Magfet

ECE 904 Microsensors Magnetic Sensors
Hongrui Jiang Electrical & Computer Engineering UW - Madison

General Information

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Chapter 5, Sze, Semiconductor Sensors Will focus on magnetic sensors based on galvanomagnetic effect, including Hall devices, magneticfield sensitive MOSFETs and heterojunction structures, and magnetoresistive sensors. Generally, a magnetic sensor is atransducer that converts a magnetic field into an electrical signal. Galvanomagnetic effect is due to Lorenz force on charge carriers on materials – here semiconductors. Silicon based predominant – IC integration. Hall effect most known galvanomagnetic effect: an electric current exposed to magnetic field produces an electric field perpendicular to the magnetic induction vector and the original currentdirection. Lorenz deflection: producing a current component perpendicular to the magnetic induction vector and the original current direction.

General Information (cont’d)
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Other galvanomagnetic effects: magnetoresistance – modulation of electrical resistance by magnetic field; magnetoconcentration or Suhl effect – a gradient of the carrier concentration perpendicular to the magneticinductor vector and the original current direction. High mobility and low carrier concentration favor galvanomagnetic effects. Hence, semiconductors, rather than metals, are preferred materials for magnetic sensors. Applications of magnetic sensors: not just magnetic field; when magnetic signal is produced as an intermediate conversion; e.g., detection of current through its own magnetic field andmechanical displacement of a magnet. Hence divided into two categories:
• Direct: magnetic sensor part of a magnetometer. Examples: geomagnetic field measurement; reading of magnetic data storage media; credit cards – identification of magnetic patterns; control of magnetic apparatus. • Indirect: magnetic field intermediate carrier for detecting nonmagnetic signals. Examples: potential-freecurrent detection for overload protection, switching of power systems; integrated power (watt-hour) meters, contactless position sensors using magnets (linear, angualr, velocity, etc.)

Ranges
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Large-scale applications where inexpensive batch-fabricated semiconductor magnetic sensors are desirable. Detection of magnetic fields in the micro- to millitesla range. Nanotesla not feasible –reserved for nuclear magnetic resonance (NMR) Picotesla fields in biomagnetometry – superconducting quantum interference devices (SQUID) Magnetic units: magnetic field strength H (Gauss); magnetic induction B – 1 tesla – 1V-s/m2; inverse of unit of carrier mobility: 1 m2/V-s = 104 cm2/V-s = 1T-1. Product of B and mobility is dimensionless – fundamentally connected to galvanomagnetic effects. Somevalues related to Si and GaAs magnetic sensors:
• • • • • Geomagnetic field: 30-60μT; Magnetic storage media: ~ 1mT; Permanent magnets in switches: 5-100 mT; Conductor carrying a 10 A current: 1 mT; Superconduction coils: 10-20 T

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Magnetic Sensor Family Tree
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Lorenz force: F = -qv × B on moving electrons: q – electron charge; v – velocity; B – magnetic induction vector B = μμ0H: μ0 –permeability of free space; μ - relative permeability of sensor material. Hence ↑μ, ↑sensitivity of sensor. High μ: ferro- or ferrimagnetic; e.g., based on magnetoresistance of NiFe thin films, magnetostriction of Ni cladding of an optical fiber, magneto-optic effect in garnets, and other combining flux-concentration device Low μ (≅1): dia-or paramagnetic materials; no amplification due to μ; allgalvanomagnetic effect based semiconductor magnetic sensors.

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More on Family Tree
Thin metal film magnetic sensors:
• Usually based on ferromagnetic materials, especially lowmagnetostriction alloy Ni81Fe19 permalloy. Example: magnetoresisitive switching of anisotropic NiFe or NiCo films.

Semiconductor magnetic sensors: galvanomagnetic effects based. Our focus. Si or III-V. Si...