Band Theory: The Effect of Temperature on Conductors & Semiconductors
- The conductivity of conductors and semiconductors is highly dependent on temperature
- For semiconductors: resistance decreases and conductivity increases with temperature
- For metals: resistance increases and conductivity reduces with temperature
- At absolute zero, the kinetic energy of all particles is zero, but conductivity is different for conductors and semiconductors:
- In semiconductors: all electrons will be held tightly by atoms and none will be available to move into the conduction band, hence semiconductors behave as insulators at low temperatures
- In metals: there will still be electrons in the conduction band, so, as long as there is a potential difference they will still be able to move and conduct
- For intrinsic semiconductors:
- An increase in thermal energy will excite more electrons from the valence band into the conduction band
- This increases the conductivity of the semiconductor, hence semiconductors behave as conductors at high temperatures
- For conductors:
- In conductors, the conduction band is already full with electrons
- Due to the overlapping valence and conduction bands, they already conduct close to maximum conductivity
- Since there are already enough electrons in the conduction band, an increase in temperature only leads to increased lattice vibrations
- Therefore, the collisions of conduction electrons with positive ions increases, which leads to higher resistance in metals
Band Theory: Light Intensity & LDRs
- A light-dependent resistor (LDR) is a type of intrinsic semiconductor
- Its resistance decreases as temperature increases
- This is because when photons fall on the LDR, electrons absorb their energy and move from the valence band to the conduction band
- The electrons in the conduction band can now form a current
- Increasing the light intensity gives rise to more photons per unit time and hence increases the number of electrons promoted to the conduction band
- Therefore, the resistance of an LDR will decrease as the intensity of light increases
