BASIC PRINCIPLES OF OPERATION · 15 operating at high temperatures. It was demonstrated that SiC-based sensors respond to a change in ambient between oxygen and propane even at 1000°C (Lundstrom et al. 2007). 6. chemFET-BASED SENSORS The schematic of a chemFET is similar to that of a MOSFET except that the gate structure is slightly modified. Based on the gate structure, the family of chemFETs can be divided into three main types, ion-sensitive field-effect transistors (ISFETs), enzymatically selective field-effect transistors (ENFETs), and basic or work function chemFETs (Wilson et al. 2001). The ion-sensitive field-effect transistor is a chemFET structure without a conductive gate. The ion- selective layer is placed on top of the insulator layer of the FET structure. The enzymatically selective fi eld-effect transistor is also a chemFET structure without a conductive gate. The chemically sensitive enzyme layer forms a part of or the entire insulator layer of the FET. The basic or work function chem- FET is a chemFET structure with a conductive gate. The conductive gate is the chemically selective layer (Wilson et al. 2001). All of these chemFET structures work on the basic principle that the surface charge changes at the interface between the insulator and the overlying layer (the overlying layer is the ion-selective layer in an ISFET, the enzyme-selective layer in the ENFET, and the gate layer in the basic or work function chemFET) (Yotter and Wilson 2004). This results in a change in the work function, and therefore, the threshold voltage changes in accordance with Eq. (1.15). Extensive research has been conducted on chemFETs for monitoring ammonium (Brzozka et al. 1997; Senillou et al. 1998), cadmium and lead (Reinhoudt 1995; Ali et al. 2000), Cu 2+ (Taillades et al. 1999), hydrogen (Domansky et al. 1998), and pH (Bausells et al. 1999; Cho and Chiang 2000). Ion-sensitive field-effect transistors were first developed in the early 1970s (Bergveld 1970). Figure 1.12 is a schematic diagram of an ISFET. As can be seen, its structure is the same as that of a MOSFET. The main component of an ISFET is an ordinary metal-oxide silicon field-effect transistor (MOSFET) with the gate electrode replaced by a chemically sensitive membrane, solution, and a reference electrode Figure 1.12. Schematic diagram of an ISFET.