It appears that the usual SiO2 from the MOSFET process does not fulfil the requirements of a high value of βint. The pH sensitivity is only about 30mV/dec depending also on the electrolyte concentration via Cdif. Therefore other layers have
been introduced such as Si3N4, Al2O3 and Ta2O5 with increased values of βint. The intrinsic buffer capacity of Ta2O5 is even so high that the value of Cdif becomes less important which means that independent of the electrolyte concentration a pH sensitivity of 58 mV/dec can be achieved over a pH range from 1 to 12.