As bandwidth continues to shrink in the wake of ever-expanding wireless functions, device designers look higher in frequency for available frequency ranges for communications, monitoring, control, and other possible short-range wireless functions and the solid-state devices that can provide them. An international team of researchers succeeded in developing a pair of monolithic microwave integrated circuits (MMICs) based on stacked-FET architectures for use at the upper edge of the millimeter-wave frequency range, with a power amplifier cell operating at 0.3 THz and a medium-power amplifier (MPA) running at 240 GHz.
The devices represent the combined efforts of researchers at the Fraunhofer Institute for Applied Solid State Physics IAF (Freiburg, Germany), the Department of Computer and Communication Technologies of the University of Extremadura (Caceres, Spain), and the Institute for Sustainable Systems Engineering, Albert-Ludwigs-University Freiburg (Freiburg, Germany).
Both circuits were fabricated with a 35-nm InGaAs-on-GaAs metamorphic high-electron-mobility-transistor (mHEMT) semiconductor technology that uses grounded coplanar waveguide (GCW) transmission lines. By employing three metallization layers rather than a two-layer process, it enhanced circuit performance while also reducing in size compared to the conventional fabrication process.
The experimental circuits were characterized with the aid of a model N5224A vector network analyzer (VNA) from Agilent Technologies (now Keysight Technologies) modified with two Oleson model V03VNA2 transmit/receive (T/R) frequency extension modules and two Picoprobe Infinity GSG 60 microwave probes for the S-parameter measurements. A frequency tripler from Virginia Diodes and an in-house driver MPA module helped push the PA circuits close to saturation at such high frequencies. Measurement results were compared to computer simulations using the Advanced Design System (ADS) simulation software from Agilent Technologies.
Measurements on the 0.3-THz PA cell yielded as much as +5.6 dBm output power at 292 GHz when input power of −5 dBm was applied. The MMIC PA cell achieves reasonably high power-added efficiency (PAE) of 5.2%. The PA cell is capable of more than +4.3 dBm output power from 280 to 308 GHz, with more than 3.3% PAE.
In the case of the 240-GHz parallel stacked FET MPA, the search was to find the limit for the maximum number of transistors beyond which the output power would not further improve. The researchers’ analysis revealed that for a 35-nm mHEMT technology and 240 GHz, they would be limited to a maximum of three stacked FETs in the MPA configuration, beyond which the output power would not increase further. The researchers added a cascade preamplifier stage to their design, to drive the last stacked FET stage into saturation. The cascade stage adds very little size to the stacked stages, while increasing the overall gain of the stacked FET amplifier.
See “Analysis and Development of Submillimeter-Wave Stacked-FET Power Amplifier MMICs in 35-nm mHEMT Technology,” IEEE Transactions on Terahertz Science and Technology, Vol. 8, No. 3, May 2018, p. 357.