Graphene-Based Sensors & Their Future Proliferation

Author : Olivier Masséglia and Matt Enderle - Paragraf

30 June 2024

Figure 1: A Hall-effect sensor using graphene on top of a 2004 article about the material’s isolation
Figure 1: A Hall-effect sensor using graphene on top of a 2004 article about the material’s isolation

When the isolation of graphene was first written about in late 2004, the promise of this ‘wonder material’ was firmly centred on the benefits it would bring to electronics, while recognising significant challenges needed to be overcome. It was noted that the performance limitations of silicon (Si) were becoming increasingly apparent and non-traditional materials should thus be explored.

Well, since then graphene-based transistors and sensors have been produced by university research teams and several commercial organisations too. However, the lack of repeatable production of monolayer graphene in a form that doesn’t require transfer (and its associated difficulties) has meant that such devices couldn’t be reproduced in large volumes with high enough yields at competitive specifications.

Direct deposition of graphene on a standard semiconductor substrate with the ability to use a conventional processes to create a sensor element and then build up other microchip layers was first done by a team at Cambridge University, back in 2015. But, evolving this process into a fully-fledged manufacturing line capable of large-scale production of sensors has taken several years more. 

With a need for high-value demand required to encourage long-term investment, developing the process is not enough. Having products that prove the process, which can be bought and tested by potential customers in volume is as important as the process itself. This is where Hall-effect sensors are emerging as a key test area for the readiness of graphene-based electronic devices. These sensors measure the magnitude of magnetic fields - with automotive, industrial, avionics, medical and renewable energy sectors all making use of them. 

Figure 2: Principle of the Hall-effect
Figure 2: Principle of the Hall-effect

Indium-antimonide (InSb), gallium-arsenide (GaAs) and Si substrates enable sensors of varying sensitivity, environmental tolerance and unit cost to be produced. There are plenty of areas in which current solutions can be improved - such as heightened accuracy, reduced noise, better stability in relation to ambient conditions (from fluctuations in temperature, magnetic field, radiation levels, etc.), lower power budget, greater durability and further miniaturisation.  

Single-layer graphene has impressive properties, including being highly electrically and thermally conductive, mechanically strong yet flexible, etc. Thanks to its robustness, environmental tolerance and sensitivity, graphene can replace current materials used in Hall-effect sensing elements and increase the potential of existing magnetic field sensing solutions.
 
Key advantages worth noting include:
- Larger magnetic field ranges (up to 30T).
- Broadened temperature ranges (exhibiting stability from mK to beyond 125°C).
- Higher resolutions (due to the larger field range without increased noise floor). 

Hall-effect sensor operation 
Hall-effect sensing relies on the deviation of an electron path in a conductive material when subjected to a magnetic field. This deviation can be picked up as a voltage across the 2 terminals - perpendicular to the electron path - known as the Hall voltage (VHall). The scaling of the device depends on the sensitivity of the sensing element to the magnetic field and the magnitude of the bias current used. The stability and repeatability of sensitivity in relation to magnetic field, temperature and mechanical stresses is paramount in ensuring a linear relationship between VHall, sensed magnetic field and bias current. 

Figure 3: Hall-effect sensor electrical model
Figure 3: Hall-effect sensor electrical model

Hall-effect sensors do not rely on magnetic material to work and therefore do not affect the magnetic field that they measure - allowing for multiple sensors to be used in proximity without cross-interference. This also means that they aren’t susceptible to magnetic field hysteresis.

The electronics model of an ideal graphene-based Hall-effect sensor (GHS) is a diamond shaped 4-resistor Wheatstone bridge, where all 4 resistances have the same value. Due to manufacturing variations, there will be a small additional resistance (dR) that represents the zero-field offset present in measurements (as shown in Figure 3). The offset is therefore expressed as a resistance and is linearly dependent on the bias current. In this configuration, connections on opposite sides can be used either for bias current or VHall pickup and are fully interchangeable (allowing for the use of full spinning current modulation techniques).
 
Advanced field sensing with spinning current
Because of its very large dynamic range, a GHS device can be employed in a wide range of applications, each requiring slightly different supporting electronics. It is common practice to use bias current modulation to remove unwanted effects that will impact on the accuracy of a symmetrical Hall-effect sensor. The spinning current modulation technique (SCMT) consists of repeating measurements fast enough that the magnetic field being measured remains constant while the physical connections are rotated across the symmetrical Hall plate. This inverts the physical effect of the plates and the acquisition chain in relation to the measured field.

From each of those wiring configurations an output is acquired. Recombining those outputs allows the user to either extract the magnetic field data with highly reduced offset and 1/f noise contribution (at frequencies lower than the spinning frequency) from the sensor and acquisition chain or highlight the offset or noise contributions.  

Figure 4: SCMT illustration
Figure 4: SCMT illustration

This means that more bias current will increase the signal-to-noise ratio (SNR), and only the thermal noise is a significant noise contributor. 4 different wiring connections can be produced by rotating the connections (as shown in Figure 4).

Additional inversion of the VHall pick-up for each of those rotation phases can be added to the 4 wiring connections to create 8 independent configurations. The comparison of the 2 demodulation alternatives in the frequency and time domain follow. This type of system is highly recommended when measuring small magnetic fields where an offset may be a significant contributor to the sensor output. 

Advantages of graphene in large fields and at cryogenic temperatures
GHS devices have emerged as the superior option for measurement of magnetic fields in cryogenic conditions, because of their compact size and reduced power requirements. Before graphene, conventional Hall-effect sensors were limited by material characteristics, excessive power dissipation and a phenomenon known as the quantum Hall-effect (QHE). Now these limitations can be avoided and field measurements of up to 30T can be made down to mK temperatures.

Figure 5: Time domain trace of the demodulated signal from a GHS device in 0T field
Figure 5: Time domain trace of the demodulated signal from a GHS device in 0T field

The 1st obstacle for a Hall-effect sensor to overcome is the thermal expansion and contraction of their components when moving between varying temperatures - which will otherwise lead to the degradation of materials and failure of the sensing function. Owing to its 2-dimensional (2D) structure, graphene is an exceptionally flexible and robust material, capable of withstanding extreme temperatures and acute thermal shocks. 

The 2nd obstacle is the QHE, which involves a loss of linearity for voltage outputs above a particular field strength in low temperatures. Once sensors encounter fields above that threshold, the corresponding readings increase in plateaus and sharp inclines, rather than in a continuous, proportional trendline. This makes the device unusable as a cryogenic magnetic field sensor. By modifying sensitivity during graphene deposition, it is possible to delay the onset of QHE so that sensors can maintain their linearity up to ~30T.

The 3rd obstacle is temperature pollution caused by excessive power dissipation. GHS devices can comfortably operate effectively at currents down to µA with nW power dissipation. In extreme cases where large magnetic fields are measured, GHS devices are capable of operating at nA currents with pW power dissipation.

Figure 6: Paragraf’s GHS devices measured in extreme temperatures and magnetic fields
Figure 6: Paragraf’s GHS devices measured in extreme temperatures and magnetic fields

The 4th obstacle is operation in a confined space at cryogenic temperatures. In common with conventional sensors, GHS devices can be produced significantly smaller than alternative solutions and can be designed to fit available space and with a possible 3-dimensional (3D) configuration, picking up fields from any direction. Because such sensors can operate at cryogenic temperatures without the need for additional inserts to protect them, they are simpler to integrate and they can be placed closer to the field of interest than alternative solutions.

Graphene-based sensing solutions
Paragraf currently supplies the market with GHS devices. These are produced using monolayer graphene that has been directly deposited on a semiconductor substrate. The company has sensors capable of operating at cryogenic temperatures and measuring small fields of <0.5T all the way up to 30T with sensitivities of between 100V/AT and 1,700V/AT.


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