Integrated pH sensors and performance improvement mechanism of ZnO-based ion-sensitive field-effect transistors

Compact ZnO-based ion-sensitive field-effect-transistor (ISFET) pH sensors were successfully fabricated by integrating the intrinsic zinc oxide (i-ZnO) sensing membrane with the ZnO-based FET. Both the i-ZnO sensing membrane and the various ZnO layers in the ZnO-based FETs were deposited using the vapor cooling condensation system. To improve sensing performances, the photoelectrochemical passivation method was applied to passivate the surface of the i-ZnO sensing membrane in the ZnO-based ISFET pH sensors. Consequently, it effectively suppressed the Fermi level pinning effect induced by the surface states and the dangling bonds resided at the surface. The sensing sensitivity of the passivated ZnO-based ISFET pH sensors was improved from 42.13 to 48.27 mV/pH compared with the unpassivated ZnO-based ISFET pH sensors. Furthermore, the sensing parameters of the ZnO-based ISFET pH sensors were deduced by fitting the site-binding model to the measured dependence of the surface potential on the pH value. The obtained sensing parameter of the passivated ZnO-based ISFET sensors was larger than that of the unpassivated sensors. Finally, the energy band alignment between the i-ZnO sensing membrane and the electrolytic solution was analyzed. It was concluded that the variation of the alignment with the pH value of the solution also contributed to the performance improvement.