Scientists have presented for the first time a bipolar transistor based on an organic semiconductor and capable of operating in the gigahertz range. To do this, they use the hydrocarbon rubrene, which in its crystalline state has similarly favorable properties to the usual silicon, as they report in the specialist magazine “Nature”. The scientists see their technology primarily in medical applications, where flexible electronics could open up new possibilities.
Transistors are among the most important components of modern electronics and are used in almost all electronic circuits. The two most common designs are called field effect and bipolar transistors and differ in the type of control and the areas of application. While field effect transistors are used with high currents and are controlled via the voltage, bipolar transistors are controlled via the current. Their field of application is the low-current range, in which higher clock frequencies are also required.
Looking for organic bipolar transistors
Both types are nowadays usually based on the semiconductor silicon. This allows the transistors to be scaled down to the nanometer range, which enables greater performance and thus extremely fast data processing. One problem with the relatively rigid technology, however, is that it is rather unsuitable for flexible components such as roll-up displays or for medical applications on or in the body.
Shu-Jen Wang and Michael Sawatzki from the Technical University of Dresden and their team have now presented an organic transistor that is intended to solve these problems. “A major challenge in the implementation of an organic bipolar transistor is to find a suitable material and a configuration that enables both the necessary n- and p-doping and sufficient mobility of the charge carriers to allow electrons and corresponding holes to flow in the desired way transport,” explains the team.
Rubrene as a semiconductor
The scientists used carbon-based rubrene for their transistor. This organic semiconductor is made up of several rings of an aromatic hydrocarbon and has long been used for organic light-emitting diodes. Its charge carriers are particularly mobile in the crystalline form of rubrene.
To construct the transistor, the researchers applied the variously doped rubrene layers required for the transistor to function on a crystalline base layer about 20 nanometers high. The structure of these layers, which are between 100 and 300 nanometers thick, is based on the high order of the crystalline base layer. Gold electrodes served as emitter and collector, and an aluminum electrode formed the basis.
1.6 gigahertz are possible
“The first realization of the organic bipolar transistor was a great challenge because we had to realize layers of very high quality and new structures. However, the excellent parameters of the device reward this effort,” says Wang. Their configuration enabled high carrier velocity of the entire transistor.
As tests showed, the bipolar transistor achieved a high transition frequency, which can be taken as a measure of the speed of the component. Previous organic-based models were only implemented as field effect transistors and had a transit frequency of 40 to 160 megahertz. In contrast, the new bipolar transistor developed by the Dresden researchers is said to have a frequency of up to 1.6 gigahertz.
“New Perspectives for Organic Electronics”
“We’ve been thinking about this device for 20 years and I’m thrilled that we’ve now been able to demonstrate it. The organic bipolar transistor and its potential open up completely new perspectives for organic electronics,” says Karl Leo, also from the Technical University and senior author of the study.