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A G R E AT E R M E AS U R E O F CO N F I D E N C E materials with high carrier mobility, which
is what's sparked much of the interest in
graphene. This one-atom-thick form of car-
bon exhibits the quantum Hall effect and,
as a result, relativistic electron current flow.
Researchers consider Hall effect measure-
ments crucial to the future of the electron-
ics industry.
Materials with high carrier mobility
allow creating devices that obtain maxi-
mized current flow at lower power levels with
faster switching times and higher bandwidth.
Hall Effect Measurements
A manipulation of Ohm's Law (Figure 2)
shows the importance of carrier mobility in
maximizing current. The current is directly
Essential for Characterizing
proportional to carrier mobility.
The options for maximizing current flow
through a device include increasing voltage,
High Carrier Mobility charge carrier concentration, the cross-sec-
tional area of the sample, or the mobility of
the charge carriers. All but the last of these
have serious disadvantages.
Robert Green Measuring Mobility
Senior Market Development Manager The first step in determining carrier
mobility is to measure the Hall voltage (VH)
Keithley Instruments, Inc. by forcing both a magnetic field perpendicu-
lar to the sample (B) and a current through
The Hall effect can be observed when the (n), Hall coefficient (R H), resistivity, mag- the sample (I). This combination creates a
combination of a magnetic field through a netoresistance (R B), and the carrier con- transverse current. The resulting potential
sample and a current along the length of the ductivity type (N or P) are all derived from (VH) is measured across the device. Accurate
sample create an electrical current perpen- Hall voltage. measurements of both the sample thickness
dicular to both the magnetic field and the As researchers develop next-generation (t) and its resistivity (r) are also required.
current, which in turn creates a transverse ICs and more efficient semiconductor The resistivity can be determined using
voltage perpendicular to both the field and materials, they`re particularly interested in either a four-point probe or the van der Pauw
the current (Figure 1). The underlying prin-
ciple is the Lorentz force: the force on a point
charge due to electromagnetic fields.
Magnetic Flux
Hall effect measurements are invaluable
for characterizing semiconductor materials
whether they are silicon-based, compound
semiconductors, thin film materials for solar
cells, or nanoscale materials like graphene.
The measurements span low resistance
(highly doped semiconductor materials,
high temperature superconductors, dilute
magnetic semiconductors, and GMR/TMR
materials) and high resistance semiconduc-
tor materials, including semi-insulating
e
GaAs, gallium nitride, and cadmium
ag
C
lt
u
telluride.
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rr
lV
e
n
A Hall effect measurement system is use-
al
t
H
ful for determining various material param-
eters, but the primary one is the Hall voltage
(VH). Carrier mobility, carrier concentration Figure 1. Illustration of Hall effect.
Hall Effect Measurements Essential for Characterizing High Carrier Mobility November 2011 1
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