tesi R. Miscioscia.pdf - EleA@UniSA

30 State of the Art
Ageing and stress factors
Dynamic/AC behaviours
These parameter are known to be relevant in the determination of
the switching speed, static and dynamic power dissipations, logic
thresholds and noise margins in digital logics based on MISFETs and
finally in the design on complementary-logic circuitry.
Keeping in mind these considerations, it’s natural to understand
why among the principal efforts of the research in material
development there is the achievement of high charge mobility values,
elevated on/off ratios and low working voltages (and threshold
voltages).
For these reasons, for a long time, the attention has been kept on
the synthesis and development of organic materials which, when
employed as channel semiconductors, would result in high field-effect
mobility and low concentration of trap states.
As technology advances and as research deepens its knowledge of
new material’s physics and chemistry, from the study of new chemical
formulations and novel processing technologies, the focus is moving
to integration and optimization-related issues switching to higher
order perspectives and to system-specific aspects like the necessity to
have the availability of both n-type and p-type OFETs but also to
make predictable controllable and repeatable the threshold voltages.
Furthermore, another fundamental issue is related to device
processing, in fact to make effective the employment of OTFTs, it has
become evident the need for fabrication workflows which fit the
realization of complementary CMOS-like circuits. From this point of
view, the evidence that the maximum operating frequencies (clockfrequencies)
are connected (from the device’s point of view) not only
to channel’s mobility but also to the capacitive load effect of the gates
on the driver circuits, recalls that aspects as gate overlaps and
geometric tolerances in layouts and then to design rules have to be
addressed [27].
Also environmental and stability issues are emerging as an
industrial scenario comes to be prospected: the need for organic
materials and devices which exhibit low sensitivity to external
contaminations, humidity, oxygen, light etc. are even more