Schottky diode gas sensors were fabricated on top of the epitaxial layer grown by three different methods, purchased from Cree Research Inc., by hot wall CVD, or by sublimation at a high growth rate. The epitaxial layers have different thickness and doping. The current-voltage characteristics of the gas sensors were compared in different gas ambient during operation in the high temperature region. The temperature dependence of the series resistance of the diodes revealed two types of carrier scattering mechanisms, impurity scattering for the sublimation epitaxial layer at 300-400degreesC and at 400-600degreesC, lattice scattering for all diodes. The ideality factor of the diode fabricated on the Cree substrate is higher than others. The higher ideality factor gives rise to a larger forward voltage change for a change in gas ambient. The amount of change in barrier height caused by a change in the ambient gas is almost the same for the three types of diodes. The value of the barrier height of the diode grown by the sublimation method is lower than for the others, which gives a higher reverse saturation current at temperatures above 400degreesC. The largest saturation current also shows the largest current change when switching between different gas atmospheres.