Analysis, design, and experimental evaluation of sub-THz power amplifiers based on GaAs metamorphic HEMT technology

The interest for devices operating in the millimeter-wave (mmW) range (30-300 GHz) and submillimeter-wave (sub-mmW) range (above 300 GHz) is increasing due to the advantages they provide such as compactness, low weight, broad bandwidth, and the capability of these frequencies to penetrate dust, smog and adverse weather conditions. The availability of compact and broadband single-chip solid state power amplifiers (SSPAs) at sub-THz (0.1-1 THz) frequencies can have a decisive impact in many applications including: instrumentation, high-resolution imaging radar and high-data rate communication. Significant improvements in high-speed transistor technologies have enabled the realization of SSPAs in the sub-THz regime. One of them is the GaAs metamorphic high electron mobility transistor (mHEMT) technology available at the Fraunhofer Institute for Applied Solid State Physics, whose processes with gate lengths of 50 nm and 35 nm achieve impressive cut-off frequencies of 380 GHz and 515 GHz. Although InP heterojunction bipolar transistor (HBT) SSPAs offer higher output power due to the higher breakdown voltage, the GaAs mHEMT is a more robust, cheaper and lower DC power consuming technology. Besides the low breakdown, there are other difficulties in the design of sub-THz SSPAs, such as high parasitic effects, losses, and stability issues. To overcome the low-breakdown voltage limitation of GaAs mHEMTs, this work investigates the potential of amplifier topologies based on stacked field effect transistor (stacked-FET) configurations. A theoretical investigation of unit power cell topologies based on different series configurations is presented, including the evaluation and classification of their performance through the definition of novel figures of merit. A design methodology suitable for the sub-THz frequency range is developed and practical examples of unit power cell SSPAs based on the different stacked-FET configurations are presented. PA cells based on a series connection of more than two transistors had previously been published only up to W-band. The results in this research work represent the first experimental demonstrations of stacked-FET topologies up to the onset of the sub-mmW range. As the number of transistors that can be stacked in series is limited, in-phase parallel combining or balanced topologies are also implemented in combination with the best stacked-FET cell configurations to achieve the required power levels. In this context, the design of compact in-phase splitters/combiners and hybrid couplers with low-loss and broadband performance is a key aspect. Different configurations of on-chip couplers and combiners are studied. This investigation also explores the potential of a novel three-metallization-layer process variant, which has allowed to develop a broadband broadside coupler, with higher isolation and compactness than the standard state of the art. As a result, an SSPA with four parallel stacked-FET cells has been demonstrated at 240 GHz, leading to improved bandwidth and power levels compared to previous results in the same technology. In a further step, an ultra-broadband balanced SSPA was packaged into a waveguide module for use as an instrumentation amplifier. These PAs, while making use of novel topology configurations, significantly increase the published performance in terms of bandwidth or power added efficiency while achieving enough output power for the considered applications.