Comparing solution-gate and bottom-gate nanowire field-effect transistors on pH sensing with different salt concentrations and surface modifications

Field-effect transistors (FETs) have been developed as pH sensors by using various device structures, fabrication technologies, and sensing film materials. Different transistor structures, like extended-gate (EG) FETs, floating-gate FET sensors, and dual-gate (DG) FETs, can enhance the sensor performance. In this article, we report the effects of using solution-gate and bottom-gate FET configurations on pH sensing and investigate the influence of different ionic concentrations of buffers in the measured signals. The surface charge of hafnium dioxide (HfO₂) affected by the buffer pH, with/without the modification of polyethylene glycol (PEG) terminated with hydroxyl groups, and the location of applied gate voltage are vital factors to the sensor performance in pH sensing. Based on the results, the solution-gate FET exhibits good pH sensitivity even in the high ionic strength solutions of bis-tris propane (BTP), and these values of pH sensitivity are close to the Nernst limit (59.2 mV/pH). In general, silane-PEG-OH modification can reduce the deviations of measured signals in pH sensing. The performance of bottom-gate FET is inferior in the BTP buffers with high ionic solutions but suitable to be operated in low ionic concentrations, such as 0.1, 1, and 10 mM BTP buffers. The size of the ions was also studied and discussed. The solution-gate FET demonstrates excellent performance under high ionic strengths, meaning a more significant potential for detecting biological molecules under physiological conditions.