Carbon nanotube RF transistors are predicted to offer good performance and high linearity when operated in the ballistic transport and quantum capacitance regime; however, realization of such transistors has been very challenging. In this paper, we introduce a self-aligned fabrication method for carbon nanotube RF transistors, which incorporate a T-shaped (mushroom-shaped) aluminum gate, with oxidized aluminum as the gate dielectric. In this way, the channel length can be scaled down to 140 nm, which enables quasi-ballistic transport, and the gate dielectric is reduced to 2–3 nm aluminum oxide, leading to quasi-quantum capacitance operation. A current-gain cutoff frequency (ft) up to 23 GHz and a maximum oscillation frequency (fmax) of 10 GHz are demonstrated. Furthermore, the linearity properties of nanotube transistors are characterized by using the 1 dB compression point measurement with positive power gain for the first time, to our knowledge. Our work reveals the importance and potential of separated semiconducting nanotubes for various RF applications.