Correlation between the isolation-feature geometry and the dc current-voltage characteristics of AlGaN/GaN heterojunction field-effect transistors (HFETs) is investigated. Although, traditional AlGaN/GaN HFETs are fabricated on cubic isolation-features (i.e. mesa) of lateral dimensions in the order of the gate-width, the reported transistors of this work have been realized on a variety of alternative isolation-feature geometries, resembling the following structures: island, fin, comb, and ladder. A link between the increase in the perimeter-to-area ratio of the top surface of the isolation-feature and the positive-shifting of the pinch-off voltage is observed. However, the variation of the pinch-off voltage does not show such a correlation with the reduction of the average distance between gate's side-wall coverage of the isolation-feature and the channel. (C) 2014 Elsevier Ltd. All rights reserved.