Digital Signal Processing Reference
In-Depth Information
2.2.2 Working Principle
The thin film of pentacene in an organic thin-film transistor (OTFT) is built with
a discrete number of monolayers, as can be seen in Fig.
2.4
. Since the conduc-
tivity in pentacene in the X-Y plane, i.e., inside one monolayer, is much higher
than the conductivity in the Z direction, i.e., in between the monolayers the cur-
rents are assumed to flow in a laminar way. Furthermore, most of the current flows
in the monolayer which is the closest to the insulator (Sancho-Garca et al.
2003
;
Klauk
2006
).
Pentacene is an ambipolar semiconductor since the accumulation regime of both
types of carriers is in principle possible, depending on the sign of the gate voltage.
Electrons and holes can move freely inside the pentacene molecule. Furthermore,
because of the close molecule stacking in the herringbone crystal structure the elec-
trons/holes can relatively easily hop from one to a neighboring molecule, which is
actually a quantum mechanical tunneling effect (Gelinck et al.
2005
). Grain bound-
aries are the most critical regions for carrier hopping and these pull the mobility
down. The hole mobility in a pentacene transistor is around 0.1-1 cm
2
/Vs which is
about three decades below the mobility in monocrystalline
Si
. The electron mobility
is much lower and even negligible in air for two reasons. First, a pentacene molecule
that possesses an electron in its lowest unoccupied molecular orbital (LUMO) is very
reactive to H
2
O molecules which are present in the ambient air (Leeuw et al.
1997
),
according to the redox reaction in Eq. (
2.3
):
C
22
H
13
+
OH
−
2
×
2
×
H
2
O
2
×
C
22
H
14
+
H
2
+
2
×
(2.3)
Second, hydroxyl groups (R
OH) at the semiconductor-insulator interface are
believed to act like
e
−
traps which immobilize the negative charge carriers (Chua et
al.
2005
). Pentacene is therefore considered, as most other organic semiconductors, to
be a p-type semiconductor only. When the pentacene transistor is biased
on
, holes are
accumulated in the channel near the semiconductor-insulator interface which starts
to conduct current. In practice, pentacene is considered as a unipolar semiconductor.
−
2.2.3 Alternative Organic Semiconductors
The research for organic semiconductors is performed by chemists and material engi-
neers all over the world. Without aiming to give a full material and chemical analysis,
a few of the most promising engineered organic semiconductors are summarized in
this section.
2.2.3.1 Buckminsterfullerene
Buckminsterfullerene, better known as Bucky balls or
C
60
, is a spherical molecule
built with carbon atoms only. This molecule, which consists of 20 hexagons and 12
