The parameter α is a dimensionless sensitivity parameter that varies between 0 and 1,
depending on the intrinsic buffer capacity, βint, of the oxide surface and the
differential double-layer capacitance Cdif. If α=1, the ISFET has a so-called Nernstian
sensitivity of precisely-59.2 mV/pH at 298K, which is also the maximum achievable
sensitivity.
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.