Deep Level Effects in N-Polar AlGaN/GaN High Electron Mobility Transistors: Toward Zero Dispersion Effects

A full characterization of deep level effects in N-polar GaN-based HEMTs has been carried out by means of Drain Current Transient Spectroscopy (DCTS). The effect of ohmic contacts (alloyed or regrown), composition of AlGaN cap layer, and of the quality of the epitaxial layers have been studied. We show that the occurrence of an "antidispersion" effect, i.e. of a transient increase of ID during pulsed measurements, is due to a negative shift of the threshold voltage, related to the ionization of Fe-related defects. Through device and epitaxy optimization, dispersion-free devices, showing a 15% transient increase of drain current were obtained.