A TFT Strategy for Polymer Circuits
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B9.59.1
A TFT Strategy for Polymer Circuits Naser Sedghi*, Munira Raja*, Giles C. R. Lloyd*, Iain Liversedge†, Simon J. Higgins†, and Bill Eccleston* *
Molecular Electronics Group, Department of Electrical Engineering and Electronics, The University of Liverpool, Liverpool, UK, † Department of Chemistry, The University of Liverpool, Liverpool, UK. ABSTRACT Although integrated circuit design principles are well understood, they lead to the possibility of radically new modes of device operation when applied to conjugated polymers. Design for speed is very important, even when this is not a primary requirement for a particular application, since supply voltage can be reduced and with it power dissipation. For all-polymer circuits, on thermally insulating plastic substrates, device operating temperature will also be reduced. This has implications for device stability. There are a number of factors that are important in increasing circuit speed. Reduction of channel length must be accompanied by a reduction of gate overlap capacitance and this makes the conventional versions of bottom gate TFT perform badly. Vertical devices are a particularly attractive proposition providing that off-currents can be maintained at a low level. One approach is to use Schottky barriers as the source and drain. Examples will be explained, as will the very unusual mode of operation of such devices. Optimum load structures will also be defined.
INTRODUCTION There is a need to consider in some detail the relationship between the kinds of required polymer thin-film transistor (TFT) circuit. This should act as a guide to the design of TFTs. Circuit demands are likely to include such features as low power dissipation, high yield and reliability and, perhaps speed. It is very instructive to consider two inverters, the first driving the second. Fig. 1 shows this configuration for the simplest load, the resistor. In so doing it is simplest to determine the design that will most meet speed requirements. This can later be traded, by reducing the supply voltage, for improvement in the yield and reliability or reduced power dissipation. Many other trade-offs may be possible. For example by increasing the supply voltage it may be possible to give increased drive capability to specific parts of a circuit.
POLYMER TFT INVERTORS For an ideal transistor with no inactive capacitance
µ dQ µC0 W L µ ∝ = 2 = 2 dt C0WL L λm
(1)
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where dQ/dt is the switching speed, µ is the field effect mobility, C0 is the capacitance per unit are of the gate insulator, W is the channel width, L is the channel length, and λm is the smallest feature size possible from a particular technology. This says that decreasing channel length is more productive in improving speed than is increasing mobility. Missing from this simplified equation is the need for high drive voltage, largely because of low mobilities. The gate of transistor B in Figure 1 is being discharged through the channel of an identical transistor A. The discharge current increases with the size of transis
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