I am using a sensing layer with sputtered-SiO2 (30nm) and IGZO as the channel in an ISFET-based pH sensor. Referring to another paper, I am measuring the current by inserting a tungsten probe into the pH solution to apply gate voltage. However, I consistently observe a trend where the threshold voltage increases at pH 4 and decreases at pH 10 in an n-type transistor. This result contradicts the theoretical expectations or other papers. Can you provide any assistance?