We fabricated 25-nm-gate lattice-matched InAlAs/InGaAs high electron mobility transistors (HEMTs) with a very short gate-channel distance. Using the two-step-recessed gate technique, we reduced the gate-channel distance to 4 nm. We found that the cutoff frequency f T increases with decreasing gate-channel distance d . This phenomenon can be explained by an increase in electron velocity under the gate with decreasing d . We obtained an f T of 500 GHz with a d of 4 nm. We also fabricated 25-nm-gate pseudomorphic In 0.52 Al 0.48 As/In 0.7 Ga 0.3 As HEMTs using the same processing technique, and obtained an f T of 562 GHz with a d of 4 nm. This f T is the highest value yet reported for a transistor of any type.
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Endoh et al. (2003) studied this question.