Title:
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Study of diodes and transistors for organic circuits
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In recent times much research has been carried out regarding the use of
organic materials to fabricate electronic devices, for example diodes and
thin film transistors. This thesis examines the use of polymers and small
molecule materials to fabricate these devices with the aim of developing a
greater understanding of their operation and to provide the basis for the
development of circuits. Various materials are used to fabricate Schottky
diodes and the measurements enable them to be compared. Equations are
developed to describe the operation of the diodes and to facilitate the
modelling of circuits.
Schottky diode characteristics were also studied at various temperatures.
This is important since many of the materials do not conduct heat very
readily and the effect of internal heating may be important. Temperature
measurements also provide useful evidence of the nature of the conduction
process and demonstrate the validity of the analytical work. Such models
often, unintentionally, have temperature as one of the variables.
Capacitance measurements were obtained for the Schottky diodes to
establish how the depletion region width alters with frequency, applied
voltage and doping. A series of expressions were also derived to establish
the theoretical relationships between the parameters and these were
checked against experimental results.
The transient response of the diodes was also modelled and compared to
experimental results. This was carried out at both a uniform temperature
and at varying temperatures.The final chapter of the thesis moves onto polycrystalline materials as
opposed to polymers. The reason for this was the higher carrier mobility. It
is practically important to have a single set of equations for both disordered
and polycrystalline material if they are to be modelled in design software.
This part of the project was to establish how the different transport
mechanisms compare and also how parameters such as Tc vary between
different kinds of material.
The polycrystalline experiments were completed using TFTs because
diodes fabricated with polycrystalline material were not available at the
time.
Having completed this work it is apparent that further work could be
undertaken to aid the analytical aspects of the subject. Polycrystalline
diodes need to be studied and compared with disordered diodes.
Assessment of the potential value of the two types, for circuit applications,
is essential, particularly from the point of view of circuit performance and
cost. This implies an extension of the work to flexible substrates. It would
also be of use to fabricate the devices in more ideal conditions to eliminate
the effects of air. A wider range of temperatures should also be used to
establish how internal heating of the devices affects the results. The effect
of temperature on device capacitance and measurements under pulsed
inputs would go someway to describing the mechanisms involved with
digital circuits.
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