Page 49 - Organic Electronics in Sensors and Biotechnology

26 Chapter One

acoustic wave devices, 91, 92 electrochemical resistive sensors, 93, 94 and
field-effect transistors with a chemically sensing gate (CHEMFETs). 95, 96
Among these sensing schemes, field-effect transistors have attracted
more and more interest due to their ability to amplify in-situ, gate-
modulate channel conductance, and allow for compatibility with
well-developed microelectronic fabrication techniques that enable
miniaturization. A simple resistive sensor probes only the change in
bulk conduction, and simple capacitive sensors probe only the change
in permittivity. Since the drain current in a FET reflects the transport
through the two-dimensional electron gas (2DEG) at the semiconductor-
dielectric interface, instead of the conduction through the bulk, a FET
sensor would directly detect the effects on 2DEG transport caused by
the analyte through the change in the drain current. The organic thin-
film transistor (OTFT) is a promising sensor device for an electronic
olfaction platform that possesses all the required features (sensitivity,
reliability, and reproducibility) at low cost. Compared to CHEMFETs
or chemiresistor sensors, an OTFT sensor can provide more informa-
tion from changes in multiple parameters upon exposure to analyte,
namely, the bulk conductivity of the organic thin film, the field-
induced conductivity, the transistor threshold voltage, and the field
effect mobility. 97
In the second portion of this chapter we will first introduce poly-
crystalline organic and polymeric thin-film field-effect transistors
and then cover such topics as the proper detection of sensing signals
truly from nanoscale active area, the geometry (for device and material)
dependence of the sensing behavior, and discussions for the sensing
mechanisms in these sensors. We will also address several aspects
of the interactions which produce sensing effects in electronic devices.
The chemical sensors made of organic or conjugated polymeric tran-
sistors are operated at room temperature, which gives an advantage
compared to inorganic oxide semiconductor sensors. Upside-down
(see Fig. 1.3) OTFT sensors use the organic semiconductor active
layer as the transducer, which interacts with airborne chemical
species, referred to as analytes. This kind of structure provides ana-
lytes a direct access to the active semiconducting layer and enables
the investigation of how the sensing behaviors depend on its mor-
phology and interface properties. The interaction given by analytes
directly affects the conductive channel of an OTFT sensor, unlike
the sensors made of inorganic MOSFETs 98–101 or the insulated gate
89
FETs (IGFETs use the polymer layer as the gate for a silicon FET )
where the sensing events occur at the gate or gate/insulator boundary
and indirectly modulate the drain current through capacitive cou-
pling. This means conductivity in the upside-down structure can be
affected by changes in mobility (as well as changes in charge density/
threshold) which is not possible in the other sensor configurations.
These upside-down organic and polymer sensors can be refreshed
by reverse-biasing the gate (a high positive voltage for p-channel,