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 x 10(-6) Omega cm(2). Meanwhile, the contact resistivity increases a little to 5.0 x 10(-6) Omega cm(2) fdr 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.