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Posted on September 16, 2008 by  & 

CDT develops organic thin film transistor software model

IDTechEx sees that the biggest opportunity for Organic Light Emitting Diodes OLEDs is when they can be large area and/or flexible. Both options are constrained today by the back plane, which is usually a-Si (amorphous silicon with hydrogen in the lattice), the same backplane as LCD displays. Therefore the two compete, with OLEDs typically costing much more given that it is the new technology. However, where LCDs cannot compete is when they can be flexible or very large area, which is possible using Organic Thin Film Transistors OTFTs as the backplane instead of the more brittle and unstable aSi. Only recently has this become feasible because OLEDs are current driven whereas OTFTs are voltage drivers, suited more to voltage driven displays such as electrophoretic displays. However, with ever improving materials OTFT drivers for OLEDs are now in development from companies such as Sony and Cambridge Display Technology (CDT), UK.
 
 
However, material developments are moving faster than it takes to create test devices, which is slow and expensive. This has now been addressed by CDT. CDT has moved a step forward in promoting the future use of OTFT technology: a new Universal Organic Thin Film Transistor (UOTFT) "SPICE" model.
 
In collaboration with Silvaco Data Systems, CDT has developed a model to design and simulate circuits using OTFT (organic thin film transistor) technology.
 
Implemented in Simucad SmartSpice, a leading analogue circuit simulator, the UOTFT SPICE model combines universal charge-based field effect transistor modelling with OTFT specific channel charge, mobility bias, temperature dependences, and nonlinear contact resistances.
 
Well known for the work on printed OLEDs (organic light emitting diodes) and flexible Polymer OLEDs, CDT has now presented the work on Organic Thin Film Transistors (OTFT) as components for OLED backplanes.
 
Jeremy Burroughes, Chief Technical Officer at CDT, pointed out that the fundamental research on OTFTs was required to develop the UOTFT SPICE model. Among other things, the observed differences in the behaviour of short- and long-channel OTFTs in terms of the effective mobility as well as the need for interface modification by several chemical and thermal treatments were used to optimize the model.
 
 
The design of organic electronic circuits depends on accurate and efficient OTFT compact models in commercial circuit simulation tools. A range of distinct features in OTFT device properties require special consideration and a different modelling approach compared to their inorganic counterparts. The most notable differences include a trap-assisted charge distribution and mobility behaviour, unipolar or bipolar charge accumulation operation mode and non-ohmic contact resistances. The existing MOSFET, as well as amorphous and poly-silicon TFT models, are therefore not suitable for the design of OTFT circuits.
 
The Partnership with US company Silvaco Data Systems a leading vendor of TCAD and EDA software is new to CDT and means that software models can predict the performance of OTFT devices rather than creating actual devices, which is expensive and slow. David Fyfe, CEO of CDT stated, it "will speed development and commercialisation of low cost rigid and flexible electronics applications of organic semiconductor technology".
 
Apart from the fundamental research on OTFTs for backplanes and the new UOTFT SPICE model, CDT is also planning to design circuits in the near future and has already started to make simple ones.
 
The project is part-funded by the UK government's Technology Strategy Board and their Technologist for Electrical Systems.
 
 
 

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Technology Analyst

Posted on: September 16, 2008

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