Methods to achieve Ohmic contact to p-GaN

In this review, the mechanism of LED light emission and the formation of barrier height on GaN-based semiconductor are analyzed, which affects the device electrical properties. After explanation of Ohmic contact physics, to achieve Ohmic contact to p-type GaN is considered as the rather difficult work. The method to improve the conductivity of contact to pGaN is concluded. The surface treatment is suggested to remove insulating native oxide on GaN surface while the annealing in oxygen ambient may have multiple effects on p-GaN structure or active carrier concentration, although the mechanism of annealing is still a controversial issue. Ni with Au cap layer is a popular scheme for p-GaN but the property could be further improved via the formation of solid solution with other divalent metal or those extracting hydrogen. Finally, the use of superlattice/strained layer like AlGaN or InGaN could create 2-D hole gas, which further boosts the carrier concentration.