imec shows GaN-on-Si MISHEMT for 5G RF
imec has developed metal-insulator-semiconductor high-electron mobility transistors (MISHEMT) on 200mm Si with high output power and energy efficiency while operating at 28GHz for 5G radio applications.
The GaN-on-Silicon depletion mode MISHEMT technology uses aluminium-nitride and gallium-nitride (AlN/GaN) outperforms other GaN MISHEMT device technology in terms of performance, while the adoption of the Si substrate provides a major cost advantage for industrial manufacturing. The lab in Belgium is also working on enhanced mode MISHEMT transistors using device stacking.
imec has been working on GaN-based devices for over a decade as they offer superior performance over CMOS devices and gallium-arsenide (GaAs) HEMTs in terms of output power and energy efficiency for next generation high density radio systems and the industry is looking at two different RF uses.
GaN MISHEMTs can be used in the power amplifier circuits operating at relatively low voltages (i.e., VDD below 10V) in consumer equipment, or in base stations where VDD voltages are higher (above 20V). For the latter case, GaN-on-silicon-carbide (SiC) devices offer the largest potential, but SiC substrates are expensive and small in size. The ability to integrate GaN HEMTs on Si offers a tremendous cost advantage and potential for technology upscaling.
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“The challenge is in achieving a high operating frequency, derived from the fT and fmax at small-signal conditions, while at the same time delivering a high output power with sufficient efficiency (derived from the devices large-signal performance),” said Nadine Collaert, imec fellow and programme director for advanced RF at imec.
“While most GaN devices are HEMTs, in this experimental study, we focused on GaN-on-Si MISHEMTs with AlN barriers as a crucial step towards addressing the demand for both high-power d-mode devices for infrastructure as well as low-voltage e-mode devices required in mobile handsets,” she said.
“These GaN MISHEMT devices, featuring a relaxed gate length of 100nm, demonstrate exceptional performance across various metrics. Specifically, for low-voltage (up to 10V) applications, these devices achieved a saturated output power (PSAT) of 2.2W/mm (26.8dBm) and a power added efficiency (PAE) of 55.5% at 28GHz, positioning our technology better than comparable HEMT/MISHEMTs out there. These results underscore the potential of our technology as a strong foundation for next-generation 5G applications.”
For 20V basestation applications, excellent large-signal performance at 28GHz is demonstrated with a PSAT of 2.8W/mm (27.5dBm) and PAE of 54.8%. “Our AlN/GaN MISHEMTs are still d-mode devices,” says Collaert. “But we know the path towards e-mode devices, through further device stack engineering.”
The key to the performance improvement is the thickness scaling of the AlN and Si3N4 layers, which are used as stop barrier layer and gate dielectric. Ultrathin stacks, for example, enable a high operating frequency, but come at the expense of trapping-induced current collapse and device breakdown in large-signal conditions.
“These fundamental studies give us a modelling platform to further optimize the design of our GaN-based material stack for specific use cases,” said Collaert.
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