The influence of the GaN buffer layer doped with carbon on the avalanche breakdown effect of normally open HEMT AlGaN / AlN / GaN transistors was studied. The avalanche breakdown was simulated in a structure where the gate length is LG = 0.3 mkm, the distance between the source and gate is LSG = 1.5 mkm, and the distance between the gate and drain is LGD = 2.2 mkm. For modeling, consider a layer doped with carbon, the thickness of which is 0.3 mkm, and the layer is located at a distance of 20 nm from the channel. The Simulation showed that with an increase in the concentration of carbon doping of the buffer, the breakdown voltage increases in the range UB = 225 – 360 (V). When the layer thickness changes to 0.4 mkm, the breakdown voltage increases in the range UB = 230 – 446 (V). For a structure where the gate length is LG = 0.8 mkm, the distance between the source and the gate is LSG = 1.0 mkm, the distance between the gate and drain is LGD = 3.0 mkm, the breakdown voltage increases in the range UB = 300 – 622 (V).