This paper presents the technologies for over-40-Gbit/s operation of InP-based HEMT ICs for future optical communication systems. High-speed interconnection using low-permittivity benzocyclobutene (BCB) film as an inter-layer insulator decreases interconnection delay and results in high-speed operation of digital circuits. A static frequency divider and a 2 : 1 multiplexer using this novel interconnection demonstrate 49-GHz and 80-Gbit/s operation, respectively. Ultrahigh-speed digital/analog ICs fabricated using the HEMTs were used in 40 Gbit/s optical transmission experiment and showed good bit-error-rate performance. A novel two-step recess process for gate recess etching considerably improves the performance of InP-based HEMTs and is found to be promising for future ultrashort-gate devices.

InP-based high electron mobility transistors (HEMTs) with gate lengths reduced to 30 nm were fabricated and characterized, and the effect of the gate recess on the high-frequency characteristics was studied. The cutoff frequency, which is regarded as a function of the gate length and the average carrier velocity in a first-order approximation, depends on the size of the gate recess when the gate length becomes short. The size of the gate recess is optimized by taking the feed-back capacitance and the parasitic resistance into account. For HEMTs having the gate recess with an InP surface, an appropriate widening of the gate recess gives a record cutoff frequency of 368 GHz for the 30-nm-gate HEMTs with a lattice-matched channel.

The authors have demonstrated AlGaN/GaN HEMTs with lightly-doped buried p-layers under the channel for the first time. A 1.5-µm-gate device showed good pinch-off characteristics, gm of 25 mS/mm, and breakdown voltage of 70-90 V. Carrier confinement by the p-n junction was confirmed by capacitance-voltage measurements. These results indicate the potential of p-layer insertion into GaN-based FETs.

We report on the first terahertz emission from a novel dual grating gate plasmon-resonant emitter fabricated with InAlAs/InGaAs/InP material systems. The introduction of InP based heterostructure material systems, instead of the GaAs based ones, in order to improve the quality factor, has successfully enhanced the THz emission intensity and realized the spectral narrowing at room temperature.

As a review of the InP HEMT technology and its applications to logic ICs, the two-step-recess gate structure, which is now widely used in high-performance InP HEMTs, and its application to optoelectronic ICs are described. This paper also covers the topic of the gate delay analysis that reveals that the parasitic delay becomes the primary cause of the gate delay in sub-100-nm gate regime. For future challenge for logic applications, ways to reduce the off-state transistor current is also discussed.