AlGaN/GaN high electron mobility transistor (HEMT) based hydrogen sensors incorporating platinum nanonetworks in the gate region were demonstrated. Pt nanonetworks with 2-3 nm diameter were synthesized by a simple and low-cost solution phase method, and applied to the gate electrode of transistor sensor. The HEMT with physically and electrically connected Pt nanonetwork gate showed good pinch-off and modulation of drain current characteristics. Compared to conventional Pt thin film AlGaN/GaN HEMT sensor, the Pt nanonetwork sensor has dramatically improved current response to hydrogen. Relative current change of Pt nanonetwork gated sensor in 500 ppm H-2 balanced with Air ambient was 3.3 x 10(6)% at V-GS of -3.3 V, while 2.5 x 10(2)% at V-GS of -2.9 V for Pt film. This results from large increase in channel conductance induced by huge catalytic surface area of nanostructured Pt networks. (C) 2013 Elsevier B. V. All rights reserved.