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Graphene magnetic sensors

Marko Spasenovic

The market for magnetic field sensors is an expanding one, with size estimates up to USD 4.16 billion in 2022. The multiple purposes of magnetic field sensors such as position detection, current monitoring, speed detection, and angular sensing allow access to a wide range of industries such as automotive, consumer electronics, healthcare and defense. A most common magnetic sensor type utilizes the Hall effect, the production of a potential difference across an electrical conductor when a magnetic field is applied. Hall effect-based sensors constitute 55% of the market share for magnetic sensors.

The key factor for determining sensitivity of Hall effect sensors is high electron mobility. As such, graphene is a highly interesting material for this application, with measured carrier mobility in excess of 200,000 cm2 V-1 s-1. Graphene Hall sensors with current-related sensitivity up to 5700 V/AT and voltage-related sensitivity up to 3 V/VT were demonstrated in graphene encapsulated in boron nitride. Such performance outpaces state-of-the-art silicon and III/V Halls sensors, with a magnetic resolution as low as 50 nT/Hz. The current practical limit for sensitivity of graphene Halls devices on industry standard wafers is around ~3000 V/AT. For comparison, state of the art Hall sensors from traditional CMOS-compatible materials have sensitivity on the order of ~100 V/AT. Even flexible graphene Hall sensors, produced on Kapton tape, reach sensitivities similar to rigid silicon Hall sensors.

Image: GFET S-10 chip with graphene FETs.

Graphene Hall sensors consist of a graphene sheet on a substrate. The sheet is patterned into a “Hall bar” geometry, with two electrodes on each side providing the source and drain for the charge carriers, and four electrodes on the lateral sides to measure the potential difference. Graphenea provides a chip that houses thirty graphene Hall-bar devices and an additional six with a standard 2-probe geometry. The standard chip offer varying channel length and width, with constant high quality evidenced by field effect mobilities in excess of 1000 cm2/Vs, residual charge carrier densities lower than 2x1012 cm-2, and quality control inspection with Raman spectroscopy and optical microscopy.

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