Often encountered in the literature dealing with label-free biosensing technologies are sensors made of semiconductor nanowires, silicon nanowires in particular. Since semiconductor nanowires are compatible with CMOS technology, they are often considered as ideal devices to detect chemical and biological elements, including DNA, viruses, and cancer markers for instance. The principle of operation and and the architecture of these nanowires are very similar to the gate-all-around junctionless FETs, except that there is no solid-state electrode over the gate insulator. These nanowires have no source/drain implant and are highly doped i.e., they are fabricated with minimal technological steps. In addition, they exhibit a high surface-to-volume ratio. These features explain why they are seen as the ultimate devices for label-free detection of ions, nucleic acids, and protein markers to say the least. More generally, nanowire FETs can be configured as highly sensitive biosensors by linking recognition or receptor molecules to their surface [189]. However, despite being used in many “bio” applications, analytical models simulating the intrinsic characteristics of such ungated nanowires are missing. Extending the approach presented in Chapter 3 by including additional features to predict the electrical properties intrinsic of semiconductor nanowires when used as biosensors [190] is the goal of this chapter.

Principle of Semiconductor-based Field-Effect Biosensors

The electrostatic interaction between a solution containing chemical species (ions, biomolecules, etc.) and the gate electrode of a field-effect transistor is a topic of interest in the molecular processes of biology. At the beginning was the so-called ion-sensitive field-effect transistor (ISFET) proposed by Bergveld in the 1970s [191]. This device was aimed at measuring ion activities in electrochemical and biological environments and in particular the action potentials that build up during neuronal activity. An overview of ISFET technology can be found in [192–195] and a typical representation is shown in Figure 13.1 [192]. The principle of operation relies on the modification of the charge density between a liquid and an ion-sensitive gate, which induces variations in the drain current.

When immersing an insulator (silicon dioxide) in an electrolyte, a transition layer is created at the interface between the silicon dioxide and the liquid.