Recessed-gate GaN MESFET using ICP-RIE for high temperature microwave applications

In this paper, for the first time, we present the DC and RF performances of recessed-gate GaN MESFETs fabricated using ICP-RIE. This recessed gate process has enabled GaN MESFETs to achieve the highest frequency results compared to those previously reported. The MESFET layer used in the study was grown on a sapphire substrate by metal organic chemical vapor deposition (MOCVD). The epilayer consisted of a 2 /spl mu/m semi-insulating GaN buffer, a 200 nm lightly-doped (/spl sim/2/spl times/10/sup 17/ cm/sup -3/) channel, and a 40 nm heavily-doped (>5/spl times/10/sup 18/ cm/sup -3/) GaN cap layer. Mesa isolation was achieved using ICP-RIE in a Cl/sub 2/-Ar plasma. The source and drain ohmic contacts were formed using alloyed Ti/Al/Ti/Au metallization. PECVD-deposited Si/sub 3/N/sub 4/ was used as an etch mask for the gate-recess etch. Then, electron beam lithography was used to pattern 1 /spl mu/m recess windows centered between the source and drain in the Si/sub 3/N/sub 4/ mask. Gate recessing was performed using ICP-RIE in a Cl/sub 2/-Ar plasma at 5 mT and -50 V bias. The drain current was monitored during recessing. The drain current decreased significantly at the beginning and saturated when the cap layer was completely removed. After recess, the Si/sub 3/N/sub 4/ was removed. The sample was then rapid-thermally-annealed to eliminate etch-induced defects. Finally, a mushroom-shaped 0.25 /spl mu/m Ni-Au Schottky gate was fabricated by electron beam lithography. The device width was 100 /spl mu/m while the source-drain spacing was 3 /spl mu/m.