Objective:

 * Resistivity of Semiconductors by Four Probe Method at Different Temperatures

 * 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.

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.

The Setup Consists of:-

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.The whole –arrangement is mounted on a suitable stand and leads are provided for the voltage measurement.

2. Sample:  Germanium crystal in the form of a chip

3. Oven: It is a small oven for the variation of temperature of the crystal from the room temperature.

4. Digital Voltmeter

In this unit, intersil 3½ digit single chip A/D Converter ICL 7107 has been used.

5.  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. 

6. 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.