Semiconductors

Recall the definition of conductors and insulators.

Conductor

Any material that has low resistance and allows electrons to flow freely

Insulator

The opposite of a conductor. It has extremely high resistance and inhibits electron flow.

Warning

Given enough potential energy, current can still flow through an insulator. An everyday example of this is lightning, which can flow through air even though air is an insulator.

There is another class of materials known as semiconductors, which can act either as a conductor or an insulator given the right conditions.

Typically, semiconductors are made with silicon (hence the name Silicon Valley) arranged in a crystal lattice structure. Silicon (chemical symbol Si) has four valence electrons and wants four more to be in its most stable state. The Si atoms can share electrons (known as a covalent bond), which leads to a regular lattice structure with each atom sharing with four other atoms.

When arranged like this, silicon is an insulator because it would take a lot of energy to “free” the valence electrons from their stable bonds and allow them to move around. However, we can make a semiconductor by doping the silicon lattice with other elements that have either 5 or 3 valence electrons (compared to Si’s 4). The amount of doping is typically small, on the order of one atom per million, but the effects of the additional charge carriers are profound.

There are two types of doping.

N-type Doping

Adds extra negative charge carriers by doping with elements with 5 valence electrons such as phosphorous, arsenic, or antimony. The extra donor electron is free to move around the material.

P-type Doping

Takes away electrons by doping with elements with 3 valence electrons like boron, gallium, or indium. The missing electron creates a positively charged “hole” in the lattice structure. The hole can move around in much the same manner as the donor electron (even though technically it is the electrons that are moving), and thus acts as a positive charge carrier.

Both types of doped material are put together to form a semiconductor device called a junction diode, which we will cover in the next section.