Magnetic field sensing behaviour of sectorial split drain field effect transistor in mechatronic application

Abstract

Magnetic sensors have a wide range of applications in proximity sensing, position sensing, etc. Once a magnetic field is present, or there is a change of magnetic field, an electronic signal will be generated from the magnetic sensor. The Hall Effect sensor is a well-known magnetic sensor where the hall voltage varies piecewise with the magnetic field strength. The advanced CMOS technology enables the integration of magnetic sensing core, biasing circuit, amplifier, etc., into one tiny chip. Therefore the power consumption, parasitic element and manufacturing cost of the integrated magnetic sensor can be reduced.

It is well-known that silicon is a material which is compatible with the integrated circuit fabrication process. However, silicon may not be a good choice for being an integrated magnetic sensor because of the low mobility property. Low mobility will degrade the performance of the magnetic sensing core, especially the magnetic field sensitivity. Recent research has improved the performance of magnetic sensing core fabricated by silicon such as the development of the metal oxide semiconductor (MOS) Hall Plate and Split-Drain Magnetic Field Effect Transistor (SD-MAGFET). These magnetic sensors are feasible for integrating with a biasing circuit, amplifier and A/D converter. Therefore a silicon magnetic sensor with high magnetic field sensitivity, reduced power consumption and less parasitic elements can be fabricated. These properties make integrated magnetic sensors popular in industrial and motion control systems, especially in mechatronic application. The operation of a mechatronic system usually involves a high operating temperature and high unwanted signal noise. Silicon magnetic sensors are capable of high performance under the above conditions.

A high magnetic field changing rate is one of the main factors which degrade the performance of magnetic sensors in mechatronic application. Especially in measuring the angular position of a magnet, a fast changing magnetic field rate will cause the magnetic sensor not to move fast enough respond to the changing magnetic field. Another factor that also strongly influences magnetic field sensitivity is the geometric dimension variation of magnetic sensors. This variation is caused by the imperfect manufacturing process and it is unavoidable.

In my research, a Sectorial Split-Drain (SSD) MAGFET is employed for investigation. An experiment is designed for investigating the magnetic field sensing behaviour of alternating magnetic fields. The magnetic field sensing resolution is studied through spectral analysis. The studies show that a MAGFET-based system can achieve better accuracy than traditional Hall Plate when sensing the alternating magnetic field. The underlying physics are also explored which leads to device selection guidelines for high speed, high accuracy angular position measurement.

Another experiment is designed to investigate the SSD-MAGFET magnetic field sensitivity variation, whereby the sensitivity variation is induced by manufacturing process variation (process-induced variation). The sector angle and magnetic field sensitivity of 965 “identical” SSD-MAGFETs are measured. A statistical model describing process-induced variation is developed. A method for obtaining the optimal sector angle is developed to give greatest immunity to process-induced sector angle variation, which provides a guideline for designers to design the SSD-MAGFET with an appropriate sector angle.