Organic bipolar transistors can keep up with the development

Scientists have introduced for the first time bipolar transistors based on organic semiconductors and capable of operating in the gigahertz range. To do this, they used the hydrocarbon rubrene, which in its crystalline state has similar beneficial properties to ordinary silicon, as reported in the special journal Nature. Scientists see their technology primarily in medical applications, where flexible electronics can open up new possibilities.

Transistors are one of the most important components of modern electronics and are used in almost all electronic circuits. The two most common designs are called field-effect transistors and bipolar transistors and differ in the type of control and area of ​​application. While field effect transistors are used with high currents and are controlled by voltage, bipolar transistors are current controlled. Their field of application is low current ranges, which also require higher clock frequencies.

Looking for organic bipolar transistor

Both current types are usually based on silicon semiconductors. This allows the transistor to be reduced down to the nanometer range, enabling better performance and therefore very fast data processing. However, the problem with relatively tight technology is that it can be used for flexible components such as: scrollable display or unsuitable for medical application on or in the body.

Shu-Jen Wang and Michael Sawatzki of the Technical University of Dresden and their team have now introduced an organic transistor designed to solve this problem. “The main challenge in implementing bipolar organic transistors is to find suitable materials and configurations that allow the required n and p doping and sufficient charge carrier mobility to allow suitable electrons and holes to flow in the required ground transport,” the team explains.

Transistor Structure: There is a positive (p), negative (n) and neutral (i) layer of rubrene between the emitter and collector. The substrate (mold) determines the crystal arrangement. The emitter and collector are made of gold, the base is made of aluminum. © Wang et al. /nature/CC by sa 4.0

Rubin as a semiconductor

The scientists used the carbon-based rubene in their transistors. These organic conductors consist of several rings of aromatic hydrocarbons and have long been used in organic light-emitting diodes. Their charge carriers are highly mobile in the form of rubrene crystals.

To build the transistors, the researchers applied multiple layers of doped rubene that required the transistors to operate on a crystal base layer of about 20 nanometers high. The structure of this layer, which is 100–300 nm thick, depends on the high order of the crystal nuclei. The gold electrode serves as the emitter and collector, and the aluminum electrode forms the basis.

1.6 GHz Maybe

“The first realization of organic bipolar transistors was a big challenge because we had to achieve very high quality coatings and new structures. However, the device’s excellent parameters are commensurate with this effort,” said Wang. The configuration allows a high carrier speed of the entire transistor.

As tests show, bipolar transistors achieve a high transition frequency, which can be considered a measure of component speed. Previous organic models were implemented only as field-effect transistors and had a transit frequency of 40 to 160 MHz. In contrast, the new bipolar transistor developed by Dresden researchers is said to have a frequency of up to 1.6 GHz.

“A New Horizon for Organic Electronics”

“We’ve been thinking about this device for 20 years and I’m glad we can now prove it. Organic bipolar transistors and their possibilities open up entirely new horizons for organic electronics, said Carl Liu, also of the Technical University and senior author of the study.

As a potential application area, researchers are looking at, for example, smart assistant devices that can record health data via sensors, process it locally, and transmit it wirelessly. (Nature, 2022; doi: 10.1038 / s41586-022-04837-4)

Source: Technical University of Dresden

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