Graphene, a one-atom-thick layer of graphite where carbon atoms are held in a hexagonal lattice reminiscent of chicken wire, has some unusual properties, one of which may allow it to replace silicon in high-speed electronics. At room temperature, graphene exhibits extremely high electron mobility—the speed at which electrons move through the material is over 100 times greater than silicon. However, because its material properties are different from traditional semiconductors, researchers have struggled to develop integrated circuits with graphene components. Now, a paper in last week's Science demonstrates graphene integrated circuitry printed on silicon wafers.
The authors of the current study developed a radio-frequency (RF) mixer, a device used for frequency conversion in communication hardware. They built this using a graphene field-effect transistor (FET) integrated with two inductors on a silicon carbide (SiC) substrate. Previous RF mixers using a graphene FET did not integrate all components into a circuit, limiting the performance of the device. For example, earlier efforts produced hardware that operated at 10MHz with a 40dB loss of signal; commercial hardware is 100 times faster and has much lower losses. Here, integrating the graphene FET with the other components onto a single wafer avoids the performance losses of previous studies, allowing the authors to target 5GHz as the operating frequency.
Graphene is easily damaged by plasma processing, a standard way of etching circuits, and that has been one of the difficulties preventing easy fabrication of graphene integrated circuits. To avoid damage, the authors layered graphene on top of SiC, then coated it with two layers of polymer (a 140nm layer of polymethyl methacrylate [PMMA] and a 20nm layer of hydrogen silsesquioxane [HSQ]). Electron beam lithography was used to define the FET gate by burning away the polymer. Oxygen plasma then removed the excess graphene, while the PMMA-HSQ layer protected the patterned graphene.
In testing the finished device, the team found that the performance peaked around 4.5GHz, compared to their 5GHz target. At an operating frequency of 4GHz, the frequency conversion loss is 27dB. More importantly, when testing over temperatures of 300-400 K, the performance showed effectively no dependence on temperature (conversion loss changed less than 1dB in either direction). This is a desirable property, as conventional semiconductor-based devices typically show severe performance loss with rising temperature.
RF mixers are critical communication system devices, so this demonstration is not purely academic. The graphene FET RF mixer shown here significantly outperforms previous graphene-based mixers and is approaching the performance of commerical GaAs devices (1.95GHz operation with 7dB loss). However, the most significant contribution of this work is the method used to integrate the graphene FET into a circuit using conventional fabrication techniques.
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