I. Introduction
High-Performance devices that operate in the millimeter-wave (30 to 300 GHz) and sub-millimeter-wave (300 GHz to 3 THz) frequency ranges will be major elements of the future communication systems. InP-based InAlAs/InGaAs high electron mobility transistors (HEMTs) are the most promising candidates, since this material system provides high electron mobilities, high saturation velocities, and high sheet electron densities. In both field-effect transistors (FETs) and HEMTs, high-speed characteristics can essentially be obtained by reducing the gate length . Several groups have fabricated InP-based HEMTs with decananometer-scale gates over the past decade. Nguyen et al. [1] and Enoki et al. [2] have reported on the good RF performance of the 50-nm-gate InP-based HEMTs they produced. In previous work [3], [4], we have also fabricated 50-nm-gate lattice-matched HEMTs (In0.52Al0.48As/In0.53Ga0.47As). We obtained a cutoff frequency of 362 GHz, which is higher than those previously reported for 50-nm-gate HEMTs [1], [2]. We explained the excellent RF performance of our HEMTs as resulting from a reduction in the expansion of the carrier depletion region toward the drain. The low temperature of the fabrication process, below 300 °C, suppressed degradation of the epitaxial structure, and this may have been the main reason for that reduction. Our results indicate that it is possible to improve RF characteristics down to the 50-nm-gate range by keeping process temperatures low, even when the fabrication process is conventional. On the other hand, a well-controlled gate technology is a strong requirement for the fabrication of gates below 50 nm in length. Suemitsu et al. have demonstrated a fine fabrication technology [5], [6] by using a fullerene-incorporated nanocomposite resist for electron beam (EB) lithography and a two-step-recessed gate structure [7]. They succeeded in fabricating 30-nm-long T-shaped gates [5], [6], and reported an of 352 GHz for the 30-nm-gate lattice-matched InAlAs/InGaAs HEMTs [5], and an of 368 GHz with the same gate length [6]. Judging from the previous results [3]–[7], a combination of well-controlled gate-technology and a low-temperature fabrication process should result in an excellent RF performance.