AlGaN/GaN high electron mobility transistors (HEMTs) have been grown on 4 and 6 in Si(I 1 1) substrates by metal organic vapor phase epitaxy (MOVPE). A record sheet resistance of 256 Omega/rectangle has been measured by contactless eddy current mapping on 4 in silicon substrates. The wafer also shows an excellent uniformity and the standard variation is 3.6 Omega/rectangle over the whole wafer. These values were confirmed by Hall-Van der Pauw measurements. In the 2DEG at the AlGaN/GaN interface, the electron mobility is in the range of 1500-1800 cm(2)/Vs and the electron density is between 1.3 x 10(13) and 1.7 x 10(13) cm(-2). The key step in obtaining these results is an in-situ deposited Si3N4 passivation layer. This in-situ Si3N4, deposited directly after AlGaN top layer growth in the MOVPE reactor chamber, not only prevents the stress relaxation in AlGaN/GaN hetero-structures but also passivates the surface states of the AlGaN cap layer. HEMT transistors have been processed on the epitaxial structures and the maximum source-drain current density is 1.1 A/mm for a gate-source voltage of 2 V. The current collapse is minimized thanks to in-situ Si3N4. First results on AlGaN/GaN structures grown on 6 in Si(111) are also presented. (c) 2006 Elsevier B.V. All rights reserved.