**1. Introduction**

The importance and demand for sensing gases, vapors, and volatile organic compounds (VOCs) have been increasing in fields such as diagnostics [1–4], environmental monitoring for industrial, agricultural, home safety, and so on [4, 5]. Various types of gas sensors and sensor arrays have been researched and developed [6–8], including field-effect transistor (FET)-based sensors. Following the report of pioneering work on catalytic-gate FETs, research on FET-based gas sensors has been extended to various types of gas-sensitive FETs. In this chapter, catalytic-gate FETs, suspended-gate FETs (SGFETs), and solid electrolyte-based FETs are introduced. Gas-sensitive FETs based on nanomaterials such as carbon nanotubes (CNTs), nanowires (NWs), graphene, and transition metal chalcogenides have also been investigated because the high surface-to-volume ratios of nanomaterials are attractive for improving sensor properties [5, 9]. These nanomaterial-based FETs are also reviewed.

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For recognition of gaseous and volatile analytes from sensing results, two main methods have been used [3]. The conventional recognition method uses selective sensors with specific receptors designed for selective interaction with target analytes [3, 6]. Another recognition method uses a combination of cross-reactive sensor arrays and pattern recognition methods [3, 6–8, 10]. These cross-reactive sensor arrays consist of gas sensors that are responsive to a broad range of analytes and have differential sensitivities. To date, various gas sensors have been applied in sensor arrays [6, 8], including gas-sensitive FETs. In this chapter, research on the combination of FET-based sensor arrays and pattern recognition methods is briefly reviewed.
