220/6514 High Impedance Hall Effect
Test System
Alternative economical ap-
The Model 220 Current Source offers
material researchers ±0.5pA/step to
±101mA DC output, combined with
14
10
Ω output resistance.
The Model 6514 Electrometer provides
>200TΩ input impedance and <3fA
input bias current.
Ordering Information
220 Programmable
6514 Programmable
Extended warranty, service, and
calibration contracts are available
ACCESSORIES AVAILABLE
2000 Digital Multimeter
6167 Guarded Input Adapter
CABLES
7024-10 Triax Cable, 3m (10 ft)
7025-10 Low Noise Triax Input Cable, 3m (10 ft)
7078-TRX-10 Triax Cable, 3m (10 ft) (2 required)
Current Source
Electrometer
proaches to Hall coefficient
and resistivity measurements
Occasionally, when working with samples
with very high resistivity, semi-insulating
GaAs, and similar materials with resistivities above 10
8
Ω, alternative system config-
HI
220
LO
urations may be able to produce more reliable data than standard, pre-configured
Hall Effect systems. Such systems demand
careful shielding and guarding, and typically include a current source, two electrometer buffers, and an isolated voltmeter.
The schematics show two suggested configurations for these high resistivity applications: one that requires manual switching and one with automated switching.
The range of the systems shown here is
very wide. The high resistance end is limited by the minimum output of the current
source. A current of 100pA can be supplied with an accuracy of about 2%. If the
resistance of each leg of the sample is no
more than 1TΩ, the maximum voltage
developed will be 100V, within the range
of the Model 220 current source and the
Model 6514 electrometer. This system will
provide good results with samples as low
The equipment configuration with manual switching
(above) was developed for very high resistance van der
Pauw or Hall Effect measurements. This measurement
system includes a Model 220 current source, two
Model 6514 electrometers (used as unity-gain buffers)
and a Model 2000 digital multimeter (DMM). The current
source is used with a Model 6167 guarded input
adapter, which minimizes the time constant of the current source and cable. A Model 7024-10 triax cable
connects the source, and two Model 7078-TRX-10s
connect the electrometers. Two insulated banana leads
are used to connect the digital multimeter to the preamp outputs for the electrometers. The insulation
resistance of the leads and supporting fixtures for the
sample should be at least 100 times the leg resistance
(R). The entire sample holder must be shielded to avoid
electrostatic pickup. If the sample is in a dewar, this
should be part of the shield.
as 1Ω per leg, if a test current level of
100mA is acceptable. Even at 100mΩ per
leg, accuracy is approximately 2%.
Leakage currents are the most important
sources of error, especially at very high
resistances. One important advantage of
this circuit is that a guard voltage is available for three of the sample terminals,
which virtually eliminates both leakage
currents and line capacitance. The fourth
HI
2000
LO
Preamp
Out
Preamp
Out
terminal is at circuit LO or ground poten-
.
tial and does not need guarding.
Call Keithley for additional guidance in
selecting equipment for specific high resistivity applications.
One Model 7152 Matrix Card, housed in a Model 7001
mainframe, is used to connect the electrometers and
the current source to the sample. Two Model 6514
Electrometers are used as unity gain buffers, and their
output difference is measured with a Model 2000 DMM.
To ensure faster measurement time, guarded measurements are made by turning the Guard switch ON for
both of the Model 6514s, and by guarding the Model
220 output using a Model 6167 guarded input adapter.
Call Keithley’s Applications department for cabling
information.
R = 1 TΩ
R
RR
R
HI
6514
LO
HI
6514
LO
HI
220
LO
HI
HI
1
2
3
4
Preamp Out
6514
V1
LO
Preamp Out
6514
V2
LO
2
1 3
4
12345
7152 Low Current Matrix Card
in 7001 Mainframe
HI
2000
V
DMM
LO
Hall Effect systems for high resistivity applications
1.888.KEITHLEY (U.S. only)
www.keithley.com
SEMICONDUCTOR
A GREATER MEASURE OF CONFIDENCE
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