92 CHAPTER 4 Sensors at the micro-scale analog devices (e.g. transistors) and that the scaling limits of sensors are therefore close to the limits of the binary switches, which were derived in Chapter 3 from basic physics. In fact, the field-effect transistor is used in many practical implementations of chemical sensors. It is shown that a nanowire- based FET performs the best for many bio-chemical sensing applications. The discussion in this chapter is focused on three types of sensors: (i) bio-electrical; (ii) bio- chemical; and (iii) bio-thermal. While other types of sensors, for example light or pressure sensors, are also essential for nanomorphic measurements, the size and scope of this book does not allow for an inclusive study of different sensors. In principle, scaling behavior of other types of sensors can also be analyzed within the barrier model framework and readers may find such an exercise worthwhile. This chapter, by its nature, is an integration of concepts from both biological and physical sciences. To aid the reader, a brief glossary of biological terminology used herein is included at the end of the chapter. 4.2 SENSOR BASICS A sensor is a device which converts an external physical stimulus into a distinguishable and processable signal, usually in electrical form [1]. Sensors are sometimes defined as a whole system,