Bipolar transistors go organic

Organic bipolar transistors
Organic bipolar transistors can also handle demanding data processing and transmission tasks on flexible electronic elements – for example here, for electrocardiogram (ECG) data. Courtesy: © Jakob Lindenthal

Researchers in Germany have created the first-ever bipolar transistor made from an organic semiconductor. The new transistor boasts outstanding performance, a vertical architecture and a high differential amplification, and could find applications in high-performance thin-film and flexible electronics where data must be analysed and transmitted at high speeds.

Transistors are used throughout modern electronics as switches to control the flow of charge carriers – electrons or holes – through a circuit. Bipolar transistors are special because they make use of both electrons and holes, and this extra capability means they are well-suited to high-speed and high-power applications. Building them from organic semiconductors, rather than inorganic ones, could give electronics designers the scope to make such high-speed and high-power devices flexible and transparent.

A team led by Karl Leo of TU Dresden has now taken a step towards this goal by constructing an organic bipolar junction transistor from highly ordered (crystalline) thin films of an organic semiconductor called rubrene. This material has a high charge mobility, meaning that charge carriers move through it extremely fast and over long distances.

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Layer by layer

Bipolar junction transistors consist of three terminals separated by semiconducting materials that are either p- or n-type. In the devices, these semiconductors are arranged alternately, in either a pnp or a npn configuration.

Leo’s group had previously made both p- and n-type rubrene films, but in the latest work, they took the additional step of engineering these films on a very thin crystalline rubrene layer around 20 nm thick. The films then act a seed for subsequent p- and n- layers as well as layers that are i-type – that is, they are neither n- or p- and thus carry neither negative nor positive charge carriers. “While such films had been made before, we are the first to electrically dope them and realize complex device stacks,” Leo explains.

Device characterization

The researchers estimate that the transition frequency of their new device – essentially, a measure of its speed – is 1.6 GHz. This is much higher than that the record for organic field-effect transistors (OFETs), which is 40 MHz for a vertically-configured device and 160 Hz for a horizontally-configured one. However, Leo notes that the device’s speed per voltage is a more relevant measure of its performance. “Here, the new device with about 400MHz/V is almost a hundred times faster than previous organic transistors,” he says.

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What is more, Leo tells Physics World that the team’s new transistors can be used to determine an important device parameter for organic materials: the minority carrier diffusion length. This parameter, which is key for optimizing device efficiency, is the distance that the minority carrier (electrons in p-type semiconductors; holes in n-type semiconductors) can travel before it recombines with a carrier of opposite charge. In silicon, this quantity can be many microns in length. The value for organics was expected to be much smaller, but in this class of materials it was basically unknown, Leo says.

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In the highly ordered layers employed in this work, the TU Dresden team determined that the minority carrier diffusion length was 50 nm, long enough to make the transistors work well. However, Leo stresses that further studies are still needed to determine which parameters of the material control this quantity and how it can be optimized.

According to the researchers, the new transistor could be used in applications such as signal processing and wireless transmission in which data must be analysed and transmitted at high speed. They are now working to reduce the leakage current in the device, which would allow them to measure its operating speed directly. “We also wish to generalize the application of the highly-ordered layer technique to other devices,” Leo reveals.

The team describe the work in Nature.