One transistor for all purposes

(Nanowerk News) Be it in a mobile, fridge or plane - transistors are everywhere. But often they are specialized for only one current range. Prof. Thomas Weitz (NIM) and his team have now developed a nanoscopic transistor of organic semiconductor material that is working perfectly at low and high currents.
Transistors are semiconductor devices that control voltage and currents in electric circuits. Electrical devices are supposed to become smaller and more effective. Consequently, the same applies for transistors. In the field of inorganic semiconductors dimensions below 100 nanometers are already standard.
Organic semiconductors are not yet able to keep up. Their performance regarding charge transport is considerably smaller. But organic structures offer other advantages. The can be printed on an industrial scale, the material costs are lower, and they can be transparently applied to flexible surfaces.
That is why Thomas Weitz and his team intensively work on the optimization of organic transistors. In their recent publication in Nature Nanotechnology ("Vertical, electrolyte-gated organic transistors show continuous operation in the MA cm-2 regime and artificial synaptic behaviour") they present transistors with an unusual structure that are tiny, powerful and above all adjustable.
By changing few parameters during the production process the physicists are able to design nanoscale devices for high or low current densities. “High current densities are crucial for realizing highly integrated high-performance electronics,” explains Thomas Weitz, “In neuronal networks and handheld devices in contrast, low power operation is critical.“

Unusual geometry

The key of success is the combination of the untypical geometry of the transistor and the special kind of gate material which acts as an on-off switch. Generally, transistors have a planar design with two electrodes arranged on the substrate in parallel. Within the new vertical construction, the electrodes have the form of stripes and cross each other.
The transistor is controlled by the so-called electrolyte gating. That means the whole transistor is surrounded by an electrolyte solution. As a function of the applied voltage the ions migrate in or out of the semiconductor material switching on and off the transistor.

Semiconductors as required

The new architecture enables the physicists to influence the electrical properties by changing various parameters. These include the electrode width, the thickness of the semiconductor material and the distance between the electrodes. In addition, the functionality of the transistor does not depend on the electrode or semiconductor material. The application of different semiconductors leads to the same performance.
Apart from that it is easier to produce such nanoscopic elements by applying the material vertically in very thin layers instead of placing it exactly side by side. Moreover, the occasionally sensitive semiconductor is applied in the end and cannot be damaged by prior production steps.

From classical transistor to artificial synapse

“Our aim was to develop a transistor design combining both, the ability to drive high currents as classical transistors, and low voltage operation as we need it for artificial synaptic operation,” the physicist summarizes. Their idea worked out and the result are vertical organic field-effect transistors with exactly selectable dimensions and an ionic gate.
Potential areas of application might be OLEDs and sensors where low voltages, high on-state current densities or large transconductances are required. Of special interest could be the application in so-called memristive elements.
“One could imagine a memristor as an artificial neuron, modeling the behavior of neurons when processing electrical signals,” explains Weitz. “By fine-tuning the geometry of a memristive device, it could be applied for different approaches such as learning processes in artificial synapses.”
Source: Ludwig-Maximilians-Universität München
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