Microsystems for Bioelectronics

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-chem ical sensing applications.

The discussion in this chapter is focused on three types of sensors: (i) bio-electrical; (ii) bio-

chemical; and (iii) bio-the rmal. 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 senso r is a device which converts an external physical stimulus into a distingui shable and

processable signal, usually in electrical form [1]. Sensors are sometimes defined as a whole system,

comprising, e.g., (i) a transducer, which generates an electrical signal in response to an external

stimulus, (ii) an electronic amplifier, which increases the intensity of the signal, (iii) a signal processor,

(iv) a display, etc. [1]. In this text, a narrower definition of a sensor is used: a device directly responding

to external stimuli, i.e. a transducer.

Examples of external physical stimuli are mechanical (e.g. pressure, motion, vibration), electrical

(e.g. voltage), thermal (e.g. temperature difference), electromagnetic (e.g. light), chemical (e.g.

presence/absence of particular chemical species), etc.

In response to the external stimulus, the sensor generates an electrical signal, which is further

processed by a control unit and eventually provides a basis for further actions by the nanomorphic cell.

In its simplest form, a sensor generates a binary YES/NO response by distinguishing between the

presence and absence of a particular external stimulus. In other words, a sensor must have at least two

distinguishable states. As was discussed in Chapter 3, creation of the distinguishable states requires

energy barriers within the sensing device. Indeed, in many cases, a sensor can be regarded as a switch,

whose barrier is deformed by different stimuli, e.g. pressure, light, temperature, presence of ions in

a solution, etc . Note that the FET discussed in Chapter 3 is indeed an electrical sensor, whose stimulus

is the presence/absence of an electrical charge on the gate electrode. In fact, a charge-sensing tran-

sistor, e.g. the FET, is usually a receiving device downstream from the sensor that is used for sign al

conditioning (e.g. amplification, filtering, etc.), as is shown in Figure 4.1.

Due to the intrinsic similarity between a sensor and the FET, the fundamental scaling limits of

sensors are, in principle, the same for the FET, which were discussed in Chapter 3. However, there are

additional constraints for sensors arising from requirements of sensitivity, selectivity, and response

time, as will be discussed in this chapter. Also, while sensors can be used in a digital (i.e. binary) mode,

in most cases they are used as analog devices (see Box 4.1). As a side note, some sensors can be

powered, at least partially, by the energy of the external stimulus. For example, a thermoelectric

temperature sensor (e.g. thermocouple/thermopile) can be used as an energy source converting heat

into electricity, as was discussed in Chapter 2.

92 CHAPTER 4 Sensors at the micro-scale