Effect of Temperature on the Resistivity of a Semiconductor

Question 1

what is the simplest way to measure the dependence on temperature of a disc thermistor

Answer 1

• connect the disc thermistor to an ohmmeter by its leads
• note down the resistance measured and this should be the resistivity at room temp (20C)
• then hold the leads firmly between your thumb and forefinger and measure the resistance then
• the temperature should be 37C, body temp

Question 2

what should be observed

Answer 2

a very significant drop in the resistance of the thermistor

Question 3

what does this disc thermistor therefore have if its resistance decreases as the temperature increases

Answer 3

a negative temperature coefficient (NTC)

Question 4

what would a decrease in the thermistors resistivity mean about the value for I in I = nAvq

Answer 4

• the value for I must have increased

- as if R = V / I and V is constant, I must increase for R to decrease

Question 5

why does I in I = nAvq increase in the first place as temperature increases in a semiconductor

Answer 5

• q and A are still constants
• however, in a semiconductor, an increase in temperature provides extra energy to release more charge carriers
• this means that n increases as temperature increases

Question 6

despite n increasing with temp, v should still be decreasing as the lattice of ions vibrate more vigorously, so why does I still increase

Answer 6

• n actually increases exponentially with the absolute temp
• meaning n shows a rapid increase as the temp rises
• as v doesnt decrease as rapidly as n increases, the overall change in I is positive

Question 7

what does it mean if a material has an NTC

Answer 7

• a negative temperature coefficient

- meaning the resistivity decreases as temperature increases

Question 8

do PTC and NTC characteristics only stick to metallic and semiconductors

Answer 8

no

Question 9

what are some examples of how the varying conductors can be one or the other

Answer 9

• semiconductors can be constructed to have a PTC
• they can also be built to have a constant resistance over a temperature range
• where the increase in n and decrease in v is balanced

Question 10

what is an example of a semiconductor that can have a constant resistance for a range of temperatures

Answer 10

• carbon resistors

- commonly found in school labs or electronic circuits

Question 11

what process is used in order to create a semiconductor like this

Answer 11

• a process called doping
• where a pure semiconductor has atoms of an impurity added to it
• the property of the creation is dependent on the relative amounts of impure atoms and pure semiconductors

Question 12

what are some examples of pure semiconductors and impure atoms

Answer 12

• germanium and silicon are pure semiconductors

- and arsenic and boron an act as atoms of an impurity