Electronic Material Properties - und Geowissenschaften ...

I D [10 -5 A]
14
12
10
8
6
4
2
0
-2
-4
-6
-8
-10
-12
Before Cycle 1
After Cycle 8
V GS = -80V
0 20 40
VDS [V]
60 80
-80 -60 -40 -20 0 20 40 60 80
with a low current hysteresis
and a steep increase in the
linear region of the transistor,
indicating ohmic contacts for
electron injection. In contrast, an
OFET built on a UV modified
PMMA dielectric exhibits initially
negligible hole and no electron
currents, as illustrated by the
output characteristic shown in
Fig. 3. However, once the OFET
is further operated in electron
accumulation, as shown by the
first quadrant of Fig. 3 at
VGS = 0 V, a significant increase
in ID between the initial and the
8 th operating cycle is visible.
This increase in ID, is due to a
quasi ambipolar hole current [6]
for │VDS│≥│VGS -Vth,p│. The
unipolar p-type characteristic of
this transistor can be confirmed when considering the third quadrant of Fig. 3, showing the
output characteristic measured at VGS = -80 V after the 8 th cycle in hole accumulation.
Surprisingly, the increase in ID occurs despite an energy difference of 2.2eV between the
work function of Ca (2.87eV) and the highest occupied molecular orbital of pentacene
(5.07eV). The pronounced S-shaped output characteristic in the linear region of the OFET
(Fig. 3) indicates the expected large contact resistance.
1/2 [10 -3 A 1/2 ]
I D
12
10
V DS [V]
8
6
4
2
0
V GS = 0V
Individual
Cycles
Linear
regression
Fig. 3: OFET ID characteristic for a UV-modified device. The
first quadrant illustrates ID at VGS = 0 V and the third
quadrant at VGS = 80 V. The arrows indicate the increase in
ID. Inset: ∆Vth during operation in el. accumulation.
The inset of Fig. 3 reveals the origin of the hole current enhancement during subsequent
electron accumulation cycles. Let us first consider the ambipolar drain current equation as
derived by Schmechel et. al. [6] for µn = 0 cm 2 V -1 s -1 and VGS = 0 V:
∆V th
wCeff
µ p
= (VDS
+ Vth,
p − ∆V
) for:│VDS│≥│VGS -Vth,p│. (1)
2l
ID th
Ceff = 10.4 nFcm -2 represents the resulting area capacitance of the SiO2 / PMMA dielectric
layer. With Eq.1 it becomes apparent that the output characteristics in the inset of Fig. 3
are subject to a positive threshold voltage shift (∆Vth), while the hole mobility remains
approximately constant. After the 8 th cycle ∆Vth saturates and reaches approximately 60 V.
This value has been derived from the output characteristic in hole accumulation. We
propose that ∆Vth is a result of trapped negative charge (nt), since it has been
demonstrated that the exposure of PMMA to UV light in air results in the formation of -
COH as well as -COOH functional end groups [4] in the near surface layer of the polymer.
In accordance with the work of Chua et. al. [5], who have recently identified -OH groups as
electron traps, the UV treatment of the PMMA dielectric therefore generates electron traps
at the semiconductor / polymer dielectric interface. Once driven in electron accumulation,
the electron current is inhibited, since the accumulated electrons are trapped. However, as
long as the trapped negative charges are residual, even under hole accumulation, they will
be compensated by positive charges (p) leading to a positive ∆Vth. To obtain a threshold
shift of ∆Vth = 60 V, p can be estimated to be:
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