ISFET pH Sensor



1.1 ISFET is a Metal Oxide Semiconductor Field Effect Transistor (MOSFET) without a gate and leave the gate dielectric layer intact. The Silicon nitride (Si3N4) gate dielectric is used as a Hydrogen ion (H+) sensing layer. The sensing action is done by the changing of its threshold voltage (turn on voltage) by the presence of H+ in a measured solution. In order to use WIPSK-S, you need a reference electrode in order to complete an electrical circuit. We also supply the reference electrode (WIPSK-RE). The 2 components together comprise 3 terminals:

1. Gate terminal (G) is the RE terminal. The gate terminal does not react with the solution being measured. It determines the change in the threshold voltage which changes with the pH level of the solution. (Before use, remove the heat shrinkable tube on one end of the RE, then the white ceramic will be seen.)

2. Drain terminal (D) is an electrical terminal of the ISFET which is biased at a certain voltage and current.

3. Source terminal (S) is an electrically grounded terminal of the ISFET.

Figure: Drawing of the two-terminal ISFET (WIPSK-S) (D and S) and the associated external reference electrode (WIPSK-RE) acting as a gate terminal (G).
The RE is made of Ag/AgCl contained in KCl solution with a ceramic salt bridge.