What is the Hall Effect?

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Current is produced by the movement of charge carriers within a material. Typically we think of negatively charged electrons as charge carriers. However, depending on the material, the charge carriers could also be positively charged holes or ions, or a mixture of different kinds of charge carriers. If a magnetic field (B) is applied perpendicular to the direction of the current (I), the charge carriers will experience a force known as the Lorentz force (FL) (see What is a Homopolar Motor?). This force acts perpendicularly to the direction of the charge carriers, causing their path to bend, meaning that opposite charges will collect on opposite edges of the material and this phenomenon is known as the Hall effect. The charge separation within the material results in the formation of an electric field and a voltage difference across the material. As the diagram below shows, the force due to the electric field (FE), opposes the Lorentz force. Once a field strong enough to balance the Lorentz force has been established, more charge carriers are prevented from moving to the edge of the material. This results in a constant voltage difference across the material, known as the Hall voltage.

The Hall effect was discovered by physicist Edwin Hall in 1879 and has since been used to reveal many fundamental principles regarding the nature of charge carriers. The sign of the Hall voltage gives the sign of the charge carriers in a material and the Hall experiments were the first to prove that currents in metals are generally carried by negatively charges electrons rather than positively charged protons. There are some situations, in semiconductors for example, where the Hall voltage is positive and in these cases, current is conducted by positively charged quasiparticles called holes. A hole can be thought of as the absence of an electron, so the movement of holes is actually caused by the movement of electrons. Therefore, you would probably expect that the Hall effect would be the same for both holes and electrons. However, there are cases where electrons behave as though they have negative mass, resulting in a positive Hall voltage, and it is easier to treat the charge carriers as positively charged holes.

The most common application of the Hall effect is as a sensor or magnetometer. Hall devices are used in a variety of situations, from measuring magnetic fields and investigating pipelines to use as rotating speed sensors in bikes and speedometers, to fluid flow sensors, current sensors, pressure sensors and even as compasses in smart phones and GPS devices.

When a current and a magnetic field is applied to the semiconductor crystal in a Hall probe, a Hall voltage is generated across the material. Because the Hall voltage is directly proportional to the size of the magnetic field, Hall devices can be used as magnetic field strength sensors. Conversely, as the output voltage varies in response to a magnetic field, if the strength of the magnetic field is already known, the level of the output voltage reveals the distance from the field. Hall devices have many advantages; they can be dust, dirt and water proof, making them very reliable and are also relatively cheap. Although one significant drawback is that they are very sensitive to stray fields, including the Earth’s magnetic field, making them useful as compasses but drastically reducing their accuracy. To overcome this problem Hall probes can be magnetically shielded, for example by placing the device in a ferrite ring which blocks out the stray fields.

A more complex application of a Hall device is in spacecraft propulsion systems. Hall effect thrusters have been around since the early 1970s when they were used to stabilize Soviet satellites and have the major advantage of being relatively low power devices. A radial magnetic field is used to trap electrons which then circulate and create an electric field due to the Hall effect. The energetic particles then ionize propellant particles and the positive ions and negative electrons are then ejected from the thruster creating thrust to move the spacecraft.

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