Hall Effect

By: Saloni Sharma

What is Hall Effect?

• Hall Effect is a process in which a transverse electric field is developed in a solid material when the material carrying an electric current is placed in a magnetic field that is perpendicular to the current. Hall Effect was discovered by Edwin Herbert Hall in 1879.

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• The principle of Hall Effect states that when a current-carrying conductor or a semiconductor is introduced to a perpendicular magnetic field, a voltage can be measured at the right angle to the current path. This effect of obtaining a measurable voltage is known as the Hall Effect.

• In the semiconductor industry, the Hall effect has enabled people to determine whether the current through a material is carried by positive particles (as with semiconductors) or negative particles (which is the case with metals). As a result, scientists have classified chemicals, developed refined semiconductor materials, and measured magnetic fields in various environments.
• Today, Hall effect devices are commonly used to measure magnetic fields by seeing the effect they have on a known current. Because a magnetic field can alter the course of a flowing unidirectional current, one side of the wire will have a greater negative charge than the other side, and that change results in measurable voltage. The voltage increases proportionately to the strength of the field

• The Hall effect is due to the nature of the current in a conductor. Current consists of the movement of many small charge carriers, typically electrons, holes, ions or all three.
• When a magnetic field is present, these charges experience a force, called the Lorentz force. 
• When such a magnetic field is absent, the charges follow approximately straight paths between collisions with impurities, phonons, etc.
• However, when a magnetic field with a perpendicular component is applied, their paths between collisions are curved, due to Lorentz’s force acting on them.

• Thus moving charges accumulate on one face of the material. This leaves equal and opposite charges exposed on the other face, where there is a scarcity of mobile charges. The result is an asymmetric distribution of charge density across the Hall element, arising from a force that is perpendicular to both the straight path and the applied magnetic field.

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• The separation of charge establishes an electric field that opposes the migration of further charge, so a steady electric potential is established for as long as the charge is flowing.

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�Hall Potential�

• The Hall effect is due to the nature of the current in a conductor. Current consists of the movement of many small charge carriers ie electrons, holes, ions or all three.
• In the absence of magnetic field these charges follow approximately straight paths between collisions with impurities, phonons etc.
• When a magnetic field is present, these charges experience a force, called the Lorentz force. However, when a magnetic field with a perpendicular component is applied, their paths between collisions are curved, thus moving charges accumulate on one face of the material. This leaves equal and opposite charges exposed on the other face, where there is a scarcity of mobile charges.
• The result is an asymmetric distribution of charge density across the Hall element, arising from a force that is perpendicular to both the straight path and the applied magnetic field. The separation of charge establishes an electric field that opposes the migration of further charge, so a steady electric potential is established for as long as the charge is flowing.

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For a simple metal where there is only one type of charge carrier (electrons), the Hall voltage V_H can be derived by using the Lorentz force

In the steady-state condition, charges are not moving in the y-axis direction. Thus, the magnetic force on each electron in the y-axis direction is cancelled by a y-axis electrical force due to the buildup of charges. The v_x term is the drift velocity of the current which is assumed at this point to be holes by convention. The v_xB_z term is negative in the y-axis direction by the right hand rule. F=q[E+(V X B)] (1)

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• In steady state,

 F = 0, so 0 = E_y − v_xB_z,

where E_y is assigned in the direction of the y-axis, (and not with the arrow of the induced electric field  E_y  as in the image (pointing in the −y direction), which tells you where the field caused by the electrons is pointing).

• In wires, electrons instead of holes are flowing, so v_x → −v_x and q → −q. Also E_y = −V_H/w. Substituting these changes gives

(2)

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• The conventional "hole" current is in the negative direction of the electron current and the negative of the electrical charge which gives 

I_x = ntw(−v_x)(−e) (3)

where n is charge carrier density, tw is the cross-sectional area, and −e is the charge of each electron.

Solving eq (3) for w and substituting in eq (2) gives the Hall voltage:

If the charge build up had been positive, then the VH assigned in the image would have been negative.

Hall Coefficient

• Hall Coefficient is given by

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• The units of R_H are usually expressed as m³/C.
• As a result, the Hall effect is very useful as a means to measure either the carrier density or the magnetic field.

Which gives

where Jc is the current density of the carrier electrons, and E_y is the induced electric field. In SI units, this becomes

•  Hall effect differentiates between positive charges moving in one direction and negative charges moving in the opposite. 
• If the current is carried by positively charged particles, they would be moving in the direction of applied electric field.

• The Lorentz’s force would move the charge carriers in the same direction regardless of their positive or negative charge.
• However positive charge produces positive hall voltage whereas negative charge produces negative hall voltage.
• Thus for the same current and magnetic field, the polarity of the Hall voltage is dependent on the internal nature of the conductor and is useful to elucidate its inner workings.
• This property of the Hall effect offered the first real proof that electric currents in most metals are carried by moving electrons, not by protons.

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�Hall effect in semiconductors�

• When a current-carrying semiconductor is kept in a magnetic field, the charge carriers of the semiconductor experience a force in a direction perpendicular to both the magnetic field and the current. At equilibrium, a voltage appears at the semiconductor edges.
• As in these materials conduction can involve significant, simultaneous contributions from both electrons and holes, which may be present in different concentrations and have different mobilities.

• For moderate magnetic fields the Hall coefficient is

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• Here n is the electron concentration, p the hole concentration, μ_e the electron mobility, μ_h the hole mobility and e the elementary charge.
• For large applied fields the simpler expression analogous to that for a single carrier type holds.

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�What Is a Hall Effect Sensor?�

• Hall Effect Sensor are magnetic components that convert magnetically encoded information—such as position, distance, and speed—so that electronic circuits can process it.
• They are generally classified based on their manner of output or means of operation. There are two types of digital sensors and analog sensors.

Types of hall effect sensors

�Digital Output Hall Effect Sensors�

• Digital Output Hall Effect Sensors are primarily used in magnetic switch applications to provide a digital voltage output. In this way, they provide an ON or OFF input signal to the system.
• The primary distinction of a digital output Hall effect sensor is its means of controlling voltage output.
• Instead of the power supply providing the saturation limits, digital output sensors have a Schmidt-trigger with built-in hysteresis connected to the op-amp.
• This switch turns the sensor output off whenever magnetic flux exceeds its pre-set limits and back on when flux stabilizes.

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�Analog Output Hall Effect Sensors�

• An analog type sensor provides a continuous voltage output that increases when a magnetic field is stronger and decreases when it is weaker.
• Thus, the output voltage or amplification of an analog Hall effect sensor is directly proportional to the intensity of the magnetic flux passing through it.

�Bipolar Hall Effect Sensors�

• This is a type of digital sensor, which operates using either positive or negative magnetic fields is called Bipolar Hall Effect Sensors.� Either the positive or negative magnetic field of the magnet activates the sensor.
• In this configuration, a switch using a bipolar Hall effect sensor is triggered in much the same way as a traditional Reed switch.
• However, the Hall effect switch has the added advantage of having no mechanical contacts, making it more durable in rugged environments.

�Unipolar Hall Effect Sensors�

• In contrast to a bi-polar sensor, this type of digital sensor is triggered only by one pole (either North or South) of the magnet. 
• Using a unipolar Hall effect sensor in a switch allows the set up to be more particular and only activate when exposed to a specific magnetic pole.

�Direct Angle and Vertical Angle Hall Effect Sensors�

• More advanced Hall effect sensors focus on components of the magnetic field other than the poles. These are of two types:
• Direct angle sensors measure the sine and cosine measurements of the magnetic field.
• Vertical angle sensors analyze the components of the magnetic field that are parallel—rather than perpendicular—to the plane of the chip.

�Automotive and Automotive Safety�

• The automotive and automotive safety industries use both digital and analog Hall effect sensors in a variety of applications.
• Examples of digital Hall effect sensor applications in the automotive industry include:
• Sensing seat and safety belt position for air-bag control
• Sensing the angular position of the crankshaft to adjust the firing angle for spark plugs
• Some examples of the use of analog type sensors include:
• Monitoring and controlling wheel speeds in anti-lock braking systems (ABS)
• Regulating voltage in electrical systems.

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�Appliances and Consumer Goods�

• The appliance and consumer goods industries integrate various types of Hall effect sensors in numerous product designs. For example:
• Digital unipolar sensors help washing machines maintain their balance during wash cycles.
• Analog sensors serve as availability sensors for power supplies, motor control indicators and shut-offs on power tools, and paper feed sensors in copier machines.

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�Fluid Monitoring�

• Digital Hall effects sensors are commonly used for monitoring flow rate and valve position for manufacturing, water supply and treatment, and oil and gas process operations.
• In fluid monitoring applications, analog Hall effect sensors are also used to detect diaphragm pressure levels in diaphragm pressure gauges.

�Building Automation�

• In building automation operations, contractors and subcontractors integrate both digital and analog Hall effect sensors.
• Digital, proximity-sensing devices are often used in the design of:
• Automatic toilet flushing mechanism
• Automatic sinks
• Automatic hand dryers
• Building and door security systems
• Elevators
• Analog sensors are used for:
• Motion sensing lighting
• Motion sensing cameras

�Personal Electronics�

• This is another area where both analog and digital Hall effect sensors continue to grow in popularity.
• Applications for digital sensors include:
• Motor control devices
• Timing mechanisms in photography equipment
• Applications for analog sensors include:
• Disk drives
• Power supply protectors�

�Applications of the Hall Effect�

• The Hall effect has applications for researchers, industrial facilities, commercial businesses, the automotive industry, and more. Hall sensors can measure voltage, current, and magnetic fields in manufacturing, inspecting, and testing purposes. Some of the most common applications of the Hall effect.
• Magnetometers
• Magnetometers, or Hall probes, measure the strength of magnetic fields, often for permanent magnets in engineering design evaluations. They can also be used to detect magnetic flux leakage in pipes and storage tanks. 

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• Magnetic Field Detection
• Magnetic field sensors and detection equipment can pick up the presence of magnetic fields and determine their magnitude. Once the fields are detected, a trigger can provide signals and data or switch power to a circuit.

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• Current and Voltage Sensing and Measurement
• Sensors use the Hall effect to detect or measure direct currents. A Hall device can detect the presence of a magnetic field.  In some cases, a Hall device can measure the voltage and determine the current, displaying it as a readable signal.

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• Position and Motion Sensing
• In the case of magnetic field detection, this function is used widely in industrial and commercial equipment and machinery.  A Hall effect sensor has the advantage of no mechanically moving components in detecting the presence of a magnetic field. They are used commonly as limit switches.
• Complex machinery and vehicles also benefit from the Hall effect. When they detect fluctuations in voltage, these sensors transmit signals that can be implemented in tachometers, anti-lock braking systems in vehicles, and material handling assemblies.

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• Ignition Timing
• The Hall effect’s capabilities to sense or control motion are crucial to proper ignition timing in internal combustion engines. Ignition timing is the precise release of a spark into a combustion chamber based on the position of the piston and the corresponding crankshaft angle.

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Thank You