670-GHz Cascode Circuits Based on InGaAs Metamorphic High-Electron-Mobility Transistors

This article reports the development of high-gain cascode amplifier circuits in the frequency range around 670 GHz. The cascode circuits are based on a 35-nm metamorphic high-electron-mobility transistor (HEMT) technology. Till date, only common-source transistors have been used in HEMT-based amplifier circuits above 600 GHz. For this reason, prospects and design considerations of cascode devices operating at this terahertz frequency band are evaluated. The design and implementation of high-gain six-stage cascode amplifier circuits is described, achieving up to 30 dB of measured gain at 670 GHz and a small-signal gain of at least 25 dB over the frequency range from 630 to 690 GHz. The gain benefit of cascode devices is demonstrated with up to 5 dB of gain per cascode stage at 670 GHz. This corresponds to the highest reported gain per stage for HEMT-based WR-1.5 circuits.