Researchers at Chalmers have for the first time demonstrated a novel subharmonic graphene FET mixer at microwave frequencies. The mixer provides new opportunities in future electronics, as it enables compact circuit technology, potential to reach high frequencies and integration with silicon technology.
A mixer is a key building block in all electronic systems – a device that combines two or more electronic signals into one or two composite output signals. Future applications at THz frequencies such as radar systems for security and safety, radio astronomy, process monitoring and environmental monitoring will require large arrays of mixers for high-resolution imaging and high-speed data acquisition. Such mixer arrays or multi-pixel receivers need new type of devices that are not only sensitive but also power-efficient and compact.
We demonstrate a subharmonic resistive graphene FET (G-FET) mixer utilizing the symmetrical channel resistance vs. gate voltage characteristic. A down-conversion loss of 24 dB is obtained with fRF=2 GHz, fLO=1.01 GHz and fIF=20 MHz in a 50 Ω impedance system. Unlike the conventional subharmonic resistive FET mixers, this type of mixer operates with only one transistor and does not need any balun at the LO port which makes it more compact.
The ability in graphene to switch between hole or electron carrier transport via the field effect enables a unique niche for graphene for RF IC applications. Thanks to this symmetrical electrical characteristic, the researchers at Chalmers have managed to build the G-FET subharmonic resistive mixer using only one transistor. Hence, no extra feeding circuits are required, which makes the mixer circuit more compact as opposed to conventional mixers. As a consequence, the new type of mixer requires less wafer area when constructed and can open up for advanced sensor arrays, for example for imaging at millimetre waves and even sub millimetre waves as G-FET technology progress.
– “The performance of the mixer can be improved by further optimising the circuit, as well as fabricating a G-FET device with a higher on-off current ratio”, says Jan Stake, professor of the research team. “Using a G‐FET in this new topology enables us to extend its operation to higher frequencies, thereby exploiting the exceptional properties of graphene. This paves the way for future technologies operating at extremely high frequencies.
A novel subharmonically pumped resistive mixer based on a G-FET has been fabricated and demonstrated at microwave frequencies. The performance of the mixer follows the theory and can be improved by further optimizing embedding impedances as well as designing a G-FET device with a higher on-off ratio. The contact resistances degrade the mixer performance, especially, for short gate lengths. Therefore, reducing the contact and access resistances is important for realizing high performance and high frequency G-FET subharmonic mixers.
Schematic picture of a subharmonic graphene-FET mixer. The LO and RF signals are fed to the gate and drain terminals, respectively, and the IF signal is extracted from the drain terminal.