Nano-IGZO layer for EGFET in pH sensing characteristics

In-Ga-Zn-O (IGZO) was widely applied in the substrate of TFT to replace a-Si in recent year. In this study, IGZO layer with thickness of 70 nm is first proposed as a pH sensing membrane directly on P-type Si substrate acting as an extended gate of conventional extended-gate field-effect Transistor (EGFET). Material criteria of extended gate electrode are low resistance and high capacitance. Therefore, Ar/O₂ ratio was modified in the rf sputtering with IGZO target. Post deposition anneal was also performed to check the sheet resistance and pH sensing performance. EGFETs were measured in standard pH buffer solution by using B1500A and constant voltage constant current (CVCC) circuit. Similar I_DS-V_GS curves including transconductance (G_m) and substrate swing (S.S.) are obtained in various sputtering conditions of IGZO compared to commercial NMOSFET in CD4007. pH application range is only between pH 2 to pH 10. IGZO-EGFET prepared by Ar/O₂ ambience of 24/1 in sputtering can have a sensitivity of 59.5 mV/pH. Lower sensitivity and linearity can be observed in the samples with RTA treament at higher temp and in O₂ ambience. N₂ anneal at 500°C can be used to improve pH sensing performance for IGZO-EGFET prepared by Ar/O₂ ambience of 20/5 in sputtering. Nano-IGZO layer is verified to be the sensing membrane in EGFET to have a high sensitivity of 59.5 mV/pH for the first time. More studies on enlargement pH application range and minimization of non-ideal effect still need to be investigated.