STUDY OF TRAPPING IN GALLIUM NITRIDE HEMTS FOR RF APPLICATIONS

In recent years new wide band-gap semiconductors are emerging for RF transistors and in many applications, they are substituting the traditional semiconductors. In particular, Gallium Nitride (GaN) and GaN alloys heterostructures are widely studied for High Electron Mobility Transistors (HEMTs). Thanks to the high electron mobility, saturation velocity and breakdown voltage of GaN, Gallium Nitride HEMTs show greater RF performances and the capability to withstand higher voltages. GaN HEMTs are already available on the market, but industries and universities are studying new epitaxial structures and growth techniques in order to optimize these devices and to reach higher frequencies and performances. In this work we have examined HEMTs exploiting both Gallium and Nitrogen polarity. We have studied the trapping mechanisms in highly scaled devices, with gate length below 0.15µm. We have found two de-trapping processes. The first is thermally activated with an activation energy around 0.6eV, it is drain stress voltage dependent and it is ascribed to a semiconductor trap. The second mechanism is a surface de-trapping process. It has slow time constants, it is weakly thermally activated (0.3eV) and it is due to a superimposition of states. We have extrapolated an empirical model that describes the recovery of threshold voltage at different drain stress voltages and temperatures. We have explore in deep the surface trapping. In particular the influence of the surface processes passivation and etching on trapping behavior and reliability during high temperature reverse bias tests (HTRB tests). We have examined the effect on trapping of the epitaxial structure, in particular barrier properties and buffer layers. A chapter of this thesis is dedicated to N-polar HEMTs. This configuration is becoming popular due to the very promising performances. In these devices we have studied the influence of the Aluminum concentration in AlGaN cap layer under the gate Schottky contact. We have found a correlation between gate leakage and trapping. This thesis is an overview of the most recent structures of GaN HEMTs for RF applications and it gives hints on the optimization of GaN HEMTs