| dc.description.abstract | In traditional AlGaN/GaN HEMTs, ohmic contacts are typically formed using Ti/Al/Ni/Au through high-temperature annealing. For example, in this study, annealing at 850 °C was used to achieve good ohmic contacts, resulting in a specific contact resistivity (ρc) of 3.26 × 10-5 Ω·cm2 and a contact resistance (RC) of 1.39 Ω·mm. By dry transferring graphene onto the AlGaN/GaN sample in the ohmic contact area, the specific contact resistivity was effectively reduced to 1.30 × 10-5 Ω·cm2. Furthermore, by combining the 2 μm horizontal line recessed pattern structure with monolayer graphene in the ohmic contact area, the lowest specific contact resistivity (ρc = 1.98 × 10-8 Ω·cm2) was achieved.
The study includes the graphene transfer process, Raman spectroscopy analysis after rapid thermal annealing at 850 °C for 30 seconds in a nitrogen environment, and basic I-V electrical measurements to analyze the changes in ohmic contact resistance with the use of graphene. The presence of graphene introduces a different conduction mechanism compared to traditional ohmic contacts. During high-temperature annealing, the formation of Ti/Al/Ni/Au alloys (such as titanium nitride) may be promoted, forming a good ohmic contact with the 2DEG channel. Graphene can form titanium carbide at the boundaries of these titanium nitride alloys, resulting in an equivalent high doping concentration, further reducing the ohmic contact resistance.
Additionally, various recessed patterns were etched into the ohmic contact area, combined with either monolayer or bilayer graphene, to fabricate AlGaN/GaN HEMTs, and their characteristics were compared. The comparison of devices with different recessed patterns in the ohmic contact area showed that those with a grid recessed pattern combined with bilayer graphene exhibited the lowest on-resistance. Finally, a DC electrical comparison of devices with two different recessed depths (above or below the 2DEG channel) combined with bilayer graphene showed that the devices with recessed depths above the 2DEG channel had lower on-resistance. Among these, the hole array recessed pattern devices with bilayer graphene demonstrated the lowest on-resistance. | en_US |