In this review article, we focus on two complementary approaches of Fourier transform infrared (FTIR) spectroscopic analysis of semiconductors, in an attempt to bridge a gap between the “atomistic model” and the “rigid band model.” First, we report on selected papers describing the chemical reactions occurring at the surface of semiconductor chemical gas sensors. Then, we mention different studies in which FTIR spectroscopy has been used to monitor the free carrier absorption in semiconductors. Finally, to correlate surface chemistry with electronic properties, we describe our approach combining both possibilities offered by FTIR spectroscopy. This approach allows one to clarify the fundamentals of the gas detection mechanisms and to obtain an unequivocal and rapid correlation between the electrical response of the chemical sensors and the infrared absorption of sensing semiconductors.