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Microstructural reactions of a Pd/Ge contact to a AlGaAs/InGaAs pseudomorphic high electron mobility transistor (PHEMT) have been investigated, and the results have been used to interpret the changes of electrical properties of the PHEMT. In the as-deposited state, a ternary phase of Pd x GaAs containing excess Ge atoms is formed at the interface of Pd/GaAs. When the ohmic metals deposited were annealed at 300°C, a PdGe compound was formed and its contact resistivity was decreased to 1.2×10-7 Ω·cm2. This is due to the preferential outdiffusion of Ga atoms to the PdGe during annealing, leaving Ga vacancies behind. Simultaneously, Ge atoms are indiffused to the GaAs and occupy the Ga vacancies, converting the interfacial GaAs layer into n+-GaAs. The low-temperature PdGe ohmic contact allows changing the sequence in the formation of source/drain and gate electrodes in the PHEMT process. The device performance was remarkably improved as the annealing temperature was increased or the contact resistivity was reduced. The PHEMTs with a gate length of 1.1 µm, annealed at 300°C, exhibit maximum transconductance of 381 mS/mm.

The effect of existence of undoped GaAs/AlGaAs cap layers on Ohmic contact resistivity in an AlGaAs/InGaAs pseudomorphic high electron mobility transistor was investigated. The Au/Ge/Ni/Au Ohmic contact formed on the undoped GaAs cap layer yields the lowest contact resistivity of 3.8×10−6 Ω cm2. Meanwhile, the contact resistivity increases a little to 5.0×10−6 Ω cm2 for the contacts formed on an n-Al0.23Ga0.77As layer exposed by removing the undoped cap layer. Both contact resistivities are comparable to those obtained using the n+-GaAs cap layer. The good Ohmic contacts obtained independent of removal of the undoped cap layer are due to the formation of the interfacial compounds, Au2Al and Au2Ga, during annealing. The interfacial compounds penetrate deep into the buried InGaAs channel, resulting in direct contacts to the channel.

The contact resistance degradation mechanism of the Pd/Ge ohmic contact on a pseudomorphic high electron mobility transistor was interpreted. The contact resistance continuously degrades at temperatures higher than 300 °C although no microstructural changes were observed. Meanwhile, elemental Ga atoms were preferentially outdiffused to the PdGe layer and Ge atoms were indiffused to the GaAs during annealing. The indiffused Ge converts the interfacial GaAs layer into n-GaAs, in which Ga vacancies are produced to maintain the charge neutrality condition. The Ga vacancy concentration further increases with annealing temperature, which plays a role in degrading the contact resistance as a result of the recombination of electrons with the Ga vacancies.