WORK FUNCTION­BASED GAS SENSORS 189 Section 3 deals with concrete transducer devices. Starting with the Lundström FET, a simple MIS-FET structure, the historical development all the way to the hybrid fabricated high-temperature floating-gate FET (HTFG-FET) and the silicon carbide FET (MISiC-FET) is presented. These devices allow stable operation even at temperatures above 400°C. Additionally, the FG and HTFG concepts are extremely flexible. Due to the hybrid mounting of the chemical-sensitive element, these devices can be adapted to the detection of a large variety of gases. Finally, a FG-FET­based hydrogen safety sensor for future automotive applications is presented in Section 4, and its new and unique detection mechanism is discussed in more detail. 2. THEORETICAL BACKGROUND: GAS ADSORPTION AND WORK FUNCTION CHANGE Suspended-gate field-effect sensors detect certain gas species by measuring the work function changes caused by their adsorption on a well-chosen chemical-sensitive layer. To explain the sensing mechanism in detail, this section discusses the principles of adsorption, the consequential work function change of a solid, and the readout by the FET-based transducer. The sensing mechanism consists of four parts (see Figure 4.2). First, there must be a certain concentration of the target gas species. Depending on this concentration, the gas molecules adsorb at a well-chosen and highly specific chemical-sensitive surface. Due to a partial electron transfer or a simple dipole creation, this adsorption comes along with a work