Digital circuits based on GaN HEMTs with threshold voltage adjusted by fluorine plasma treatment

We report on development of digital circuits fabrication technique based on depletion mode AlGaN/GaN high electron mobility transistors (HEMTs) with adjusted threshold voltage. Transistors fabrication technique is based on fluorine plasma treatment of AlGaN/GaN on Sapphire HEMT gate region. Plasma treatment process based on CHF3/O2 gas mixture was developed. Post-treatment annealing process was applied to improve carrier mobility, which degrades after plasma treatment due to plasma etching induced damage. Physical etching of top heterostructure layer allowed us to control maximum drain-source saturation current. We have fabricated AlGaN/GaN HEMTs with threshold voltage Vth= −1.36 V, drain-source saturation current Idss=170 mA/mm and transconductance gm=20 mS/mm. The voltage level shifter (LS) which transfer input voltage levels VIH=3÷5 V and VIL=0÷0.4 V to output voltage levels VOH= −0.4÷0 V and VOL= −40 ÷ −35 V was developed and fabricated using Ni-AlGaN/GaN HEMT Schottky diodes. Maximum supply current of LS is 10 mA. In this paper we discuss adjusted threshold voltage AlGaN/GaN HEMT fabrication technique and introduce design and measurement results of HEMTs and LS.