Back-Channel Electrolyte-Gated a-IGZO Dual-Gate Thin-Film Transistor for Enhancement of pH Sensitivity Over Nernst Limit

A dual-gate amorphous indium–gallium–zinc oxide (a-IGZO) thin-film transistor (TFT) was studied to obtain a pH sensitivity of
$\sim 160$
mV/pH, which is above Nernst limit (59 mV/pH) with a low value of operating voltage (−5 to 5 V). The utilization of the active layer itself as the sensitive surface constitutes the dual-gate TFT structure with the back-channel gated with the electrolyte. The TFT characteristics were optimized by annealing the IGZO film in varying temperatures in oxygen ambience. A double-layered a-IGZO was used to sandwich the source/drain electrodes to eliminate the issues related with the high contact resistance and critical passivation of source/drain electrodes to protect from the exposure of the electrolyte. The single-gate electrolyte-gated thin film transistor showed the pH sensitivity of 24 mV/pH, which was enhanced by 6.7-fold by its dual-gate operation. The operation of dual-gate TFT ion-sensitive field-effect transistors (ISFETs) with varying only bottom gate voltage eliminates the requirement of a reference electrode necessary in single-gate ISFETs. Such dual-gate ISFETs obviate the need of an additional high-
$K$
dielectric needed for the dual-gate TFT ISFETs and could also be fabricated on flexible substrates. These structures with the high sensitivity of
$\sim 160$
mV/pH and requiring low (
$\sim 2~\mu \text{L}$
) analyte solution could be the potential candidates for utilization as chemical and bio-sensors.