TUM researchers Janio Venturini (left), Tom Nilges (right) and Anna Vogel (middle) in front of a measuring device for determining crystal structures. Photo: Andreas Heddergott/TUM.
TUM researchers Janio Venturini (left), Tom Nilges (right) and Anna Vogel (middle) in front of a measuring device for determining crystal structures. Photo: Andreas Heddergott/TUM.

Diodes allow directed flows of current. Without them, modern electronics would be inconceivable. Until now, they had to be made out of two materials with different characteristics. A research team at the Technical University of Munich (TUM) in Germany has now discovered a material that makes it possible to create a diode with a simple change in temperature.

Manufacturing a diode generally involves combining two semiconducting materials with different properties. In most cases these are modified forms of silicon, produced by adding different elements to silicon to create the desired characteristics, a process known as doping.

Doping with phosphorus, arsenic or antimony, which adds free electrons to the material, is called n-type doping. The n refers to the excess of negatively charged electrons. Boron, aluminum and gallium, by contrast, bind electrons from the silicon, resulting in an excess of positively charged holes. This material is termed p-doped. Combining n-conducting and p-conducting materials produces a diode that permits electric current to flow only in one direction.

“We have now found a material which we can cause to be n-conducting or p-conducting simply by changing the temperature,” says Tom Nilges, professor of synthesis and characterization of innovative materials at TUM, and senior author of a paper on this work in Advanced Materials. The researchers have been able to show that a temperature change of just a few degrees is enough to bring about this effect – and that a functioning diode can be created with a temperature gradient.

“When the material is at room temperature, we have a completely normal p-conductor,” explains Nilges. “If we then apply a temperature gradient, we can simultaneously generate an n-conductor in the heated areas.” Handily for practical applications, this effect occurs at room temperatures. “To generate a diode, a local temperature rise of just a few degrees is enough – in our case from 22°C to 35°C.”

For Nilges, the elimination of the need for doping is not the only advantage. “Every diode that is produced is always there. With our material that is not the case: with the temperature gradient, the diode also disappears. If it is needed again, it is enough to create a temperature gradient. If we think about the range of applications for diodes, for example in solar cells or in every kind of electronic component, the potential of this invention becomes evident.”

The search for the perfect material involved 12 years of work, which culminated in the team’s discovery of the coinage metal chalcogenide halide Ag18Cu3Te11Cl3, which consists of the elements silver, copper, tellurium and chlorine. The researchers came across this class of compounds when exploring thermoelectric materials, which generate electricity from heat. One material they studied displayed the p-n switching effect. However, this was observed only at temperatures of around 100°C, which is unsuitable for practical applications.

After extensive analysis and experimentation, the researchers discovered, in Ag18Cu3Te11Cl3, a material that displays the desired effect and is also suitable for applications at normal temperatures. “Other research groups have also discovered this switching effect in various materials, but so far nobody has managed to convert it into a specific application,” explains Nilges.

As a next step, the researchers plan to show that their material can also be used to create transistors, which, like diodes, contain both p- and n-conductors.

This story is adapted from material from the Technical University of Munich, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.