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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.