireless communication because of its low attenuation of less than 10dB/km.”

To meet those requirements, NTT and Tokyo Institute of Technology demonstrated a 300GHz device with a 100Gb/s wireless data transmission at a link distance of 2.2 m. With the device, researchers also achieved 120Gb/s, 9.8 m wireless data transmissions.

To develop this device, low loss monolithic microwave integrated circuits (MMICs) with waveguide transition techniques were introduced.

To make the device, researchers developed an InP substrate. On the substrate, researchers have developed a composite channel based on indium gallium arsenide (InGaAs). The channel is sandwiched between two buffer layers.

The InP-HEMT itself is based on a lateral structure. The source, gate and drain are on top of the transistor. The gate pitch is 80nm.

Researchers also used through-substrate-vias (TSVs) for the backside metalization process. “Because the wavelength of a 300GHz wave (1-mm in free space) is comparable to the InP wafer thickness (600um), the 300GHz electromagnetic (EM) wave can propagate through the InP substrate in the form of the substrate mode (S-mode) This causes significant issues with MMICs such as amplifier’s oscillation,” Hamada said. “To cut out S-modes, we use substrate thinning and a dense TSV formations in MMICs. The substrate is thinned to a 55um thickness.”

At IEDM, CEA-Leti and STMicroelectronics presented a paper on the development of sub-6GHz power amplifiers based on a RF SOI process.

RF SOI is the radio-frequency (RF) version of silicon-on-insulator (SOI) technology. RF SOI processes are used for RF components in smartphones.

A smartphone consists of digital and RF chips. Based on CMOS, the digital part consists of the applications processor and other devices.

The RF components are integrated into a RF front-end module, which handles the transmit/receive functions. The front-end module consists of a number of components, including power amplifiers, antenna tuners, low-noise amplifiers (LNAs), filters, and RF switches.

The power amp provides the power for a signal to reach a destination. LNAs amplify a small signal from the antenna, while filters prevent any unwanted signals from entering the system. LNAs and filters are based on various processes.

Switch chips and tuners, meanwhile, are based on RF SOI. RF switches route signals from one component to another, and tuners help the antenna adjust to any frequency band.

Typically, the power amps are based on GaAs. “Today, a key challenge is placed on the power amplifier (PA) and its reliability because of the high power handled by the device,” said Xavier Garros of CEA-Leti, in the paper at IEDM. “This paper proposes, for the first time, a deep combined analysis of both performance and reliability of a RF SOI transistor for a PA.”

Researchers have developed a silicon-based RF SOI power transistor for 4G and sub-6GHz power amplifiers. The technology delivers a +31dBm output power with 74% of power added efficiency (PAE) and 18dB of gain.

Stacked PA transistors have been fabricated based on a 130nm RF SOI process. The device is an interleaved transistor, which is made from L=0.3μm NMOS body-contacted transistors and LDMOS in a cascode configuration, according to the companies.

The transistor exhibits a ft of 32GHz at VDD=2.5V. The gate oxide is 5nm thick, which guarantees a 3.7V breakdown voltage, according to researchers. For information, click here.

Boron nitride RF switches
At IEDM, the University of Texas at Austin and the University of Lille presented a paper on RF switches based on hexagonal boron nitride (hBN) materials.

Crystalline hBN enables a thin RF switch device with a single monolayer. With these material, the RF switch demonstrated a low insertion loss (≤0.2dB) with high isolation (≥15dB) up to 110 GHz.

“Importantly, operating frequencies cover the RF, 5G, and mmWave bands, making this a promising low-power switch for diverse communication and connectivity front-end systems,” said Myungsoo Kim from the University of Texas in the paper. “Compared to other switch technologies based on MEMS, memristor, and phase-change memory (PCM), hBN switches offer a promising combination of non-volatility, nanosecond switching, power handling, high figure-of-merit cutoff frequency (43THz), and heater-less ambient integration. Our pioneering work suggests that atomically-thin nanomaterials can be good device candidates for 5G and beyond.”

For more information, click here.

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