Magnetic Sensors by KuenHsien Wu

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Magnetic Sensors by Kuen-Hsien Wu

• Galvano-magnetic effect
• Lorentz deflection
• Lorentz force on charge carrier ?carrier
  deflection
• Magneto-resistance
• Modulation of resistance by a magnetic field
• Magneto-concentration
• Producing a gradient of carrier concentration
  perpendicular to the magnetic inductor vector and
  original current direction

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Magnetic Sensors and Effects
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Main Magnetic Sensors

• Hall Plates
• Integrated Hall Sensors
• Hall devices
• MAGFET
• Magneto-transistor
• Magneto-diode
• Carrier-domain Magnetometer
• Super Magneto-resistor

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Hall Plate
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Geometric Effects

• VH -GIBrn(qnt)-1
• rn scattering factor
• Geometric correction factor G
• Describe the shapes effect of the plate
• G depends on
• Plate length
• Plate width
• Contact size
• Position of the sensor contact
• Hall angle ?H

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Biasing and Amplification Circuitry

• Hall-voltage operation is preferred in mordern
  Hall devices.
• Biased with a constant current source.
• The left sensor contact is virtually grounded by
  an operational ampliier (OA)
• The full Hall voltage appears at the right sensor
  contact.
• Without the OA, a large common-mode voltage will
  appear at the amplifier input.

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Sensitivity

• Absolute sensitivity
• SA
• Supply-current related sensitivity
• SI
• Supply-voltage related sensitivity
• SV

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Limiting Effects

• Noise
• Offset Voltage
• Temperature Coefficient
• Nonlinearity

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Integrated Bulk Hall Sensor
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Integrated Hall Switch

• A binary output signal is produced.

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Vertical Hall Device
Equipotential line
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Differential Amplification Magnetic Sensor (DAMS)

• With a Magnetic induction, the Hall voltage
  appears across the base region.
• If the two emitters are kept at the same
  potential, the Hall voltage acts as the
  differential emitter-base voltage of the
  transistor pair.
• Under proper bias conditions, this results in a
  corresponding collector-current difference, which
  can be converted into a final voltage difference
  by load resistors.

Base Region
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Magnetic Field-Effect Transistor (MAGFET)

• The surface inversion layer or channel of a
  MOSFET can be used as the active region of a Hall
  sensor.
• This device exploits the Hall effect and the
  Lorentz deflection of carriers in the inversion
  layer.
• Such a device is compatible with MOS bias and
  signal-conditioning circuitry.
• Disadvantages
• High 1/f noise
• Low channel mobility ? Magnetic Heterojunction
  Device (2DEG)
• Surface instability

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Hall MAGFET
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Dual-Drain MAGFET

• A magnetic induction perpendicular to the
  inversion layer produces a current imbalance.
• ?ID ID1 ID2
• ?IDG?nch(L/W)B ID

Split-Drain MOSFET
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Magnetic Heterojunction Device
2DEG
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Magnetotransistor (MT)

• Lorentz deflection
• Lorentz force deflects minority carriers toward
  one collector and away from the other collector.
• Injection modulation
• The magnetic induction acting on the majority
  carriers moving in the base region creates a Hall
  voltage, which modulates the emitter-base voltage
• Creating an asymmetry in the minority-carrier
  injection.
• MTs
• Vertical Magnetotransistor
• Lateral Magnetotransistor
• Suppressed-Sidewall-Injection MT (SSIMT)

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Vertical Magnetotransistor

• The Lorentz deflects the injected carriers in the
  base and the subsequent epi-layer causing a
  collector-current imbalance
• ?IC IC1 IC2
• ?ICG?nch(L/WE)B ICO

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Lateral Magnetotransistor(sensitive to
perpendicular field)

• The two n base contacts are used to create an
  accelerating field across the large base region.
  (different from the vertical MT)
• Due to the accelerating voltage, most minority
  carriers injected from the emitter are directed
  towards the two collectors and only a small
  amount flows into the substrate.

Large Base Region
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Lateral Magnetotransistor(sensitive to parallel
field)

• The device has only one collector and uses the
  substrate as a second collector.
• The minority carriers flowing laterally through
  the base region are deflected either towards the
  collector or the substrate.
• Thus, the ratio IC/IS is modulated by the
  magnetic field.

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Suppressed-Sidewall-Injection MT (SSIMT)

• ?IC IC1 IC2 ? B
• The highly-doped n guard ring surrounding the
  emitter prevents the lateral injection of
  minority carriers from the emitter into the base.
• Improving the sensitivity
• An accelerating field is formed between the guard
  and the base contacts to boost the magnetic
  response.
• The substrate current deflection also cooperate
  the ?IC formation

? B
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Magnetodiode (MD)
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Integrated Magnetodiode
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Carrier-Domain Magnetometer (CDM)

• Carrier Domain
• A region of high, nonequilibrium carrier density.
• A CDM
• exploiting the action of Lorentz force on the
  charge carriers moving in the domain.
• This force moves the entire carrier domain
  through the semiconductor or modulates a domain
  migration caused by some other effect.
• Detection the domain motion provides information
  on the magnetic field.

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Vertical Four-Layer CDM

• A perpendicular magnetic field produces a
  displacement of the domain, thus resulting in the
  current imbalance in Ip1 andIp1 (or In1 andIn1).
• The current imbalance indicates the domain
  displacement, and hence the presence of the
  magnetic field.

Carrier domain
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Circular, Horizontal Four-Layer CDM

• Under the action of the magnetic induction, the
  domain travels around the circumference of the
  structure.
• The frequency of this rotation is proportional to
  the applied magnetic induction.
• This generation of a frequency output is a unique
  feature of thr circular CDM.
• Disadvantage
• High threshold field
• Large temperature coefficient

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Circular, Horizontal Three-Layer CDM

• No threshold magnetic induction is required.
• Operated in the collector-emitter breakdown
  regime with short-circuited emitter and base
  contacts.
• The angular frequency of the carrier domain
  rotation is modulated by the magnetic field.
• Disadvantages
• High current (need cooling)
• Breakdown voltage is not precise.

short-circuited emitter and base contacts
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Supermagnetoresistor

• The sensor operates at the temperature of 77K and
  responses to very small fields (below 10 mT)
• A week magnetic field will disturb the
  superconductivity of a superconductor material.
• This leads to an abrupt change in the resistance
  of the sample with magnetic field.