Page 118 - Organic Electronics in Sensors and Biotechnology

Strain and Pressure Sensors Based on OFET 95

The low thickness of the semiconductor is due to the fact that this
device does not work in inversion mode; i.e., the carriers that accumu-
late in the channel are the same that normally flow in the bulk in the
off state. Therefore, if the semiconductor were too thick, the off cur-
rent would be too high and the switching ability of the device would
be compromised. For the same reason, this structure is used only for
low mobility semiconductors, as high mobility semiconductors would
give rise, even in thin film devices, to high off currents. An individual
I –V curve has a linear region at V << V − V and a saturation region
d d d g t
at V > V − V , where V is the device threshold voltage.
d g t t
The basic device performances are described by the field-effect
mobility μ and the on/off ratio. The equations developed for inor-
ganic semiconductor devices are often used to extract the value of the
3
field-effect mobility from the I–V curves. Strictly speaking, these
equations apply only to TFTs that exhibit constant charge carrier
mobility; this is not the case for polycrystalline OTFTs. Thus, extracted
mobility values should be considered only as estimations of the real
value. Typical extracted values are in the 10 to 10 cm /(V · s) range,
−1
2
−3
2
but they can be as high as 1 to 10 cm /(V · s) with pentacene. 4
The on/off ratio, defined as the ratio of I in the on (V > V ) and
d g t
off (V << V ) states, indicates the switching performance of the device.
g t
A low off current is desirable to ensure a true switching of the tran-
sistor to the off state; this is achieved by keeping the doping level of the
organic semiconductor as low as possible. Because of non-intentional
doping, this is not easily accomplished with polycrystalline organic
active layers. However, on/off values as high as 10 , suitable for
6
most applications, have been reached with several different organic
materials. Another important parameter is the threshold voltage V ,
t
which marks the passage from the off to the on conductivity regime. 5
As organic semiconductors can in principle support both hole and
3
electron conduction, in principle V should be zero. But, as a matter
t
of fact, trap states for electrons or holes heavily affect the transistor
performance and, in particular, the value of V . Therefore, the thresh-
t
old voltage V corresponds to the voltage required to fill the trap
t
states in the organic material or at the interface with the gate dielec-
tric. To obtain a sensing device from an OTFT, it is necessary to have
6
a specific sensitivity to the parameter to be sensed. In chemical sensors,
typically, the sensing mechanism is due to a chemical interaction
between the chemical compound to sense and the semiconductor
itself, normally affecting its mobility. In detectors for physical
2
parameters (for instance, deformation or pressure sensors), the exter-
nal stimulus must reversibly affect one of the different layers of the
device and result in a variation of one or more of the electronic param-
eters (mobility, threshold voltage, etc.) that can be extracted from the
output curves. In the next paragraph, we describe pressure/strain
sensors that can be obtained starting from an organic field-effect
transistor.