Tuesday, April 27, 2004

ODCAD] A play of Material in Next Century

In next century, what do we expect for the hi tech industry? The
industry from tele-comunication in wire or wire less, to hand held
electronic device. A lot of expectation is there. Well, one important
factor to meet the expectations is the electronic materials. Si based
technology may not meet all of the exepectation such as flxible
display device. Organic material, epecially polymer play important
role. A good article on www.mrs.org/publications/bulletin has a
detailed discussion about the electronic materials and its
application in hi tech.

ODCAD from OD Software Incorporated (ODSI)(http://www.odcad.com/)-the expert and tookit provider of electronic material, device.

Wednesday, April 14, 2004

[ODCAD] Mobility Effect :Junction of Organic Semiconductor, Electrode

The charge carrier mobility is critical information for the simulation (modeling) of electronic devices such as OLED, TFT etc. Its value of organic semiconductor is usually much lower than crystal Si material. This low mobility has impact to the transistor (see "Organic TFT Transistor: Interface and Performance"). Also, it has impact to the electrical behavior of the other device.

For a layered structure device, say a simple three layer DIODE device: bottom electrode, middle semiconductor, top electrode. Assume the bottom junction is ohmic, then the diode is due to the top junction. One popular approximate equation is Schottky junction model. The reversed current J0 measured for the junction is usually 6 order (or higher) less than what the model predicts (see "Reversed Current in Schottky Junction"). What is the reason to cause this?

There are quite few reasons for this. One important effect is due to the slow mobility of the charge carrier. A complete model considering charge injection and charge diffusion is Thermionic Emission-Diffusion model (Sze 2nd Edition). In this model, the mobility effect (drift velocity) is trivial if it is large enough compared with thermal charge carrier velocity. Otherwise, the injected current is proportion to the drift velocity that is the product of mobility and field. Dr. Scott from IBM lab in San Jose, CA) has done a set of experiments and the results have confirmed the mobility effect.

This does tell us that the current can be dependent on the mobility even it is at junction control. For device engineer, he (she) has to design (choose) the material to ensure the current obtained from the device can meet the requirements.

More related articles can be found in Electronic Device Group (click the link to join the group).

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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.

Copy right owned by OD Software Incorporated (ODSI)- the expert, and tool kit provider of electronic material, devices

Wednesday, April 07, 2004

All info is currently posted on Electronic Device Group. Click the link at the right to visite the site.