Deep level effects in GaN based HEMTs for radiofrequency applications. Different solutions for mitigation of short channel effect

In this thesis we have investigated some of the most common solutions for the mitigation of short channel effects based on the confinement of electrons within the channel layer. In the devices considered in this study, confinement was achieved (i) by means of a carbon or iron doped buffer layer, (ii) by adopting nitrogen polar GaN, and (iii) thanks to the AlN nucleation layer acting as a back-barrier in “buffer-free” devices. Each technology has been experimentally characterized in order to investigate the presence of deep levels in the structure and identify their nature. More in detail, we carried out a series of measurements including DC, Pulsed IV and Drain Current Transient Spectroscopy measurements. Moreover, a circuit level model was developed, capable of predicting the dynamic variation, in time, of the VTH of an AlGaN/AlN/GaN HEMT under off-state stress. The model was validated by means of experimental measurements, with good results. Further refinements are required to enlarge the number of cases in which this model can be adopted.