SENSITIVITY CONTROL OF ISFETs BY CHEMICAL SURFACE MODIFICATION

The response of ISFETs (ion-sensltlve field-effect transistors) to concentrations of ions, especxally H+ ions, 1s determined by the type of gate surface Both the number of active surface sites and (proton) assoclatlon and dlssoclatlon constants influence the sensltlvlty In the case of a chemically-moddled gate surface, a new surface 1s formed, which generally has a different sensltlvlty It 1s shown that the original pH response of the gate oxide can be either lowered or mcreased, depending on the reactivity of the added groups In general, coverage with apolar groups and reduction of the number of sites result m a lower pH response, while addltlon of basic or acldlc groups as well as an increase of active sites give a higher pH response Using the extended site-dlssoclatlon model, which describes the behavlour of a surface composed of two types of sites, theoretical curves for surface potential versus pH are calculated Measurements with chemlcallytreated S102 and Ta,05 ISFETs conclusion has been drawn that both the point of zero charge changed confnm the theoretical expectations The by a proper choice of chemical treatment, (pzc) and the pH-msensltlve range can be