First vertical bipolar organic transistor runs at 1.6GHz
Researchers in Germany have developed the first high performance vertical bipolar organic transistor, opening up the development of flexible, organic logic designs.
Organic field-effect transistors (FET) have been developed over the last twenty years for applications such as screen drivers, but are restricted to the low-to-medium megahertz range from the low charge carrier mobility.
Instead, the bipolar junction transistor offers both low capacitance and contact resistance, but has challenges with the miniaturization and process integration. As are result, organic bipolar junction transistors (OBJTs) have not yet been realized, mainly because they rely on minority carrier diffusion through a thin and precisely doped base layer, says lead researcher Shu-Jen Wang from the Dresden Integrated Centre for Applied Physics and Photonic Materials (IAPP).
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The team at Dresden developed an OBJT based on crystalline films of n- and p-type doped rubrene. In contrast to common furnace-grown single crystals, these films are made directly on the surface of a substrate and are so are compatible with mass production. Rubrene, or 5,6,11,12-tetraphenyltetracene, the glowing material in light sticks, is widely used for organic LEDs and flexible displays.
The device uses a vertical stacking of a rectangular emitter electrode at the bottom, a finger-like structured base electrode in the middle and a rectangular collector (top) electrode. The distancing between adjacent fingers of the base electrode and the width of each base finger itself are crucial.
The final device is of pnp type, with an n-doped base, as the researchers expect the p-type minority diffusion length to be higher owing to higher mobility. As is common for organic diode-like devices, intrinsic films are added in between p- and n-doped films to improve reverse leakage behaviour, ending up with a pinip structure. Emitter and collector electrodes are made from gold to facilitate efficient hole injection, whereas the base electrode consists of aluminium for better electron injection.
A thin film of n-doped C60 is added on the emitter side of the base electrode to further facilitate electron injection. Additional layers of intrinsic and weakly doped material can be added on top of the base electrode to minimize base–collector leakage.
The highly crystalline films with improved mobilities and low capacitance of the vertical design enable ultrafast operation in the gigahertz range, says the researchers. The transconductance of the OBJT is as high as 0.1 S, in the range in which devices show amplification, whereas the capacitance is around 10 pF. This results in a transition frequency of 1.6 GHz, which is similar to the speed of operation found for the single, rubrene-based diodes and is a step of 10 to 40 times the performance of state of the art of organic transistors.
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