Conductors and semiconductors

​​In a nutshell

In conductors positive ions are surrounded by free electrons which are able to move and create a current. In semiconductors the majority of the electrons is stuck in the valence band and needs energy to be freed.

Conductors

Conductors are made of lattices (crystal structure) of positive ions surrounded by free electrons. When one end of the conductor is made positive and the other negative, which is what happens when you connect a component to a power source, these free electrons start moving towards the positive terminal creating a current. 

The speed of this overall movement is slow due to the electrons constantly colliding with the positive ions and each other. These collisions are what create electrical resistance.

Increasing the temperature in a conductor makes the ions and electrons vibrate more which leads to more collisions and therefore an increase in resistance. If you increase the current in a wire it means that electrons are moving faster which causes more collisions and increases temperature and resistance.

Semiconductors

Semiconductors are materials with a smaller number of free electrons. Most of the electrons are contained in the valence shell (last electron shell) of the atoms and can be released by giving them energy. This energy can come in the form of temperature (NTC thermistors) or even light (LDRs).

When opposite charges are applied on both sides of a semiconductor, the free electrons start moving towards the positive charge. If energy is added into the system more electrons will be released from their valence shells and will also add to the current therefore reducing the resistance. This energy can be gained in different ways depending on the material.

For some semiconductors this energy comes in the form of temperature. When it is increased the energy of the system will increase and thus more electrons will be released from their shells and decrease the resistance. NTC thermistors use these types of semiconductors to function, their relationship between resistance and temperature is shown below:

Note: An increase in temperature does not always mean a decrease in resistance for semiconductors (e.g. PTC thermistors) but you can assume it does for your exams.

For semiconductors used to make LDRs, the energy needed to release electrons comes in the form of light intensity. This means that in brighter conditions an LDR releases more electrons which decreases its resistance. The relationship between light intensity and resistance in an LDR is shown below:

Conduction holes

Semiconductors are able to create a current in another way. When an electron is released from a valence shell it effectively leaves a positive "hole" which electrons from other atoms can jump into leaving other holes. This jumping of electrons creates a movement of positive holes in the direction opposite to that of the free electrons and contributes to the current flow of the semiconductor. 

Note: not all exam boards cover conduction holes so you don't need to worry about it too much!