Performance enhancement mechanisms of passivated InN/GaN-heterostructured ion-selective field-effect-transistor pH sensors

The InN/GaN heterostructure with high sheet electron concentration was utilized to fabricate the ion-selective field-effect-transistor (ISFET) pH sensors. To improve the sensing performances of the InN/GaN-heterostructured ISFET pH sensors, the PEC passivation treatment was applied to reduce the surface states resided on the sensing membrane and to suppress the Fermi level pining effect. Consequently, the passivated InN/GaN-heterostructured pH sensors exhibited a linear pH response with a sensing sensitivity of 52.04 mV/pH, while the unpassivated one showed a nonlinear behavior, especially in low pH (<7) region, with an average sensitivity of 34.75 mV/pH. The improvement of the sensing response in acid solution was attributed to the decrease of the surface states density on the sensing membrane by the PEC passivation function. Furthermore, it was demonstrated that the experimental data can be well described by the site-binding model. Based on the model, the sensing parameters were derived. The resulting sensitivity parameter value for the passivated sensor was 1.18, obviously larger than the value 0.37 for the unpassivated one. The selectivity coefficients of the passivated InN/GaN-heterostructured pH sensors for interfering ions of sodium and potassium are -7.62 and -7.901, respectively.