Page 117 - Organic Electronics in Sensors and Biotechnology

94 Cha pte r T h ree

terms of switching speed are not required. For these reasons, organic
semiconductor-based devices offer very interesting opportunities for
sensor applications due to the low cost and easy fabrication tech-
niques, and the possibility of realizing devices on large and flexible
areas on unusual substrates such as paper, plastic, or fabrics. In fact,
being able to obtain large sensing areas is certainly a benefit for a
wide set of applications, and using printing techniques for creating
sensing devices on unusual substrates could certainly widen the set
of possible applications where sensing is required. For instance, many
body parameters can be measured by using non-invasive sensors,
among them geometric and mechanically related parameters such as
respiration rate and amplitude, heart rate, blood pressure, position,
detection of falls, monitoring of various daily activities, etc.
In the following sections, we will focus our attention on organic
semiconductor strain/pressure sensors. We will present the state of
the art of technologies and applications and will show how to take
advantage of a flexible, free-standing dielectric film for obtaining
sensors on conformable, large surfaces.

3.2 Working Principles of Organic Field-Effect
Transistor Sensors
In a field-effect device, the input signal is a voltage, applied through
a capacitive structure to the device channel. This signal, named gate
voltage, modulates the current flowing into a narrow portion of semi-
conductor (the channel) comprised between two ohmic contacts, the
source and drain. A field-effect sensor is based on the idea that in the
presence of the parameter to be sensed (whatever it is, for instance, a
chemical compound), the current is reversibly affected (even by keep-
ing constant the voltages applied to gate and drain) and this modifi-
cation can be exploited to detect the parameter itself. An organic
(semiconductor based) field-effect transistor—(OFET) or organic
thin-film transistor (OTFT)—is usually realized in a thin film configu-
ration which is a structure that was developed for the first time for
1
amorphous silicon devices. Several reviews have described how
OTFTs operate, so we will present only the most salient aspects. 2
An OTFT is composed of a multilayered structure where the gate
capacitor is formed by a metal, an insulator, and a thin organic semi-
conductor layer. On the semiconductor side, two metal contacts,
source and drain, are used for extracting a current that depends both
on the drain-source voltage and on the gate-source voltage. The con-
ductance of the organic semiconductor in the channel region is
switched on and off by the gate electrode, which is capacitively coupled
through a thin dielectric layer. The gate bias V controls the current I
g d
flowing between the source and drain electrodes under an imposed
bias V .
d