8
CHAPTER 1:
Introduction
MeasureReady™ M91 FastHall™ Measurement Controller
1.5.3 Hall Voltage
When the combination of a magnetic field and a flowing current in a material
produces a new voltage, the Hall voltage, it is called the Hall effect. The Hall effect is
illustrated in FIGURE 1-4 and can be explained as a long thin material with a current,
I, flowing along the length. The current, I, is the total charge per second and is related
to the velocity (V
x
) by I = neV
x
w t, where n is the volume concentration of the carriers
in the material and V
x
is the velocity of the carriers. If a static (DC) magnetic field is
present in the z direction (into the plane of the paper in FIGURE 1-4), then there is a
force on the charges given by -eV
x
B. This force, called the Lorentz force, pushes the
carriers along the curved lines in FIGURE 1-4.
Since no current can flow through the boundary, the charges build up on the edges of
the material and creates the Hall voltage across the width of the material. When the
force from the Hall voltage exactly balances the Lorentz force, no more carriers hit
the edge of the material. The force from the Hall voltage is eV
H
/w, where w is the
width of the sample. Setting this equal to the Lorentz force gives:
V
H
IB
tne
--------
=
The Hall voltage can be either positive or negative. If the Hall voltage is positive, the
carriers in the material are holes. If the Hall voltage is negative, the carriers are
electrons.
The result that the Hall voltage is proportional to the magnetic field is the basis for
Hall magnetic field sensors. In our application, we turn this around and use the Hall
effect and the known magnetic field to study the properties of the materials.
FIGURE 1-4
Hall effect
H
1.5.4 Resistivity
The resistivity of a material does not depend on the size or shape of the sample. It also
does not depend on the position of electrical contacts used in the measurement. The
fact that resistivity is independent of these properties complicates the measurement.
Over the years, many protocols have been developed to meet these requirements, the
most common being for van der Pauw (section 1.5.6.3.2).
Summary of Contents for Measure Ready M91 FastHall
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Page 64: ...54 cHAPTER 3 Operation MeasureReady M91 FastHall Measurement Controller ...
Page 156: ...146 cHAPTER 4 Computer Interface Operation MeasureReady M91 FastHall Measurement Controller ...
Page 160: ...150 cHAPTER 5 Options and Accessories MeasureReady M91 FastHall Measurement Controller ...
Page 174: ...164 cHAPTER 6 Service MeasureReady M91 FastHall Measurement Controller ...
