Enhancement mode RF Transistors in GaN-on-Si Technology

Gallium nitride high electron mobility transistors (GaN HEMTs) offer a compelling combination of high-frequency operation, power handling capability, and efficiency - making them the preferred choice for a wide range of RF applications, where performance, size, and power efficiency are critical factors. Using cost-effective and sustainable integration of GaN HEMT on silicon (Si) substrate, imec is developing GaN-on-Si RF HEMTs relevant for 6G-and-beyond wireless communication.  The goal of this PhD is to develop enhancement mode transistors in the 6-24GHz band (FR3) with industry-relevant specifications.

One of the challenges for such transistors is to achieve E-mode, i.e. make the threshold voltage positive. To this end, innovations at material level include (i) back barriers (e.g., AlGaN, C-GaN, InGaN) in the epitaxial stack, and (ii) employing thin (sub-50 nm) GaN channel. To alleviate the performance losses associated with the above architecture, imec has recently demonstrated regrown source/drain contacts with record low resistivity [S. Banerjee et al., https://doi.org/10.1002/pssa.202400069]. In parallel, work is ongoing to reduce the current leakage associated with thin barriers, using high-K gate dielectrics. Still, the state-of-the-art E-mode devices require numerous areas of improvement which is the scope of this PhD.

The candidate is expected to identify the present challenges to achieving E-mode RF transistors, and propose innovative material and device strategies, for example:

• Investigating various GaN-on-Si buffer architectures and transistor configurations (MOSHEMT, MISHEMT) - using TCAD-based simulation tools.
• Characterizing the various RF stacks grown at imec in state-of-the-art 200 mm metal organic chemical vapor deposition (MOCVD) reactor, with in-house and external metrology facilities.
• Developing stacks with thin (graded) GaN channel, various back barriers and capping layers, optionally with highly doped (n⁺) source/drain regrowth.
• Improving the state-of-the-art process flow for RF transistors.
• Proposing material & device solutions to mitigate thermal losses in next-gen RF transistors.