|
|
|
|
|
|
Introduction
The resistivity measurements of semiconductors
can not reveal whether one or two types of carriers are present;
nor distinguish between them. However, this information can
be obtained from Hall Coefficient measurements, which are
also basic tools for the determination of carrier density
and mobilities in conjuction with resistivity measurement.
Theory
As you are undoubtedly aware, a static magnetic field has
no effect on charges unless they are in motion. When the charges
flow, a magnetic field directed perpendicular to the direction
of flow produces a mutually perpendicular force on the charges.
When this happens, electrons and holes will be separated by
opposite forces. They will in turn produce an electric field
( h)
which depends on the cross product of the magnetic intensity,
 , and the
current density, J.
h
= R x
Where R is called the Hall coefficient.
Now, let us consider a bar of semiconductor,
having dimension, x, y and z. Let
is directed along X and
along Z then h
will be along Y, as in Fig. 2.
Then we could write
Where Vh is the Hall voltage appearing between
the two surfaces perpendicular to y and I = yz
Hall Effect experiment consists
of the following:
1. (a) Hall
Probe (Ge Crystal); (b) Hall
Probe (InAs)
2. Hall Effect Set-up
(Digital), DHE-21
3. Electromagnet, EMU-75
or EMU-50V
4. Constant Current
Power Supply, DPS-175 or DPS-50
5. Digital Gaussmeter,
DGM-102
Hall Probe
| (a)
Hall Probe (Ge Crystal) |
|
|
Ge
single crystal with four spring-type pressure contacts
is mounted on a sunmica-decorated bakelite strip. Four
leads are provided for connections with measuring devices.
Technical
details
Material:
Ge single crystal n or p-type as desire
Resistivity:
8-10 W.cm
Contacts:
Spring type (solid silver)
Zero-field
potential: <1mV (adjustable)
Hall
Voltage: 25-35mV/10mA/KG
It
is designed to give a clear idea to the students about
Hall Probe and is recommended for class room experiment.
A minor drawback of this probe is that it may require
zero adjustment. |
Indium
Arsenide crystal with 4 soldered contacts is mounted
on a PCB strip and covered with a protective layer.
The Hall Element is mounted in a pen-type case and a
4-core cable is provided for connections with the measuring
device and current source.
Technical
details
Contacts:
Soldered
Rated
Control Current: 4mA
Zero
Field Potential: <4mV
Linearity
(0-20KG): ±0.5% or better
Hall
Voltage: 60-70mV/4mA/KG
The
crystal alongwith its four contacts is visible through
the protective layer. This is mainly used as a transducer
for the measurement of magnetic field. |
Hall Effect Setup
(Digital), DHE-21
DHE-21 is a high performance instrument
of outstanding flexibility. The set-up consists of a digital
millivoltmeter and a constant current power supply. The Hall
voltage and probe current can be read on the same digital
panel meter through a selector switch.
| (i) Digital
Millivoltmeter |
(ii) Constant
Current Power Supply |
|
Intersil 3½ digit single chip A/D Converter ICL 7107
have been used. It has high accuracy like, auto zero
to less than 10µV, zero drift of less than 1µV/°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. Digital
voltmeter is much more convenient to use in Hall experiment,
because the input voltage of either polarity can be
measured.
Specifications
Range: 0-200mV (100mV minimum)
Accuracy: ±0.1% of reading ±1 digit
|
This power supply, specially designed for Hall Probe,
provides 100% protection against crystal burn-out due
to excessive current. The supply is a highly regulated
and practically ripple free dc source.
Specifications
Current: 0-20mA
Resolution: 10µA
Accuracy: ±0.2% of the reading ±1 digit
Load regulation: 0.03% for 0 to full load
Line regulation: 0.05% for 10% variation
|
|
Top
|
|
