Advanced Down-Scaling Technology and its Physical Mechanism for 515 GHz GaN HEMT

GaN-based high electron mobility transistors (HEMTs) are considered suitable devices for power amplification in the millimeter wave frequency range owing to their notable advantages, including a high 2DEG concentration, a high electron saturation velocity, and a high breakdown electric field. In order to facilitate the development of future 6G communications, there is a pressing need for millimeter wave amplifiers operating at frequencies exceeding 100 GHz. To address this requirement, GaN HEMTs must possess a maximum oscillation frequency (f
max
) surpassing 400 GHz. However, achieving this objective presents a considerable challenge in terms of advanced down-scaling technology and cost management. In this report, sapphire substrates and MOCVD were employed to produce an ultra-thin 4 nm AlN polarization layer on the 300 nm GaN channel layer to suppress short channel effect through vertical scaling. Using lateral scaling, T-gates with gate length less than 70 nm and S-D space ∼250 nm were fabricated to reduce the HEMT intrinsic delay. Selective area regrowth (SAG) of n
⁺
-GaN source/drain electrodes was also implemented in order to mitigate access resistance and minimize parasitic delay. These components are crucial in achieving the successful fabrication of GaN HEMTs reach f
max
of around 515 GHz with corresponding fT of around 150 GHz. To the best of the author's knowledge, our scaled HEMTs exhibit the highest fmax among reported HEMTs on sapphire. This study highlights the advantageous characteristics of GaN HEMTs on sapphire substrates, namely their ability to operate at high frequency and their high cost-effectiveness, making them a promising choice for millimeter-wave applications.