Impact of Channel Thickness on the Large-Signal Performance in InAlGaN/AlN/GaN HEMTs With an AlGaN Back Barrier

The impact of varying the GaN channel layer thickness (t(ch)) in InAlGaN/AlN/GaN HEMTs with C-doped AlGaN back barriers is investigated. tch was 50, 100, and 150 nm, and the gate length of the fabricated HEMTs ranged from 50 to 200 nm. It is found that short-channel effects (SCEs) are significantly mitigated with a small tch. For HEMTs with a gate length of 50 nm, the drain-induced barrier lowering changes from 40 to 93 mV/V as tch is increased from 50 to 150 nm. On the other hand, it is shown that dispersive effects are more severe for a smaller t(ch), as demonstrated by a sixfold increase in the dynamic ON-resistance for t(ch) = 50 nm compared to t(ch) = 150 nm. The tradeoff between dispersion and SCEs is reflected in large-signal measurements at 30 GHz. The 50-nm channel, mainly limited by dispersion, exhibits an output power of 3.5 W/mm. The thicker channels reach a maximum of around 5 W/mm, but for different gate lengths due to the difference in severity of the SCEs. This paper elucidates the interplay between SCEs and dispersion related to t(ch), its consequences for the large-signal performance and for the limitation in downscaling of the gate length.