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AlGaN/GaN 70-nm-gate high-electron mobility transistors (HEMTs) fabricated using either ion implantation or conventional mesa isolation are compared. Although the resulting devices display comparable dc characteristics, the isolation process influences the RF and pulsed I-V characteristics. Although others have described implant-isolated GaN HEMTs, published reports focused on limited performance metrics, such as the gate leakage current. The present multiparametric study explicitly contrasts the performance of ion-implanted devices to otherwise identical mesa-isolated deep-submicrometer high-speed AlGaN/GaN HEMTs, in terms of transistor cutoff frequencies, small-signal model parameters, microwave noise performance, gate leakage currents, and large-signal pulsed I-V characteristics. We find that implant isolation can bring compelling advantages in terms of bandwidth, microwave noise performance, and tighter parametric distributions.