AlGaN/GaN Heterojunction Based Hall Sensors for Magnetic Field Sensing over Wide Temperature Range

Abstract

Hall sensor has proved to be an attractive solution for sensing requirements in electric machines for direct measurement of fields or indirect estimation of physical quantities such as current, speed and torque. Current probes, which measure terminal currents and switching currents in power converters for protection, monitoring and closed-loop current control, typically use Hall-effect sensors. Recently there has been a demand for electric machines with high operational speeds and high power densities for use in electric vehicles, power generation and precision machining applications. High speed machines operate with greater reliability using active magnetic bearings as they eliminate friction and guarantee longer machine life. Research in high-power and high-speed machines and active magnetic bearings can be aided by direct measurement of the internal magnetic field distribution. These electromechanical devices can operate in harsh environments and require stable Hall sensing operation at extreme temperatures. Most commercially available Hall sensors are based on silicon and have a limited operating temperature range. For field sensing at extreme temperatures, wide band gap-based materials offer a viable alternative. This work evaluates Hall-effect sensing using AlGaN/GaN hetero-junctions grown on Si substrates for extreme temperatures. Hall-effect sensors are fabricated using AlGaN/GaN heterojunctions grown on Si substrates. The square-shaped Hall-sensing element is realised by means of a simple fabrication methodology employing shadow masking. An array of greek-cross shaped Hall effect sensors is batch fabricated on a single GaN-on-Si wafer. A process flow for batch fabrication is proposed. In particular, an insulating layer of SixNy, deposited initially in the process, is shown to result in a lower sheet resistance of the Hallsensing elements. The fabricated samples are extensively characterised at temperatures ranging from 75 K to 500 K and at magnetic field strengths up to 2 Tesla. Notwithstanding wide fluctuations in sheet resistance and carrier mobility with the operating temperature, the plot of sensitivity against temperature is reasonably flat. The operating temperature range from 75 K to 500 K spans those of the military grade, industrial grade and commercial grade Hall sensors. Additionally, the fabricated sensors can also be used for field sensing in a cryogenic environment. Small variations in sensitivity, however exist. It is suggested that these variations can be compensated using the terminal measurements such as the transresistances. The geometrical correction factors of the fabricated sensors are also studied over the complete temperature range of interest. It is shown to be very close to unity and exhibit a variation as small as 2%. The offset voltage in the Hall sensor output and its dependence on the biasing currents and operating temperatures are of particular interest in this study. The offset voltage of each of the characterised samples shows a linear dependence on the bias current and a non-linear dependence on the sample temperature. For a given sample, the offset voltage is shown to vary with a change in its biasing configuration. The method of current spinning is shown to nullify the offset at any operating temperature, field or bias current. A micro-controller based electronic subsystem is developed to implement the current spinning scheme to cancel the offsets in the sensed Hall signal. The subsystem achieves the necessary signal amplification and filtering of the Hall voltage. In addition, the subsystem estimates the field and provides a visual read-out of the same. The square-shaped sensing element along with the electronic subsystem has been integrated into a suitable package for use as a magnetic field probe in the air-gap of an electromagnet or a magnetic circuit. A Helmholtz coil based magnetic field producing setup is used for testing and calibration of the electronic subsystem.