Ohmic Contacts with low contact resistance for GaN HEMTs

GaN-based high electron mobility transistor (HEMT) is a promising candidate for high-frequency and high-power applications due to its outstanding material properties, such as high electric breakdown field and high peak electron drift velocity. A low contact resistance (R_c) contact is essential for the device performance including output power, power efficiency, frequency response and noise performances. To obtain low contact resistances, several studies using different metallization schemes have been demonstrated. A standard Ti/Al/Ni/Au metal stack is a conventional ohmic contact of GaN HEMTs. Ti reacts with AlGaN to form TiN, which results in the creation of nitrogen vacancies which act as donors in AlGaN layers. The resultant N-type doped AlGaN and the conductive TiN facilitate tunneling mechanism of carriers at the interface.