Effect of Temperature on the Resistivity of a Semiconductor

Introduction

A thermistor is a resistance thermometer, or a resistor whose resistance is dependent on temperature. The term is a combination of “thermal” and “resistor”. It is made of metallic oxides, pressed into a bead, disk, or cylindrical shape and then compressed with an impermeable material such as epoxy or glass. This means that they are made out of semiconductor material, which shows changes in resistance depending on changes in temperature. Due to their predictable characteristics and long-term stabilityit can be measured by using a small and measured direct current or dc passed through the thermistor in order to measure the voltage drop produced. Thermistors are an incredibly accurate category of temperature sensors.

In this experiment we expect that as temperature decreases as the resistivity increases

Theory

In this experiment we are to prove direct evidence for the effect of the temperature on the resistivity of a semiconductor thus, the thermistor. Some devices made of semiconductors decrease their resistance as the temperature increases due to the extra energy, which makes the atoms release electrons, permitting them to move more easily, this in turn reduce the resistance. T is for temperature and the unit is Kelvin (K). R is for resistance the unit is Ohm’s (Ω).Resistance thermometry is a temperature measurement technique that uses the change in electrical resistance of a material as its temperature changes.

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They are two types of thermistors. The less used is theresistance temperature detectors (RTD .The most suitable type of thermistor used is the negative temperature coefficient (NTC) thermistor, which consists of two terminal solid-state electronic mechanisms that displays a huge, expected difference in resistance matching to changes in the absolute body temperature. A simple estimate for the bond between resistance and temperature for a NTC thermistor is the use of the exponential approximation between both. It is based on the easy curve fitting to the experimental data and uses two points on the curve to determine the value of the value of β. The equation relating resistance to temperature using β is:

Where;

R = thermistor resistance at temperature T

A = constant of equation

Β = beta, the material constant

T = thermistor temperature (k)

Health and safety

Health and safety instructions are mainly common sense precautions but one has to be careful. In addition, flyers are printed out pasted on notice boards, windows and doors make it clearer and obvious not one to miss the principal instructions of each experiment. Instructors are there to constantly remind and read out instructions of the laboratory use and to report and possibly solve any predicted damage. In this experiment we take these precautions;

• Ensure that lab coats are worn
• Make sure that the beaker is held from the top rather than the bottom to avert unpropitious effects of the heat at the bottom.
• Switch the digital multimeter on and off where necessary
• Ensure beaker has no linkages
• Avoid parallax error on the thermometer and beaker by observing it at eyelevel.
• Assure lab equipments are kept well on the table to avoid falling down on to feet i.e. hot water on feet

Apparatus Used

In order to do this experiment we require to posses these apparatus;

2 x 4 mm connecting leads, one Digital multimeter set on resistance range, Source of sultry dihydrogen monoxide; an electric kettle, Frozen dihydrogen monoxide cubes or Crushed frozen dihydrogen monoxide, A Thermistor 1 thermometer, Stirrer,1 x 500 ml beaker

Technique

I put dihydrogen monoxide in an electric kettle for it to boil while I placed thermistor wrapped with a thermometer in the beaker, utilized the leads and crocodile clips designated for the purport of the annexation of the cables. I now situated the beaker at the base of the retort stand, integrated boiling dihydrogen monoxide received from the electric kettle in to it to a 300 ml mark. I then ascertained that the retort is opportunely in position such that the thermometer does not all to deep into the boiling dihydrogen monoxide. I adjusted the multimeter to a range of 20KΩ as verbalized in the instructions and poured in some cold dihydrogen monoxide / crushed frozen dihydrogen monoxide. I now accumulated and record the data of the immersion heater in a table and the temperature and resistance every 5°C is recorded warm the dihydrogen monoxide. The electric kettle is then further used to increment temperature. Crushed frozen dihydrogen monoxide was put from time to time to avail expedite the rate at which the temperature decremented.

Results

From the experiment, for every 5°c vicissitude in temperature the resultant value of the resistance is quantified. From table below we revealed that two values for resistance were taken (R1 and R2) which are in ohms (Ω). This was done to ascertain precision and precision in readings is precise

Additionally, from the table it is observed that the logarithm of the average resistance is taken, it is represented as Ln(R) in the table. Additionally, it is descried from table 1 that as the temperature decremented the resistance incremented.

1 it is withal descried that as the temperature decremented the logarithm of the average resistance incremented.

Discussion

From the first graph of average resistance against temperature it is pragmatic that the graph gives a curve whereas the second graph, which is the graph of logarithm of the resistance against temperature, gives a linear graph.

Likewise, from the graph it is observed that as the temperature decreases the resistance increases. This implicatively insinuates that the thermistor used is a negative temperature coefficient (NTC) thermistor. It is additionally observed from table that as the temperature decremented the value of the logarithm of the average resistance incremented. This might have been done to speed up the process therefore altering some of the values.

In addition, the thermistors have advantages over thermocouple. They have a higher signal to noise ratio compared with that of a thermocouple and due to their smaller size they respond quicker to temperature difference. However, thermistors are insubstantial are not easily changeable and are expensive times when clemency is required

Conclusion

The values gotten shows the resistivity increases as the temperature decreases. It is believed that the calibration process presented here represents a

Reliable technique by which to achieve the required temperature-measurement.

One way to describe the curve of an NTC thermistor is to measure the slope of the resistance versus temperature (R/T) curve at one temperature. By definition, the coefficient of resistance is given by;

α =1⁄R * dR / dT

References

1. Ametherm.com, (n.d.). What is an NTC Thermistor | Ametherm. [online] Available at: http://www.ametherm.com/thermistor/what-is-an-ntc-thermistor [Accessed 29 Oct. 2014].
2. Ametherm.com, (n.d.). Thermistor – Temperature Sensing | Ametherm. [online] Available at: http://www.ametherm.com/thermistor/ [Accessed 29 Oct. 2014].
3. Analog Technologies, (n.d.). High Stability Miniature Thermistor. [online] Available at: http://www.analogtechnologies.com/document/ATH10K1R25.pdf [Accessed 29 Oct. 2014].
4. Markedbyteachers.com, (n.d.). Resistance – Ohm’s Law – GCSE Science – Marked by Teachers.com. [online] Available at: http://www.markedbyteachers.com/gcse/science/resistance-ohm-s-law.html [Accessed 29 Oct. 2014].
5. Measurement Specialties, Inc., (n.d.). [online] Available at: http://precisionsensors.meas-spec.com/default.asp [Accessed 29 Oct. 2014].
6. Precisionsensors.meas-spec.com, (n.d.). Sensors | Measurement Specialties, Inc. [online] Available at: http://precisionsensors.meas-spec.com/default.asp [Accessed 29 Oct. 2014].
7. Teamwavelength.com, (n.d.). Thermistor Basics. [online] Available at: http://www.teamwavelength.com/info/thermistors.php? [Accessed 29 Oct. 2014].

Sanusi Dangote

Group S6

PHYSICS LAB REPORT