Micro pH Sensors and Biosensors Based on Electrochemical Field Effect Transistors

Abstract

A study on ion-sensing using field effect transistor (FET) was begun by Bergveld in the 1970s [1–3]. The ion-sensitive (IS) FET is now widely used as a miniaturized pH sensor, commercialized by some companies. First, the principle and structure of the ISFET are introduced in this section. A basic design of ISFET is shown in Fig. 10.1 a. ISFET has silicon substrate with field-effect structures such as electrolyte/IS layer/(insulator)/semiconductor structures; the space charge region in the semiconductor is modulated depending on the gate voltage (V _g), same as a typical metal-oxide-semiconductor (MOS) FET. A typical bias V _g versus drain-source current (I _ds) characteristic of the device that has silicon nitride/silicon dioxide/silicon is shown in Fig. 10.1 b. This characteristic is quite similar to the MOSFET. A prominent difference between ISFET and MOSFET is that the gate voltage for the operation of the device is applied by an electrochemical reference electrode through the electrolyte in contact with the gate insulator. The threshold voltage (V _th) could shift according to the value of the pH of the solution. In the MOSFET, the V _th would shift depending on the change in the space charge region in the MOS capacitor structure by the application of V _g. On the other hand, the V _th in ISFET would shift according to the change in the surface potential in the electrolyte/IS layer interface. Therefore, the IS layers and their interfaces in ISFET play an important role in the performance of pH responsibility. It is well-known that the silicon nitride surface shows a good pH response in solution. The silicon nitride layer is often formed by plasma-enhanced chemical vapor deposition (PECVD), which is generally formed at the thickness of 100–500 nm. The V _g vs. I _ds, characteristics of the silicon nitride-based ISFET indicate a good pH responsibility of 58 mV/decade that shows Nernstian response (Fig. 10.1 c). The shift of the V _th depends on the changes of surface potential at electrolyte/silicon nitride interface. On the silicon nitride surface immersed in aqueous solution, both amphoteric Si–OH sites and basic Si–NH₂ sites (Fig. 10.1 d) are produced by hydrolysis. These sites directly interact with the solution to either bind or release hydrogen ions, leading to bear a certain surface charge on the nitride surface that was opposed to an ionic charge in the solution. This formed a double-layer capacitance across which the potential drop occurs. Therefore, the threshold voltage shifted accompanied by the pH change in solution.