Temperature Dependence of Annealed and Nonannealed HEMT Ohmic Contacts Between 5 and 350 K

HEMT performance critically depends on contact resistance, particularly in cryogenic applications, where the semiconductor resistance contributions are reduced, thanks to lower optical phonon scattering rates. The literature offers little about the temperature dependence of contact resistances in HEMTs. We report a temperature-dependent comparison of the annealed and nonannealed ohmic contacts for low-noise InP HEMTs. Contact resistances of annealed Ge/Au/Ni/Au and nonannealed Ti/Pt/Au ohmic contacts were characterized as a function of temperature between 5 and 350 K. Side-by-side comparison exposes the differences of nonannealed versus annealed structures with respect to the contact and sheet resistance. Whereas both contact types show equivalent performances at cryogenic temperatures for recessed cap structures reminiscent of practical HEMTs, our annealed contacts display a significant increase in contact resistance with increasing temperature, in marked contrast to the near temperature independence found in our nonannealed contacts. The finding is of importance since the source resistance directly enters the transistor minimum noise figure F_min in field-effect transistors. Additional contributions to the parasitic sheet resistance of ohmic, overlay, and electroplated interconnect metals are also characterized as a function of temperature.