The effect of carbon doping on crystal quality and electrical isolation has been investigated in GaN based structures grown by ammonia assisted molecular beam epitaxy on silicon substrates. A dramatic improvement of the GaN resistivity is obtained when a doping level of several 10(18) cm(-3) is reached. This improvement is however accompanied with a degradation of the crystal quality that is itself dependent on the density of threading dislocation present in the underlying layers. The analysis of the electrical behavior of transistors with thin GaN buffer layers shows that electron trapping occurs when carbon is introduced, except in the case of a structure with an AlN spacer at the interface between the AlGaN bar. rier and the GaN channel. Clear correlations between the amplitude of such trapping effects and the amount of carbon introduced in the buffer layers have been observed. (C) 2012 Elsevier Ltd. All rights reserved.