Digital Signal Processing Reference
In-Depth Information
Table 2.5
Summary of the comparative study on resistors in organic electronics technology
Architecture
Metal wire
Pentacene resistor
Linear region transistor
R per square (
/
)
0.8
19
270
/mm
2
R per Area
20k
-
-
Reliability
High
Very low
Moderate
Employability
Low
Low
Moderate
2.7.1.4 Discussion
A resistor that is used in an analog circuit typically has a value that is in the same range
or larger than the voltage-current ratio in that circuit. At the moment for organic elec-
tronics technology, which suffers from a low mobility compared to standard CMOS,
this voltage-current ratio is located in the 1-100M
range. Three different resistor
architectures, the metal line resistor, the pentacene resistor and the transistor biased
in the linear region have been discussed in Sects.
2.7.1.1
to
2.7.1.3
. The metal line
resistors are probably the most robust and linear resistors yet they have resistances
which are two or three decades below the useful range in organic electronics technol-
ogy. Consequently, the required area to build the resistors is far too large. Pentacene
resistors built with a pure pentacene film have shown a very high resistance per
square, which is probably even too high. Moreover, pentacene resistors are expected
to depend on the threshold voltage and therefore their resistance is unpredictable and
unreliable in organic electronics technology. Finally, transistors biased in the linear
regime have been employed as resistors. These give nice measurement results in the
desired resistive range. Their drawback is that the linearity is only ensured when
V
SG
−
V
T
is larger than
V
SD
and furthermore in practical application the source
contact must be connected to a virtual ground node in order to keep
V
SG
fixed. In
Table
2.5
a summary of the comparative resistor study is given.
2.7.2 Capacitors
Large rectangular metal plate capacitors can be constructed using the available gold
layers in the technology. In Eq. (
2.24
) the capacitance per area
C
A
of such a capacitor
built with metal layers
M
1
and
M
2
is given.
85 pF/mm
2
C
A
=
(2.24)
Since the permittivity is a fixedmaterial constant, the variations on the capacitance
are only determined by the parameter
d
in an inversely proportional way hence only
small relative variations of the capacitance are expected. Furthermore, a parasitic
leakage path is experienced in the capacitor. This leakage is experimentally deter-
