Gate-to-drain separation and transistor cutoff frequency in wet etched AlGaN/GaN HFETs

The variation of cutoff frequency f^sub T^ with gate-to-drain separation L^sub GD^ in 0.2 µm gate AlGaN/GaN HFETs fabricated by low-damage isolation techniques is studied. The transistor delay τ^sub T^ = 1/2τf^sub T^ increases linearly with LGD due to the rise in drain resistance with increasing L^sub GD^. The resulting apparent gate-drain resistance is higher than the post-process low-field value, and is dependent upon the fabrication process for devices implemented on the same layers, probably reflecting different levels of surface damage.