This paper gives first of all the definition of the work function of an electronic as well as an ionic conductor. FET-type gas sensors known from the literature are considered in view of this basic theory and the parameter of the work function which is responsible for the sensing properties is noticed. This appears not always to be possible for the various types of gas sensors, in this case the gas FET, the SGFET and the IGFET. In contrast to this ambiguity, the sensing parameters of the recently developed MOSFET, meant for application in an electrolyte, can clearly be identified. As an example an oxygen sensor based on the developed MOSFET is described.