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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