Metalorganic chemical vapor phase deposition of thick, crack-free GaN on Si can be performed either by patterning of the substrate and selective growth or by low-temperature (LT) AlN interlayers enabling very thick GaN layers. A reduction in dislocation density from 1010 to 109 cm-2 is observed for LT-AlN interlayers which can be further improved using monolayer thick SixNy in situ masking and subsequent lateral overgrowth. Crack-free AlGaN/GaN transistor structures show high room temperature mobilities of 1590 cm2/Vs at 6.7 × 1012cm-2 sheet carrier concentration. Thick crack-free light emitters have a maximum output power of 0.42 mW at 498 nm and 20 mA. © 2002 Elsevier Science B.V. All rights reserved.