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AlN‐based HEMTs grown on silicon by ammonia‐assisted molecular beam epitaxy (NH3‐MBE) are demonstrated and studied. As shown by photoluminescence, the very thin GaN channel (35–55 nm‐thick) is compressively strained on the 250 nm‐thick relaxed AlN buffer layer grown on silicon substrate. The structure is then completed by an 8 nm‐thick AlN barrier and 2 nm‐thick GaN cap. Despite an ultrathin total epilayer (only ≈300 nm‐thick), a high 2DEG density (≈2.7 × 1013 cm−2) is measured by Hall effect. Room temperature mobility values, as high as 636 cm2 V−1 s−1, are measured. They are higher than those reported on similar structures grown on sapphire, SiC and bulk AlN substrates, showing the interest of growing such structures on silicon substrate. Also, low contact resistance values are obtained, as low as 0.18 Ω mm, by using a basic fabrication process. [ABSTRACT FROM AUTHOR]

We report a state-of-the-art performance of deep sub-micrometer gate length AlN/GaN High Electron Mobility Transistors using a thick in situ SiN cap layer. With 120 nm gate length large-signal load-pull measurements showed a peak power-added-efficiency (PAE) above 45%. To the best of our knowledge, this represents the highest PAE for GaN HEMTs at 40 GHz, especially when using a sub-10 nm barrier thickness. Furthermore, state-of-the-art peak output power density above 6 W mm−1 at 40 GHz has been achieved in pulsed mode. This is the highest output power ever reported for sub-10 nm barrier thickness Q-Band GaN devices. The performance improvement is attributed to the thick in situ SiN cap layer and optimized electron confinement allowing higher voltage operation and lower dispersion under high electric field. [ABSTRACT FROM AUTHOR]