In this study, we have shown that it is possible to deposit a silicon oxynitride membrane on silica ISFETs by plasma enhanced chemical vapor deposition (PECVD) and to develop a solid-state device for pH detection which works in a differential mode. Silicon technology using a new encapsulation technique and a new geometry of the ISFETs is described. The PECVD oxynitride membrane is deposited without damaging the structure. A nernstian stable pH response is obtained : 57.4 +/- 0.4 mV . pH-1 between pH 2 and 8.3 and it is only slightly affected by the concentration of Na+ ions. Moreover, this silicon oxynitride membrane can be grafted with a long alkyl chain silane in order to make a REFET with a sensitivity of 36.3 +/- 0.3 mV . pH-1 between pH 3.3 and 8.1. The pH response of the solid-state device obtained (pH ISFET + REFET + platinum pseudo-reference electrode) is 23.4 +/- 0.2 mV . pH-1 from pH 2 to 8.