RF Electronics
In the field of GaN-based RF components, the academic and industrial communities aim to demonstrate components operating at frequencies above 40 GHz with performance metrics such as the highest possible output power and added power efficiency. This research is being conducted in a particularly challenging economic and strategic context, where it is essential to develop solutions that ensure independence, particularly from materials that are critical and/or costly to manufacture.
The RF research at CRHEA fits perfectly into this context, as it is partly focused on the development of epitaxial heterostructures on large-area silicon substrates (up to 200 mm in diameter) with the aim of providing lower-cost solutions. By drawing on its long-standing and unique experience in the field of epitaxy on silicon substrates (which began in the late 1990s) and by incorporating new materials such as ScAlN and NbN, CRHEA is positioning itself to play a major role in this field. To remain both innovative and competitive, the SEMI team is currently developing the following four approaches:
Building on the work conducted at CRHEA on silicon substrates using molecular beam epitaxy (MBE),[1] the epitaxial structures grown on 200 mm Si wafers as part of the Hypergan project (a collaboration between CRHEA, IEMN, and EasyGaN) have now achieved unprecedented performance levels.[2,3] For a total thickness of less than one micrometer (ultra-thin HEMT structure), an output power (Pout) of 2.5 W/mm and a PAE of 19.2% were achieved at 40 GHz.[3]
A promising alternative is also being developed at CRHEA, which involves replacing the (Al,Ga)N barrier with a thin layer of ScAlN to ensure a high carrier density, thereby maintaining high power density even at high frequencies. The SEMI team is also studying structures based on NbN, a material that enables nitrogen polarity in nitride materials, which theoretically can significantly improve transistor performance for RF applications. Furthermore, the incorporation of NbN into a III-nitride heterostructure is also under investigation to demonstrate a metal-based transistor that is expected to perform significantly better than the “conventional” HEMT for RF applications.
In parallel with the various approaches developed using molecular beam epitaxy (MBE), the SEMI team has also identified the mechanisms underlying the propagation losses that limit the RF performance of structures fabricated on silicon via organometallic vapor phase epitaxy (EPVOM). [4] To avoid this parasitic conduction, an innovative alternative using a 3C-SiC buffer layer was proposed, which significantly reduced propagation losses (less than 0.8 dB·mm⁻¹ at 40 GHz). [5] The use of this buffer layer is all the more promising given that an HEMT structure fabricated by EPVOM at CRHEA, directly on a SiC substrate, recently achieved 3.8 W/mm of output power with a PAE of 42.8%. [6] Note that this demonstration utilized selective EJM epitaxy of heavily N+-doped GaN to obtain non-alloyed ohmic contacts.
At the national level, CRHEA’s RF activities are supported by strong collaborations with IEMN, LN2, and 3-5 Lab, as well as more recent partnerships with academic institutions such as GREMAN and GREYC, and industrial partners such as STMicroelectronics. By hosting NOVASiC (a subsidiary of SOITEC) and the startup EasyGaN (a CRHEA spin-off), CRHEA can bridge the gap between academic research and industrial projects through the manufacture of small- to medium-scale production runs, which is a key factor in evaluating the performance and relevance of the various solutions under consideration.
- [1] - Assessment of transistors based on GaN on silicon substrate in view of integration with silicon technology – Semicond. Sci. Technol. 28, 094003 (2013).
- [2] - Submicrometer-Thick Step-Graded AlGaN Buffer on Silicon with a High-Buffer Breakdown Field – Phys. Stat. Sol (a) 220, 2200846 (2023) – https://doi.org/10.1002/pssa.202200846
- [3] - Low Trapping Effects and High Blocking Voltage in Sub-Micron-Thick AlN/GaN Millimeter-Wave Transistors Grown by MBE on Silicon Substrate – Electronics 12, 2974 (2023) – https://doi.org/10.3390/electronics12132974
- [4] - Electrical activity at the AlN/Si Interface: identifying the main origin of propagation losses in GaN-on-Si devices at microwave frequencies – Sci Rep 10, 14166 (2020) – https://doi.org/10.1038/s41598-020-71064-0
- [5] - Metalorganic Chemical Vapor Phase Epitaxy Growth of Buffer Layers on 3C-SiC/Si(111) Templates for AlGaN/GaN High Electron Mobility Transistors with Low RF Losses – Phys. Stat. Sol (a) 217, 1900760 (2020) – https://doi.org/10.1002/pssa.201900760
- [6] - Performance improvement with non-alloyed ohmic contacts technology on AlGaN/GaN High Electron Mobility Transistors on 6H-SiC substrate – Microelectronic Engineering 276, 111998 (2023) – https://doi.org/10.1016/j.mee.2023.111998
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