Thursday, April 08, 2004

[ODCAD] Organic TFT Transistor: Interface and Performance

The transistor based on organic semiconductor may not be able to replace Si technology in fast response device product such as CPU. However, for many other applications such as in address cell circuit for display device such as OLED (PLED, LED), its performance is good enough.

The current from drain Id and transconductance gm are proportion to the charge carrier mobility. Both Id and gm are two important characters to show the performance of the device. Large Id means low resistance of the device, which result in small size of the device. Large gm means high amplifying capability that also result in the reduction of device size.

For organic semiconductor, its mobility is usually much smaller than crystal Si material. The best value obtained so far is 5 cm2/V-S (some lab claims that 10 cm2/V-s). This organic material is at crystal phase. For polymer semiconductor, the mobility is even lower. The best charge carrier mobility in polymer is 0.05 cm2/V-s.

The main reason for the low mobility of the organic semiconductor is due to lack of ordered material structure like Si crystal. For TFT transistor, the important area to decide the device performance is the interface between the dielectric and the organic semiconductor. It is about 10 A thickness of the interface whose charge carrier mobility is critical.

This gives a good task for engineers. They should design the method to manufacture the device in the way that can ensure highly ordered organic semiconductor at the junction (interface). For polymer semiconductor, the direction of the order (the conjugated bond direction) should be along the charge carrier transportation direction.

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