We review the AlGaN/GaN high electron mobility transistor (HEMT) activities in the Millimeter-Wave Electronics Group at ETH-Zürich. Our group's main thrust in the AlGaN/GaN arena is the extension of device bandwidth to higher frequency bands. We demonstrated surprising performances for AlGaN/GaN HEMTs grown on high-resistivity (HR) silicon (111) substrates, and extended cutoff frequencies of 100 nm gate devices well into the millimeter (mm)-wave domain. Our results narrow the performance gap between GaN-on-SiC (or sapphire) and GaN-on-silicon and establish GaN-on-Si as a viable technology for low-cost mm-wave electronics. We here contrast the difference in behaviors observed in our laboratory between nominally identical devices built on high-resistivity silicon (HR-Si) and on sapphire substrates; we show high-speed devices with high-cutoff frequencies and breakdown voltages which combine fT,MAX × BV products as high as 5–10 THz V, and show AlGaN/GaN HEMTs with fT values exceeding 100 GHz on HR-Si. Although the bulk of our activities have so far focused on AlGaN/GaN HEMTs on HR-Si, our process produces excellent device performances when applied to GaN HEMTs on SiC as well: 100 nm gate transistors with fT > 125 GHz have been realized at ETH-Zürich.