Active solid-state devices (e.g. – transistors – solid-state diode – Organic semiconductor material
Reexamination Certificate
1999-06-02
2002-02-05
Crane, Sara (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Organic semiconductor material
C257S289000, C257S295000, C257S310000
Reexamination Certificate
active
06344660
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to the field of organic thin film field effect transistors (TFT), in particular to flat panel liquid crystal displays using such transistors.
BACKGROUND AND PRIOR ART
Thin film field effect transistors (TFT) used in liquid crystal display (LCD) applications typically use amorphous silicon (a-Si:H) as the semiconductor and silicon oxide and/or silicon nitride as the gate insulator. Recent developments in materials have led to the exploration of organic oligomers such as hexathiophene and derivatives, and organic molecules such as pentacene (G. Horowitz, D. Fichou, X. Peng, Z. Xu, F. Garnier,
Solid State Commun
. Volume 72, pg. 381, 1989; F. Garnier, G. Horowitz, D. Fichou, U.S. Pat. No. 5,347,144) as potential replacements for amorphous silicon as the semiconductor in thin-film field-effect transistors.
The highest field effect mobility in thiophene-oligomer-based TFT's is usually about 0.06 cm
2
V
−1
sec
−1
(F. Garnier, R. Hajlaoui, A. Yassar, P. Srivastava,
Science
, Volume 265, pg. 1684, 1994), which is substantially lower than the mobility of standard a-Si:H TFT's. Only in the case that the organic insulator cyanoethylpullulane was used, was a higher field effect mobility measured (0.4 cm
2
V
−1
sec
−1
, F. Garnier, G. Horowitz, D. Fichou, U.S. Pat. No. 5,347,144). However, that insulator exhibits some undesirable characteristics such as inferior dielectric strength, mobile charges, (G. Horowitz, F. Deloffre, F. Garnier, R. Hajlaoui, M. Hmyene, A. Yassar,
Synthetic Metals
, Volume 54, pg 435, 1993) and sensitivity to humidity. Hence it is not suitable for use as a gate insulator in the fabrication of practical TFT devices. Field effect mobility up to 0.6 cm
2
V
−1
sec
−1
has recently been achieved in pentacene based TFT's with SiO
2
as the gate insulator (Y. Y. Lin, D. J. Gundlach, T. N. Jackson, 54
th
Annual Device Research Conference Digest
, 1996 pg. 80), making them potential candidates for such applications. Major drawbacks of these pentacene-based organic TFT's are high threshold voltage, high operating voltages required to achieve high mobility and simultaneously produce high current modulation (typically about 100 V when 0.4 &mgr;m thick SiO
2
insulator is used), and high sub-threshold slope, S, which is approximately 14 V per decade of current modulation (Y. Y. Lin, D. J. Gundlach, T. N. Jackson, 54
th
Annual Device Research Conference Digest
, 1996, pg. 80) as compared to about 0.3 V per decade of current modulation achieved in a-Si:H based TFT's (C.-Y. Chen, J. Kanicki, 54
th
Annual Device Research Conference Digest
, 1996, pg. 68). Reducing the thickness of the gate insulator would improve the above mentioned characteristics but there is a limit to the decrease of the insulator thickness, which is imposed by ease of manufacturing and reliability issues. For example in the current generation of TFT LCD devices the thickness of the TFT gate insulator is typically 0.4 &mgr;m.
The electrical characteristics of TFT's having pentacene as the semiconductor, a heavily doped Si-wafer as the gate electrode, thermally grown SiO
2
on the surface of the Si-wafer as the gate insulator, and Au source and drain electrodes, are adequately modeled by standard field effect transistor equations (S. M. Sze “
Physics of Semiconductor Devices
”, Wiley, N.Y., 1981, pg. 442), as shown previously (G. Horowitz, D. Fichou, X. Peng, Z. Xu, F. Garnier,
Solid State Commun
. Volume 72, pg. 381, 1989; C. D. Dimitrakopoulos, A. R. Brown, A. Pomp,
J. Appl. Phys
. Volume 80, pg. 2501, 1996). The pentacene used in these devices behaves as a p-type semiconductor.
FIG. 1
, cited from Y. Y. Lin, D. J. Gundlach, T. N. Jackson, 54
th
Annual Device Research Conference Digest
, 1996, pg. 80, shows the dependence of the current flowing between the source and drain electrodes (I
D
) on the voltage applied to the drain electrode (V
D
), at discrete voltages applied to the gate electrode (V
G
). When the gate electrode is biased negatively with respect to the grounded source electrode, pentacene-based TFT's operate in the accumulation mode and the accumulated carriers are holes. At low V
D
, I
D
increases linearly with V
D
(linear region) and is approximately given by the equation:
I
D
=
W
C
i
L
μ
(
V
G
-
V
T
-
V
D
2
)
V
D
(
1
)
where L is the channel length, W is the channel width, C
i
is the capacitance per unit area of the insulating layer, V
T
is a threshold voltage, and &mgr; is the field effect mobility. &mgr; can be calculated in the linear region from the transconductance:
g
m
=
(
∂
I
D
∂
V
G
)
V
D
=
const
=
W
C
i
L
μ
V
D
,
(
2
)
by plotting I
D
vs. V
G
at a constant low V
D
and equating the value of the slope of this plot to g
m
.
When the drain electrode is more negatively biased than the gate electrode (i.e. −V
D
≧−V
G
), with the source electrode being grounded (i.e. V
s
=0), the current flowing between source and drain electrodes (I
D
) tends to saturate (does not increase any further) due to the pinch-off in the accumulation layer (saturation region), and is modeled by the equation:
I
D
=
W
C
i
2
L
μ
(
V
G
-
V
T
)
2
.
(
3
)
FIG. 2
a shows the dependence of I
D
on V
G
in saturation (Y. Y. Lin, D. J. Gundlach, T. N. Jackson, 54
th
Annual Device Research Conference Digest
, 1996, pg. 80). The field effect mobility can be calculated from the slope of the {square root over (|I
D
+L |)} vs. V
G
plot.
FIG. 2
b
shows a plot of the square root of I
D
vs V
G
. A mobility of 0.62 cm
2
V
−1
sec
−1
can be calculated from this plot. The sub-threshold slope, S, is approximately 14 volts per decade of current modulation (Y. Y. Lin, D. J. Gundlach, T. N. Jackson, 54
th
Annual Device Research Conference Digest
, 1996, pg. 80).
OBJECT OF THE INVENTION
It is an object of this invention is to demonstrate TFT structures that overcome the need to use high operating voltages in order to achieve the desirable combination of high field effect mobility, low threshold voltage, low sub-threshold slope, and high current modulation, without having to reduce the thickness of the insulator. Such structures contain an inorganic high dielectric constant gate insulator layer (for example, barium strontium titanate) in combination with an organic semiconductor (for example, pentacene).
It is another object of this invention to produce organic TFT structures wherein the high dielectric constant gate insulator is deposited and processed at temperatures compatible with glass and plastic substrates (150 to 400° C.), which are substantially lower than the processing temperatures of these materials when they are used for memory applications (up to 650° C.).
SUMMARY OF THE INVENTION
The proposed TFT structures make use of a high dielectric constant thin film gate insulator, an organic semiconductor such as pentacene, and a metal, conducting polymer, highly doped high conductivity material or a combination thereof as the gate, source, and drain electrodes.
There are many candidate materials with high dielectric constant that can be used as gate insulator layers in the above structures, including but not restricted to Ta
2
O
5
, Y
2
O
3
, TiO
2
, and the family of ferroelectric insulators, including but not restricted to PbZr
x
Ti
1−x
O
3
(PZT), Bi
4
Ti
3
O
12
, BaMgF
4
, barium zirconate titanate (BZT) and Ba
x
Sr
1−x
TiO
3
(BST). These materials have been studied and used in the past in combination with inorganic semiconductors mainly for memory device applications (P. Balk,
Advanced Materials
, Volume 7, pg. 703, 1995 and references therein) but never in combination with organic semiconductors. Typically these insulators are annealed at 600° C. or higher to achieve dielectric constant (&egr;) values exceeding 150.
In general the proposed structure uses a
Dimitrakopoulos Christos Dimitrios
Duncombe Peter Richard
Furman Bruce K.
Laibowitz Robert B.
Neumayer Deborah Ann
Crane Sara
International Business Machines - Corporation
Morris Daniel P.
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