By Ed Korczynski

To fulfill the promise of the Internet of Things (IoT), the world needs low-cost high-bandwidth radio-frequency (RF) chips for 5th-generation (5G) internet technology. Despite standards not being completely defined yet it is clear that 5G hardware will have to be more complex than 4G kit, because it will have to provide a total solution that is ultra-reliable with at least 10 Gb/second bandwidth. A significant challenge remains in developing new high-speed transistor technologies for RF communications with low power to allow IoT “edge” devices to operate reliably off of batteries.

At the most recent Imec Technology Forum in Antwerp, Belgium, Nadine Collaert, Distinguished MTS of imec, discussed recent research results from the consortium’s High-Speed Analog and RF Program. In addition to working on core transistor fabrication technology R&D, imec has also been working on system-technology co-integration (STCO) and design-technology co-integration (DTCO) for RF applications.

Comparing the system specifications needed for mobile handsets to those for base-stations, transmitter power consumption should be 10x lower, while the receiver power consumption needs to be 2x lower. Today using silicon CMOS transistors, four power amplifiers alone consume 65% of a transmitter chip’s power. Heterogeneous Bipolar Transistors (HBT) and High Electron Mobility Transistors (HEMT) built using compound semiconductors such as gallium-arsenide (GaAs), gallium-nitride (GaN), or indium-phosphide (InP) provide excellent RF device results. However, compared to making CMOS chips on silicon, HBT and HEMT manufacturing on compound semiconductor substrates is inherently expensive and difficult.

Heterogeneous Bipolar Transistors (HBT) and High Electron Mobility Transistors (HEMT) both rely upon the precise epitaxial growth of semiconductor layers, and such growth is easier when the underlying substrate material has similar atomic arrangement. While it is much more difficult to grow epi-layers of compound semiconductors on silicon wafers, imec does R&D using 300-mm diameter silicon substrates with a goal of maintaining device quality while lowering production costs. The Figure shows cross-sections of the two “tracks” of III-V and GaN transistor materials being explored by imec for future RF chips.

III-V on Silicon and GaN-on-Silicon RF device cross-sections, showing work on both Heterogeneous Bipolar Transistors (HBT) and High Electron Mobility Transistors (HEMT) for 5G applications. (Source: imec)

Imec’s High-Speed Analog/RF Program objectives include the following:

• High-speed III-V RF devices using low-cost, high-volume silicon-compatible processes and modules,
• Co-optimization with advance silicon CMOS to reduce form factor and enable power-efficient systems with higher performance, and
• Technology-circuit design co-optimization to enable complex RF-FEM modules with heterogeneous integration.

5G technology deployment will start with speeds below 6GHz,  because technologies in that range have already been proven and the costs are known. However, after five years the frequency will change to the “mm-wave” range with the first wavelength band at ~28GHz. GaN material with a wide bandgap and high charge-density has been a base-station technology, and it could be an ideal material for low-power mm-wave RF devices for future handsets.

This R&D leverages the III-V on silicon capability that has been developed by imec for CMOS:Photonic integration. RF transistors could be stacked over CMOS transistors using either wafer- or die-stacking, or both could be monolithically co-integrated on one silicon chip. Work on monolithic integration of GaN-on-Silicon is happening now, and could also be used for photonics where faster transistors can improve the performance of optical links.