Measurement Of Magnetoresistance of Semiconductors

DESCRIPTION

It is noticed that the resistance of the sample changes when the magnetic field is turned on. The phenomenon, called magnetoresistance, is due to the fact that the drift velocity of all carriers is not same. With the magnetic field on; the Hall voltage V = Eyt = |v × H| compensates exactly the Lorentz force for carriers with the average velocity; slower carriers will be over compensated and faster one undercompensated, resulting in trajectories that are not along the applied field.

This results in an effective decrease of the mean free path and hence an increase in resistivity

Here the above referred symbols are defines as: v = drift velocity; E = applied electric field; t = thickness of the crystal; H = Magnetic field

DESCRIPTION OF THE EXPERIMENTAL SET-UP

Experimentental Set-up for Magnetoresistance

The set-up consists of the following:

       1. Four probe arrangement, LSDMR-ARR-RM
       2. Sample: (Ge: n-type) 
       3. Constant Current Source, LSCCS-01 
       4. Low Current Source, LSLCS-02 
       3. Digital Microvoltmeter, LSDMV-001
       6. Electromagnet, LSEMU-75V 
       7. Constant Current Power Supply, LSDPS-175M
       8. Digital Gaussmeter, LSDGM-102

1. Four Probe arrangement

It consists of 4 collinear, equally spaced (2mm) and individually spring loaded probes mounted on a PCB strip. Two outer probes for supplying the constant current to the sample and two inner probes for measuring the voltage developed across these probes. This eliminates the error due to contact resistance which is particularly serious in semiconductors. A platform is also provided for placing the sample and mounting the Four Probes on It.

2. Sample
 Ge Crystal (n-type) dimensions: 10 x 10 x 0.5mm. 
 (Standard Sample included to enable the user to check the functioning of the setup)

3. Constant Current Source

a) Constant Current Source, Model : LSCCS-01 
 (for low resistivity to medium resistivity samples)

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 ripples free d.c. source. The constant current source is suitable for the resistivity measurement of thin films of metals/ alloys and semiconductors like germanium.

Specifications

b) Low Current Source, Model : LSLCS-02 
 (for high resistivity samples)

Low Constant Current Sources are needed, when the sample resistance, is large. As in the case of silicon wafers or high resistivity film deposits. Large resistance makes the measurement prone to pickups from the mains. This problem is reduced to very low level by using the battery instead of mains. Since the current requirement is very small, the batteries should have a reasonably long life. An internal voltage reference of 2.5 volt ensures reliable operation even when the batter voltage falls. A ten turn potentiometer makes the current adjustment very easy.

Specifications

 4. D.C. Microvoltmeter LSDMV-001
 
 5. Electromagnet, LSEMU-75V 
 
 6. Constant Current Power Supply, LSDPS-175M 
 
 7. Digital Gaussmeter, LSDGM-102