Page 128 - Organic Electronics in Sensors and Biotechnology

Strain and Pressure Sensors Based on OFET 105

2.4 × 10 –3
2.0 × 10 –3
1.6 × 10 –3
1.2 × 10 –3
8.0 × 10 –4
4.0 × 10 –4
0 2 4 6 8 10 12
Pressure (kPa)
(a)

–23
–24
Average V t (V) –26
–25

–27
–28
–29
0 2 4 6 8 10 12
Pressure (kPa)
(b)

FIGURE 3.13 (a) Average mobility vs. pressure (three samples); (b) average
threshold voltage vs. pressure (three samples). (Reprinted from Ref. 16.
Copyright 2007, with permission from Elsevier.)

the reduced performance of bottom-contact devices when compared
to top-contact devices. An example of the obtained results is shown
in Figs. 3.14 and 3.15.
The threshold voltage decreases with pressure in bottom-contact
devices while in top-contact devices it is pressure-insensitive. This
observation confirms the role of insulator/semiconductor and metal/
semiconductor interfaces in determining the pressure sensitivity of
the device. On the other hand, mobility has a very similar behavior in
top-contact and in bottom-contact devices, indicating a direct contri-
bution of the semiconductor mobility in the channel to the observed
sensitivity. Even if a complete explanation of the observed sensitivity
needs additional investigation, it seems likely that the effect is quite
reasonable since the transport properties of organic molecular sys-
14
tems follow the hopping model rather than the band model. Under
compressive strain, the energy barrier for hole hopping decreases
due to the smaller spacing between molecules, resulting in an increase