BASIC PRINCIPLES OF OPERATION · 19 and hydrocarbons as well as an indirect response to oxygen molecules, the effect of which is to decrease the direct response. Besides Si, compound semiconductors such as GaAs, InP, GaN, Ga 2 O 3, and SiC have been alterna- tively employed as substrate materials for Schottky diode­type hydrogen sensors (Chen and Chou 2003; Trinchi et al., 2004). GaN, Ga 2 O 3, and SiC have a wide bandgap, and hence operating temperatures up to 900°C are achievable, as compared to silicon substrates, for which operating temperatures are, limited up to 250°C (Trinchi et al. 2003). 8. CATALYTIC SENSORS The catalytic sensors widely known as "catalytic bead" or "pellistors" were among the first chemical sensors. Such detectors have been in widespread use for more the 50 years in portable, transportable, and fixed multipoint gas alarms. State-of-the-art catalytic sensors are stable, reliable, accurate, rug- ged, and have a long operating life. The output is linear because the platinum wire has a good linear coefficient of thermal resistance. Although many design improvements have been made to detectors of this type over the years, in essence, the basic concept has not changed (Symons 1992; Miller 2001; Korotcenkov 2007). In operation, the pellet and consequently the catalyst layer is heated by passing a current through the underlying coil. In the presence of a flammable gas or vapor, the hot catalyst allows oxidation to occur in a chemical reaction similar to combustion. Just as in combustion, the catalytic reaction releases heat, which causes the temperature of the catalyst together with that of its underlying pellet and coil to rise. This rise in temperature results in a change in the electrical resistance of the coil, and it is this change in electrical resistance which constitutes the signal from the sensor. The sensor temperature rise can be detected via an increase in the Pt coil resistance, typically by incorporating the sensing element in a Wheatstone bridge circuit. A measurement voltage is applied across both arms of the bridge, and the resistance in each arm is matched so that the potential differ- ence measured across the center of the bridge is zero. Any change in the resistance of the platinum wire will now result in a change in this measured voltage. The detection elements may take various forms according to the origin of their design. Possible versions of detection elements in simplified form are presented in Figure 1.15. The simplest form is Figure 1.15. Schematic views of various pellistors. (Reprinted with permission from Korotcenkov 2007b. Copyright 2007 Elsevier.)