What are computers without silicon

Extremely flat transistor without silicon

Several billion transistors are now switching in highly developed processors. The chip structures based on the semiconductor silicon can shrink even further, but the minimum switching voltages required can hardly be reduced and are reaching their physical limits. With a new type of transistor made from germanium and molybdenum disulfide, scientists have now achieved significantly lower values. As they report in the journal “Nature”, their technology could in future be used to develop more economical chips with an extremely high transistor density.

“Our transistor has the potential to provide a new avenue for ultra-dense, low-power circuits,” write Deblina Sarkar and her colleagues from the University of California at Santa Barbara. For their prototype, instead of the established semiconductor silicon, they used a wafer-thin layer of doped germanium. They linked these with a layer of molybdenum disulfide that was only a few atoms thick. They completed this unusual chip architecture with an insulating dielectric and the three contact electrodes common for transistors.

For the switching process, the conduction band of molybdenum disulfide was lowered a little with a basic voltage. This enabled electrons from the valence band of germanium to tunnel through a potential barrier. The experiments by Sarkar and colleagues showed that this so-called tunnel transistor (TFET) could be switched with low voltages of around 100 millivolts. Compared to conventional transistors, their TFET managed with a tenth of the electrical power. In addition to lower power consumption, a processor made from tunnel transistors would have the advantage of generating significantly less waste heat.

Many years of development work are required before such tunnel transistors can be arranged in high density in a processor. However, this prototype already has the properties that chip developers consider necessary on their roadmap for progressive miniaturization. The task now is to develop suitable processes for mass production in order, for example, to be able to integrate the extremely flat and therefore two-dimensional molybdenum disulfide layers in high quality and quickly into a chip architecture.

A working group from the Kavli Institute for Nanoscale Science at Cornell University took this route a few months ago. With an optimized vapor deposition process, they created molybdenum disulphide layers only three atomic layers thick and a few square centimeters in size. From these they made an area of ​​200 field effect transistors. In the future, this experience could also be used for the production of chips with numerous tunnel transistors.