Deep level effects and on-wafer reliability have been evaluated in 0.15 μm AlGaN/GaN HEMTs differing for buffer compensation (C or Fe) and for epitaxial structure (C-doped monolayer devices with only C-doped region; C-doped bilayer samples with a barrier between the channel and the C-doped region). In monolayer devices trapping effects lead to the presence of a long time constant (50s) in drain current transients, related to the presence of CN defects; interaction with deep levels in C-doped buffer leads to knee voltage walkout, R _on increase and recoverable IDS degradation. Negative trapped charge in the gate-drain region reduces the electric field with respect to bilayer devices. Results from semi-on and on-state step stress show that by using a ‘Bi-layer’ AlGaN barrier, the interaction between C-related deep levels and channel hot electrons can be effectively reduced, thus alleviating trapping effects and parametric degradation, and giving the opportunity of fully exploiting the improved confinement offered by the C-doped buffer, at the expenses of a tolerable decrease of breakdown voltage.