The thermal stability of nonalloyed Mo/Au and Ti/Pt/Au contacts on n(+) InGaAs, ranging in thickness from 3 to 50 nm, was investigated for a heterojunction bipolar process. Layers of highly silicon doped InGaAs were grown on n(+) GaAs by molecular beam epitaxy, without a compositionally graded layer between the InGaAs and GaAs. The as-deposited contact resistance exhibits a strong dependence on InGaAs thickness. For In0.5Ga0.5As, as the InGaAs thickness increases from 6 to 20 nm, the contact resistance decreases from 8X10(-7)Omega cm(2) to as low as 2x10(-7)Omega cm(2). Ohmic contacts with various InGaAs thicknesses, InGaAs composition, and contact metallurgies were heat treated at 300 degrees C for times of 2 h or more. The contact resistivity of MoAu on In0.5Ga0.5As contact layers that are thinner than 20 nm shows poor thermal stability, increasing as much as 160% for a 6 nm thick layer of InGaAs. Contacts on a pseudomorphic InxGa1-xAs layer of the same thickness, but with a lower In mole fraction of x=0.35, exhibit better thermal stability, increasing by only 15% in contact resistivity with heat treatment. Stable contacts can also be obtained by making the In0.5Ga0.5As thickness at least 20 nm. Cross-sectional transmission electron microscopy (TEM) of 20 nm InGaAs films heat treated at 300 degrees C for 2 h reveals a virtually unchanged Mo-InGaAs interface. TEM analysis of TiPtAu contacts shows that the Ti/InGaAs interface and the InGaAs layer thickness become increasingly nonuniform with thicker titanium layers. This physical degradation correlates to an increase in the contact resistivity. The increase in contact resistivity can be minimized by reducing the Ti thickness to 5 nm or less. For extended anneals at 300 degrees C for accumulated times of 397 h, MoAu on 20 nm of In0.5Ga0.5As demonstrates better stability in contact resistance than Ti (5 nm)/Pt/Au on 20 nm of In0.5Ga0.5As.