Ohmic contacts to GaN materials

In this review article, a comprehensive study of the mechanisms of Ohmic contact formation on GaN-based materials is presented. After a brief introduction on the physics of Ohmic contacts, a resume ofthe most important results obtained in literature is reported for each of the systems taken in considera-tion (n-type GaN, p-type GaN and AlGaN/GaN heterostructures). The optimal metallization schemes andprocessing conditions to obtain low resistance Ohmic contacts are presented, discussing the role of thesingle metals composing the stack and the modification induced by the thermal annealing, either on themetal layers or at the interface with GaN. Physical insights on the mechanism of Ohmic contact forma-tion have been gained by correlating the temperature dependence of the electrical parameters with amorphological/structural analysis of the interface. In the case of the AlGaN/GaN systems, the influence ofthe heterostructure parameters on the Ohmic contacts has been taken into account adapting the classicalthermionic field emission model to the presence of the two dimensional electron gas (2DEG). Finally, thestate of the art of "Au-free" metallization to AlGaN/GaN heterostructures is also presented, being thislatter a relevant topic for the integration of GaN technology on large scale Silicon devices fabs.