Resistivity of Semiconductors by
Four Probe Method at Different Temperatures and Determination
of the Band-gap
The Four Probe Method is one of the standard
and most widely used method for the measurement of resistivity
of semiconductors. The experimental arrangement is illustrated.
In its useful form, the four probes are collinear. The error
due to contact resistance, which is specially serious in the
electrical measurement on semiconductors, is avoided by the
use of two extra contacts (probes) between the current contacts.
In this arrangement the contact resistance may all be high
compare to the sample resistance, but as long as the resistance
of the sample and contact resistances are small compared with
the effective resistance of the voltage measuring device (potentiometer,
electrometer or electronic voltmeter),the measured value will
remain unaffected. Because of pressure contacts, the arrangement
is also specially useful for quick measurement on different
samples or sampling different parts of the same sample.
Description of the experimental
1. Probes Arrangement
It has four individually spring loaded probes. The
probes are collinear and equally spaced. The probes are mounted
in a teflon bush, which ensure a good electrical insulation
between the probes. A teflon spacer near the tips is also
provided to keep the probes at equal distance. The whole –arrangement
is mounted on a suitable stand and leads are provided for
the voltage measurement.
Germanium crystal in the form of a chip
It is a small oven for the variation of temperature
of the crystal from the room temperature to about 200°C
4. Four Probe Set-up, DFP-02
The set-up consists of three units in the same cabinet.
(i) Multirange Digital Voltmeter
In this unit, intersil 3½ digit single chip
A/D Converter ICL 7107 has been used. It has high accuracy
like auto zero to less than 10mV, zero drift of less than
1mV/°C, input bias current of 10pA max. and roll-over
error of less than one count. Since the use of internal reference
causes the degradation in performance due to internal heating,
an external reference has been used.
& X10 (0-2V)
at X1 range
of reading ±1 digit
digit, 7 segment LED (12.5mm height) with auto polarity
and decimal indication
of 1 on the left & blanking of other digits
(ii) Constant Current Generator
It is an IC regulated current generator to provide
a constant current to the outer probes irrespective of the
changing resistance of the sample due to change in temperatures.
The basic scheme is to use the feedback principle to limit
the load current of the supply to preset maximum value. Variations
in the current are achieved by a potentiometer included for
that purpose. The supply is a highly regulated and practically
ripple free d.c. source. The current is measured by the digital
or ±1 digit
||0.03% for 0 to full
||0.05% for 10% chang
(iii) Oven Power Supply
Suitable voltage for the oven is obtained through
a step down transformer with a provision for low and high
rates of heating. A glowing LED indicates, when the oven power
supply is ‘ON’.
The experimental set-up is complete in all
Typical results obtained from this set-up
are shown in the figure.