Influence of cap-layer doping on ohmic contacts for InP based HEMT structures

Ohmic contacts to lattice matched InP based HEMT structures with both doped and undoped GaInAs cap-layers have been investigated. Contact resistances as low as 0.09 Ωmm were achieved using annealed Ni---Ge---Au ohmic contacts. It is reported that the contact resistance is independent of the doping of the thin (50–100 Å) cap-layer. This indicates that it is the bandgap and not the doping of the cap-layer, which determines the contact resistance to the HEMT structure. Furthermore it was shown, that the contact resistance is reduced for lower sheet resistances of the 2DEG.