AlxGa1-xN/GaN high electron mobility transistor (HEMT) structures with different Al concentrations were grown by metal organic vapor phase epitaxy (MOVPE) on silicon substrates. The influence of both Al concentration and a SiN surface passivation has been investigated. HEMT devices have been processed by standard fabrication steps using optical lithography. An increase of two-dimensional electron gas (2DEG)-sheet carrier concentration and mobility with increasing Al concentration has been observed. Thus, DC as well as small-signal RF performance could be improved with increasing Al concentration. Passivated Al0.31Ga0.69N transistors with a gate length of 1 mu m exhibit a saturation current density and peak transconductance of 660 mA/mm and 180 mS/mm, respectively. Investigation of Schottky contact characteristics revealed an increase of gate leakage current for increasing All content. The cut-off frequency and maximum frequency of oscillation were 7 and 9 GHz, respectively. Pulsed measurements of unpassivated and passivated HEMTs carried out at different quiescent bias points demonstrate the influence of the dielectric layer reducing trapping effects associated with the AlGaN surface. (c) 2006 Elsevier B.V. All rights reserved.