A generalized theory of an electrolyte-insulator-semiconductor field-effect transistor

A model of surface ionization and complexation of surface hydroxyl groups on the gate insulator surface is adapted in conjunction with electronic device physics to arrive at a generalized theory for the current-voltage characteristics of an electrolyte-insulator-semiconductor field-effect transistor (EISFET) in electrolyte solutions. EISFET's that employ thermally grown silicon dioxide were tested in simple electrolytes that contain Na
⁺
, K
⁺
, and Li
⁺
ions titrated in a
p
H range from 2 to 9. Experimental results show good agreement with the theory. The model successfully explains
p
H sensitivity, as well as the ion interference effect, of the EISFET working as a
p
H sensor. From this model, it is conluded that, among all the electrolyte parameters associated with an EISFET, the surface site density of the hydroxyl groups N
s
and the separation of surface ionization constants
\Delta pK
are the primary factors to consider when employing EISFET's as
p
H sensors. For high sensitivity and good selectivity, large N
s
and small
\Delta pK
values are required.