INVESTIGATING THE USE OF INDIRECT SENSING TECHNIQUES

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INTRODUCTION

High velocity of charge carriers are required to generate a strong Hall effect [1]. To achieve this, it is required to have either a high electric field or a high mobility of quasi- electric carriers neither of which could be effectively achieved in the metal conductors used at the time of its discovery in 1879. The invention of semiconductor technology created a suitable candidate to simultaneously achieve these two qualities and consequently much improved signal levels. Semiconductors consist of low doped materials that possess quasi-electric carriers in low densities making it possible to apply a high electric field without causing thermal degeneration of the host material [1]. The need to sense the generated Hall voltage requires an appropriate contact point into the Hall device. Low ohmic contacts are typically used for this purpose but their intrinsic high doped properties contradict the fundamentals of achieving a strong Hall effect, specifically at its point of contact into a Hall device.
The result is a diminution of the Hall voltage creating the need to introduce factors that explain the reduction of the theoretical maximum Hall effect achievable. Much research has been done over the decades to find methods of reducing this factor by manipulating parameters directly under the influence of a designer. These methods mainly include changing the geometrical shape to harness the various advantages that accompany them in specific technologies [2]. The problem is reintroduced however in vertical Hall sensing devices due to the limiting influence a designer has on the geometries in the vertical direction in CMOS technologies.
With three dimensional Hall sensors [3], nanomagnetic logic devices [4] and nanobead sensing [5, 6] applications becoming ever more popular, there is continuous pressure to search for the fundamental sensing limits [7, 8] to increase the performance of such devices in various applications. OBJECTIVE The objective of this thesis is to research the feasibility of using indirect sensing techniques as an alternate approach to sense the Hall voltage in a Hall effect device. The goal is to prove whether or not an indirect sensing approach can reduce or eliminate the effects associated with traditional direct sensing techniques ultimately forming the basis upon which the research hypothesis will be rejected or accepted. 1.3 JUSTIFICATION FOR THE RESEARCH Up to now, it appears from literature that the only method of sensing the Hall effect in Hall devices is through the use of highly doped low ohmic contacts [1, 2, 3, 4, 5, 6, 7, 8]. Popovic [1] mathematically explores the Hall effect under the influence of the Lorentz force. His investigation illustrates a combination of effects appearing between the geometrical limits of infinitely long and short Hall plates.
These effects lead to a diminution of the Hall voltage in a finite length Hall device due to non-perfect current confinement, contributed by the presence of the physical electrical contacts. The contacts cause a short circuiting effect at both the current supply, as well as the sensing terminals. The result is no Hall field at the current supply contacts and a reduction of the described Hall electric field in the direct vicinity of the sensing contacts as a result of a reduction in current density. These effects are modelled by a geometrical correction factor namely the ratio between the Hall voltage generated on an arbitrary shaped Hall plate and the Hall voltage generated on an infinitely long Hall plate, both influenced by the same magnetic field and under the same conditions [1, 7]. Vertical Hall effect devices are particularly adversely affected by this effect [7].
This research aims to challenge traditional thinking and methods with regards to the sensing of the Hall effect, itself the cause of a reduction in performance, in a semiconductor Hall device. From the literature pre-study it can be deduced that if it were possible to use a non-intrusive method to measure the Hall voltage generated by a Hall device under the influence of the Lorentz force, it may be possible to reduce the geometrical correction factor effect caused by the sensing contacts themselves. By succeeding in this, an increase of the geometrical factor seems possible leading to higher sensitivities between identical geometrical Hall devices using direct and indirect sensing techniques.

TABLE OF CONTENTS :

• CHAPTER 1 INTRODUCTION
  • 1.1 INTRODUCTION
  • 1.2 OBJECTIVE
  • 1.3 JUSTIFICATION FOR THE RESEARCH
  • 1.4 RESEARCH QUESTIONS AND HYPOTHESIS
  • 1.4.1 Research questions
  • 1.4.2 Hypothesis
  • 1.5 METHODOLOGY
  • 1.6 CONTRIBUTION
  • 1.7 PUBLICATIONS LEADING FROM THIS RESEARCH
  • 1.8 OUTLINE
  • 1.9 CONCLUSION
• CHAPTER 2 HALL EFFECT THEORY
  • 2.1 INTRODUCTION
  • 2.2 THE HALL EFFECT: AN ANALYSIS
  • 2.2.1 Terminology
  • 2.2.2 Galvanomagnetic sensors
  • 2.2.3 The Lorentz force on quasi-free carriers in condensed matter
  • 2.2.4 An accurate approach
  • 2.3 GENERAL CHARACTERISTICS OF A HALL DEVICE
  • 2.3.1 Sensitivity
  • 2.3.2 Offset
  • 2.3.3 Noise analysis
  • 2.3.4 Spatial resolution
  • 2.3.5 Temperature and cross sensitivity
  • 2.3.6 Linearity
  • 2.3.7 Frequency and time response
  • 2.3.8 Stability and Drift
  • 2.3.9 Geometry
  • 2.3.10 Testing
  • 2.4 THEORETICAL LIMITS
  • 2.5 MODELLING FOR SIMULATION
  • 2.6 APPLICATION, TRENDS AND STATE OF THE ART
  • 2.6.1 Sensor classification
  • 2.6.2 General
  • 2.6.3 Linear position sensor
  • 2.6.4 Positioning
  • 2.6.5 Energy measurement
  • 2.6.6 Bimolecular microbeads
  • 2.6.7 Force
  • 2.6.8 Material characterization
  • 2.6.9 Temperature
  • 2.6.10 Track Ball
  • 2.6.11 Automotive
  • 2.7 HALL SENSOR PATENT REVIEW
  • 2.8 CONCLUSION
• CHAPTER 3 METHODOLOGY
  • 3.1 RESEARCH APPROACH
  • 3.2 RESEARCH METHODOLOGY
  • 3.3 RESEARCH METHODS
  • 3.3.1 The IC process
  • 3.3.2 Design, modelling, simulation and layout
  • 3.3.3 Measurement setup and equipment
  • 3.4 CONCLUSION
• CHAPTER 4 HALL PLATE DESIGN AND ANALYSIS
  • 4.1 INTRODUCTION
  • 4.2 TECHNOLOGY
  • 4.2.1 Overview
  • 4.3 HALL PLATE DESIGN
  • 4.3.1 Basic Hall effect characteristics
  • 4.4 VERTICAL BJT
  • 4.5 NOISE ANALYSIS
  • 4.6 CONCLUSION
• CHAPTER 5 LAYOUT, FABRICATION AND MEASUREMENT RESULTS
  • 5.1 INTRODUCTION
  • 5.2 LAYOUT DESIGN
  • 5.3 TEST SETUP
  • 5.4 HALL CURRENT MEASUREMENT RESULTS
  • 5.5 STATISTICAL ANALYSIS
  • 5.5.1 Geometrically affected Hall signal
  • 5.5.2 Statistical results
  • 5.5.3 Interpretation
• CHAPTER 6 CONCLUSION
  • 6.1 INTRODUCTION
  • 6.2 THE OUTCOMES OF INDIRECT SENSING TECHNIQUES IN CMOS HALL DEVICES
  • 6.2.1 Research question
  • 6.2.2 Research question
  • 6.2.3 Research question
  • 6.2.4 The hypothesis
  • 6.3 IMPLICATIONS ON EXISTING THEORY
  • 6.4 RECOMMENDATIONS FOR FUTURE RESEARCH
  • 6.5 CONCLUSION
  • REFERENCES

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