Spurious Mode Suppression in the Design of GCPW Submillimeter-wave Power Amplifiers

This work investigates the problem of spurious mode propagation in a grounded coplanar waveguide (GCPW) environment at frequencies between 200 GHz and 335 GHz. Design strategies focused on minimizing undesired effects are explored through full-wave electromagnetic (EM) analysis and experimental results from different test structures. It is shown that a lambda/13 distance between via-holes should be chosen to avoid unwanted resonances at these high frequencies. The critical role of via-holes is also demonstrated through the experimental comparison of two power splitters. Finally, the need of closed RF pads to avoid any propagation of parasitic modes is experimentally shown in an application example of a power amplifier (PA) cell based on a 35 nm GaAs metamorphic high electron mobility transistor (mHEMT) technology. The PA cell with closed pads achieves a 0.5 dB bandwidth of 28 % with small-signal gain levels larger than 5.2 dB.