A low-resistance Pd/Ge/Ti/Au ohmic contact to a high-low doped GaAs field-effect transistor

Pd/Ge/Ti/Au ohmic contacts have been studied for application to high-low doped GaAs metal-semiconductor field-effect transistors (MESFETs). The interfacial reaction of the Pd/Ge/Ti/Au contact is investigated using X-ray diffraction, Auger depth profile, and transmission electron microscopy. The good Pd/Ge/Ti/Au ohmic contact with the lowest contact resistivity of 2.8 × 10^-6 Ω cm² is obtained after annealing at 340°C, which is two times lower than that of the Pd/Ge contact. This is due to formation of AuGa through fast in-diffusion of Au toward the GaAs substrate. The AuGa compound enhances creation of more Ga vacancies, followed by incorporation of Ge into the Ga vacancies, and it allows the contact to be formed directly on the buried high-doped GaAs layer. The MESFET with the Pd/Ge/Ti/Au ohmic contact displays good d.c. characteristics. This supports the fact that the Pd/Ge/Ti/Au ohmic contact is well suitable for application to high-low doped GaAs MESFETs due to its low-resistance characteristics and good surface morphology.