AlGaAs/InGaAs pseudomorphic high electron mobility transistor using Pd/Ge ohmic contact

Microstructural reactions of a Pd/Ge contact to a AlGaAs/InGaAs pseudomorph ic high electron mobility transistor (PHEMT) have been investigated, and th e results have been used to interpret the changes of electrical properties of the PHEMT. In the as-deposited state, a tertiary phase of PdxGaAs contai ning excess Ge atoms is formed at the interface of Pd/GaAs. When the ohmic metals deposited were annealed at 300 degreesC, a PdGe compound was formed and its contact resistivity was decreased to 1.2 x 10(-7) Ohm . cm(2). This is due to the preferential outdiffusion of Ga atoms to the PdGe during ann ealing, leaving Ga vacancies behind. Simultaneously, Ge atoms are indiffuse d to the GaAs and occupy the Ga vacancies, converting the interfacial GaAs layer into n(+)-GaAs. The low-temperature PdGe ohmic contact allows changin g the sequence in the formation of source/drain and gate electrodes in the PHEMT process. The device performance was remarkably improved as the anneal ing temperature was increased or the contact resistivity was reduced. The P HEMTs with a gate length of 1.1 mum, annealed at 300 degreesC, exhibit maxi mum transconductance of 381 mS/mm.