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W
V
2
(
)
I
=
µ
C
V
−
V
V
−
ds
(small V
ds
)
(1)
ds
(
linear
)
n
SiN
gs
T
ds
L
2
W
(
)
2
I
= µ
C
V
−
V
(large V
ds
)
(2)
ds
(
saturated
)
n
SiN
gs
T
L
2
where C
SiN
is the capacitance of the gate insulator, W the channel width, and L the
channel length. The dielectric constant
ε
r
of our 150ºC silicon nitride is 7.4 and
r
/
d
SiN
= 1.82 x 10
-8
F cm
-2
, where
we calculate C
SiN
=
o
is the permitivity of
vacuum and d
SiN
is the thickness of the gate dielectric. From the plots, we see that
both Eqs. (1) and (2) are well obeyed in their respective regions. In the linear cur-
rent regime (Fig. 4a), we obtain V
T
= 2.84 V and a mobility of 0.47 cm
2
V
-1
s
-1
. In
the saturation current regime (Fig. 4b), we obtain V
T
= 1.89 V and a saturated mo-
bility of 0.45 cm
2
V
-1
s
-1
. The subthreshold slope, extracted from the exponential
region of the curve for V
ds
= 10 V, S = dV
gs
/d(log I
ds
), is ~ 0.5 V/decade, and it is
typical of a-Si:H TFTs. These values demonstrate that good quality a-Si:H TFTs
can be fabricated on thin foils of polyimide.
ε
ε
ε
o
5. CONCLUSIONS
We fabricated good quality amorphous silicon thin-film transistors on 51
µ
m
thick Kapton E at the maximum process temperature of 150
C. In the TFT fabri-
cation, each layer is optimized for the best electrical performance, and for low
built-in mechanical stress. Therefore, the foil is flat after TFT fabrication. The
off-current is ~ 2 x 10
-12
A, the on-off current ratio ~ 10
7
, the threshold voltage
~ 2 V, the mobility ~ 0.5 cm
2
V
-1
s
-1
, and the subthreshold slope ~ 0.5 V/decade.
Since a-Si:H TFTs are the main building blocks for thin-film electronic circuits,
we have demonstrated that amorphous silicon based electronics on organic poly-
mer substrates is feasible.
°
Acknowledgement
We thank DuPont for the information about Kapton and other polyimides. The au-
thors are grateful to the National Science Foundation, DARPA, and the New Jer-
sey Commission on Science and Technology for financial support.
REFERENCES
1. H. Kuma, T. Iwakuma, F. Moriwaki, M. Fukuda, T. Sekiya, M. Araki, Y. Saito, M. Gomyo and
T. Mita, 2001 Digest of Technical Papers of the Society for Information Display 32, 12 (2001).
2. J.K. Mahon, ibid., p. 22.
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