Sensors and Actuators A 279 (2018) 278–283 Contents lists available at ScienceDirect Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna Towards a low current Hall effect sensor a b a,∗ Yossi Sharon , Bagrat Khachatryan , Dima Cheskis a The Physics Department, Ariel University, Ariel 407000, Israel b The Physics Department, Technion, Haifa 3200003, Israel a r t i c l e i n f o a b s t r a c t Article history: Many modern electronic devices utilize linear Hall sensors to measure current and the magnetic field, as Received 30 January 2018 well as to perform switching and latching operations. Smartphones, laptops, and e-readers all work with Received in revised form 10 June 2018 very low (sub-milliampere) currents. To perform a switching function in low-power devices, however, Accepted 11 June 2018 Hall sensors must work in the microampere regime. This paper demonstrates, for the first time, the ability Available online 18 June 2018 of a standard Hall detector to work linearly in the microampere regime between√ 0 and 0.7 Tesla. To do so, we developed a current source with RMS noise on the order of 10–100 pA/ Hz. An optimized electronic Keywords: circuit with minimal connections feeds current to the Hall sensor, and the Hall voltage is measured with Hall sensors Magnetism an industrial nanovoltmeter. After cooling this system down to temperatures as low as 77 K, we found mostly 1/f noise. In this regime the thermal noise was negligible. We demonstrate the capabilities of this Low current system by precisely measuring the slope of the Hall effect with a four-point probe at current intensities of 100, 10, and 1 ␮A. We expect that our system can work as a microampere Hall sensor using external voltage detectors. © 2018 Elsevier B.V. All rights reserved. 1. Introduction output current of the Hall sensor is in the mA regime. This current level gives a good signal-to-noise ratio (SNR), and its Hall voltage Many modern devices, such as smartphones, tablets, e-readers, can be easily detected. This paper presents work towards the devel- GPS units, and heart rate monitors, are controlled remotely and opment of a Hall sensor that can work in the microampere regime operated continuously for long stretches of time. These features and still react while a device is in the sleep mode. In order to achieve come with a practical design constraint: the devices spend most of this goal, it is necessary to first greatly reduce SNR and then find their time in a low-power sleep mode, using a battery or a DC low- a voltage sensor capable of detecting the signal. The present work power bus to deliver current in the sub-milliampere (mA) regime. focuses on the first factor. We demonstrate for the first time a lin- When transmitting information, the devices switch to high-power ear DC Hall sensor working in the microampere regime, with a SNR radio frequency (RF) activity, and the current increases to the mA similar to the mA regime used by current technologies. We mea- or even Ampere scale. To control these switching behaviors, precise sure magnetic fields in the range 0–0.7 Tesla, for three very low DC sensors must be used that are capable of measuring both low and current sources. high currents. Usually, noninvasive current control is done using The basic relation between Hall voltage, current and magnetic Hall sensors. The current flowing through the Hall sensor creates a field is: perpendicular Hall voltage, which is proportional to the current and = detectable through the material of the sensor. Weak magnetic fields VHall RH BI/t (1) can be measured by various techniques, such as superconducting quantum interference (SQUID) devices [1] or magnetoresistance where RH is Hall resistivity, B is the magnetic field, I is the electrical sensors (“giant”, “anomalous”, or “tunneling”, respectively denoted current and t is a thickness of material. GMR, AMR, or TMR) [2–4]. Hall sensors are also cheap, so are used RH is equal to: in many devices. However, to conserve battery life, the energy used by different types of sensors needs to be minimized. Usually, the = RH rh/nq (2) where n is the density of charge carriers, q is the carrier charge, and ∗ rh is the Hall scattering factor that different from one, depending Corresponding author. on the material and the dominant charge carrier mechanism. E-mail address: [email protected] (D. Cheskis). https://doi.org/10.1016/j.sna.2018.06.027 0924-4247/© 2018 Elsevier B.V. All rights reserved. Y. Sharon et al. / Sensors and Actuators A 279 (2018) 278–283 279 Fig. 1. Current source. 2. Current source As was previously described, our main innovation is to use a power source with very low noise. This device is shown in Fig. 1. We use a 9 V battery as the source of voltage, and disconnect the measurement circuit completely from the external network. The load on the source should be no more than 9 V, and ideally much less, in order to reduce noise. To create a constant current through the sample, we use a Texas Instruments lm 234 current source and adjustable system. The formula which connects output current and resistivity R2 on an adjustable wire is shown below: = Iout (227 ␮V/K)/R2 (3) If the voltage load is no larger than a few millivolts, and fluctu- ations in the load are small, then the noise in the current is very small. In order to test our sensor, we built an experiment which per- forms Hall effect measurements on an industrial GaAs sensor. 3. Noise treatment Fig. 2. Current source noise. Having reduced the current by three orders of magnitude, it is necessary to understand how the signal-to-noise (SNR) ratio changes. In order to measure small magnetic fields with the same accuracy, we need to reduce the noise level. DC systems mainly We place our circuit consisting of current source and Hall sensor harbor two types of noise: the non-thermal 1/f (flicker) noise, inside a cryostat. Its connections are a coaxial cable, Fisher con- and broadband noise which depends on temperature. Our system nectors and cryogenic wires. The nanovoltmeter is connected in a includes three main parts: the current source, the circuit compris- similar manner, and will have its own noise. The noise generated ing current source and sensor, and the detector. This paper focuses by the coaxial cables, connectors, and the Hall sensor itself has a on the first two. First we need to understand the noise in the cur- thermal origin. We perform measurements at 300 K and at 77 K in rent source, then we will look at the noise in the electronic circuit order to see how this thermal noise influences our system. that includes the Hall sensor. In electronic circuits, the main type of broadband resistance Fig. 2 illustrates the noise spectral density as function of fre- noise at room temperature in DC circuits is Johnson–Nyquist noise. quency in the range of 10 Hz–100 kHz in the logarithmic scale in The noise level of the sensor voltage equals [5]: our system: According to definition, the 1/f noise decreases until some “cor- ner frequency” is reached, and then levels off to a constant value. = V 4kbTRf (4) This figure clearly shows that the corner frequency is around 100 Hz for the 3 and 5 mA current sources. At 100 ␮A, the corner frequency where k is the Boltzmann constant, T is the temperature, R is the lies between 10 and 100 Hz. As expected, 1/f noise climbs in these b resistance, and f is the frequency bandwidth. plots as we move left from the corner frequency. Although this is Laboratory power supplies are designed to provide currents not shown, we expect that√ the noise spectral density for the 100 ␮A and waveforms over a wide range of intensities and frequencies. source reaches 100 pA/ Hz for some frequency around 1 Hz. For the They are therefore built with many electronic components, which 10 ␮A source, the noise curve is flat and we expect that it does not increase the noise in the system. Moreover, the alternating voltage change drastically between 1 and 10 Hz. The 1/f noise curve for the of the power supply grid introduces additional noise. The standard 1 ␮A source is not shown, but we expect that will be lower than the way to reduce noise is to decrease the bandwidth using a modu- 10 ␮A curve. lated signal and a lock-in amplifier. In our case, we instead use a DC For comparison, the Keithley 6221 current source√ has approved battery to supply a small amount of current without introducing broadband RMS noise levels on the order of nA/ Hz. The level of any noise from extraneous electronic components or the external 1/f noise achieved by our source is 3 orders of magnitude smaller grid. than any industrial current source connected to the external grid. 280 Y. Sharon et al. / Sensors and Actuators A 279 (2018) 278–283 Table 1 Hall coefficients for 1 ␮A, 10 ␮A, 100 ␮A current sources at 300 K. Current 1 ␮A 10 ␮A 100 ␮A 100 mA (manuf.) VH/T 0.371 ␮V/T 3.955 ␮V/T 39.85 ␮V/T 39.93 mV/T (VH/T) 1.5 nV/T 6.5 nV/T 81 nV/T 80 ␮V/T (VH/T)/VH/T*100% 0.42% 0.16% 0.21% 0.2% from 77 K to 300 K.