Bipolar transistors made from organic materials for the first time

Move over silicon – digital circuits have a flexible new friend

Scientists in Germany claim to have developed bipolar transistors from organic materials, opening a path for flexible and transparent electronics.

The study, led by Shu-Jen Wang, post-doctoral researcher Technische Universität Dresden, built an organic bipolar junction transistor using doped rubrene. That could help the semiconductor industry to make the switch to organic materials, increasing access to a wide library of materials for building electronic devices.

Transistors are the basis of today's digital circuits and, at a simple level, allow one signal to control another. They can amplify a signal, or switch between 'on' and 'off' states, through control of a current of charge carriers – which are either electrons or their positive counterpart (holes), or both.

There are two broad classes of transistors: field-effect transistors and bipolar transistors. Most transistors are made from inorganic material – most commonly silicon. Over the decades researchers have begun exploring using organic material to build transistors — which could create flexible or transparent systems – but only field-effect transistors have been successfully ported to the new set of materials.

"Achieving high mobilities [of charge carriers] is straightforward for inorganic materials, such as silicon, but more difficult for organic materials. Nonetheless, since the first fabrication of an organic transistor, an organic solar cell and an organic light-emitting diode in the 1980s, tremendous progress has been made in the field of organic electronics, in particular in the OLED display industry," explained Julie Euvrard and Barry Rand of the Department of Electrical and Computer Engineering at Princeton University.

"Organic bipolar junction transistors have not been attempted before because the mobility of charge carriers is low in organic compared with inorganic semiconductors," they said.

"The challenge, then, is to fabricate a functional bipolar junction transistor by engineering an organic material with high mobilities for both [types of] carriers," the commentators said. As the low-mobility characteristic of organic materials arises in part from a lack of crystalline order, the researchers explained these films were engineered on a thin (around 20 nanometres) crystalline template of an organic semiconductor known as rubrene.

As a side benefit of the work, the researchers were able to measure the material's semiconductor property known as the minority-carrier diffusion length, which had not previously been probed in organic semiconductors.

"Wang and colleagues' study suggests that the organic bipolar junction transistor might present a means of accessing this fundamental parameter, enabling a better understanding of these materials and enhancement of existing technologies," Euvrard and Rand said.

In the paper, published in Nature on Wednesday, the researchers speculated that organic semiconductors could support thin-film electronics because of their low cost, biocompatible carbon-based materials and deposition by simple techniques such as evaporation or printing. The approach could "enable organic semiconductor devices to be used for ubiquitous electronics, such as those used on or in the human body or on clothing and packages.

"Our results open the door to new device concepts of high-performance organic electronics with ever faster switching speeds," they concluded. ®

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