Page 124 - Organic Electronics in Sensors and Biotechnology

Strain and Pressure Sensors Based on OFET 101

pentacene film was vacuum-sublimed, and finally 60 nm thick Au
drain and source electrodes were evaporated through a shadow
mask. For comparison a capacitor was also manufactured simultane-
ously in the same base film. The authors observed large changes in
the drain current that cannot be explained only by the deformation of
the device structure. Then in the analysis of the electrical character-
istics, changes in the structural parameters of the transistors (namely,
channel width W and length L and the thickness of the gate insulator)
were taken into account to evaluate possible variations in the mobility
of the transistor. The transfer characteristics of the realized transistor
and the mobility variations observed are shown in Fig. 3.8 as a func-
tion of strain or bending radius. The mobility increases monotonically
when the strain is changed from tension to compression passing
through the flat state.

–15 4.6
(a) 8.1
17
–10
I ds (μA) –5 Compression R (mm)

0
17
(b) 10.5
I ds (μA) –10 Tension 5.9
4.6
–5
R (mm)
0
20 10 0 –10 –20 –30 –40 –50
(V)
V gs
Bending radii (mm)
4 5 6 10 30 30 10 6 5 4
1.2 1.2
(c) (d )
1.1 1.1
μ/μ 0 1.0 1.0 μ/μ 0

0.9 0.9
Tension Compression
0.8 0.8
–2 –1.5 –1 –0.5 0 0.5 1 1.5 2
Strain (%)

FIGURE 3.8 Transfer curves on (a) compressive and (b) tensile strains. Mobility
as function of strain or bending radius in (c) tensile strain and (d) compressive
14
strain. Solid and open circles refer to transistors whose source-drain current
path is, respectively, parallel and perpendicular to the strain direction. (Reprinted
with permission from Ref. 14. Copyright 2005, American Institute of Physics.)