Hydrogen ion sensors based on indium tin oxide thin film using radio frequency sputtering system

Indium tin oxide (ITO) thin films were deposited onto Si and SiO2/Si substrates using a radio frequency sputtering system with a grain size of 30-50 nm and thickness of 270-280 nm. ITO/Si and ITO/SiO2/Si sensing structures were achieved and connected to a standard metal-oxide-semiconductor field-effect transistor (MOSFET) as an ITO pH extended-gate field-effect transistor (ITO pH-EGFET). The semiconductor parameter analysis measurement (Keithley 4200) was utilized to measure the current-voltage (I-V) characteristics curves and study the sensing properties of the ITO pH-EGFET. The linear pH voltage sensitivities were about 41.43 and 43.04 mV/pH for the ITO/Si and ITO/SiO2/Si sensing structures, respectively. At the same time, both pH current sensitivities were about 49.86 and 51.73 µA/pH, respectively. Consequently, both sensing structures can be applied as extended-gate sensing heads. The separative structure is suitable for application as a disposable pH sensor.