Temperature-Dependent Characteristics of AlN/Al_0.5Ga_0.5N High Electron Mobility Transistors with Highly Degenerate n-Type GaN Regrown Ohmic Contacts

Herein, the temperature-dependent characteristics of AlN/Al_0.5Ga_0.5N high electron mobility transistors (HEMTs) with highly degenerate n-type GaN (d-GaN) ohmic contacts fabricated via pulsed sputtering deposition (PSD) are investigated. The ohmic contacts formed at the source and drain regions demonstrate low resistivity within the temperature range of 77–473 K. It is found that the temperature dependence of the contact resistance (R_C) for the AlN/Al_0.5Ga_0.5N HEMTs with regrowth d-GaN contacts is attributable to the energy barrier induced by the Fermi level difference between the d-GaN contacts and Al_0.5Ga_0.5N channel layer. Notably, the R_C between the d-GaN contacts and the channel layer is 1.4 and 0.15 Ω mm at 77 and 473 K, respectively. Temperature-dependent device on-resistance coincides with the temperature variation of the two-dimensional electron gas sheet resistance because the contact resistance is negligibly low in the studied temperature range. Moreover, AlN/Al_0.5Ga_0.5N HEMTs with d-GaN contacts exhibit excellent switching characteristics, achieving an I_ON/I_OFF ratio of ≈10⁶, even at high temperatures up to 473 K. The results indicate that the PSD d-GaN contact technique has potential to overcome the limitations of ohmic contacts for high Al-composition AlGaN electronic devices, especially for next-generation high-power-density radio frequency transistors operating at high temperatures.