Before we can move directly into a discussion about how semiconductors work and why silicon is the best choice of semiconductor material, it is necessary to discuss the movement of electrons through a material and their overall velocity of movement within than material.

Drift Velocity of Electrons through a conductor.

The "average speed" at which the "free" charges are moving in the wire is called the drift velocity.

In a wire, collisions of the conduction charges with impurities, imperfections, and vibrations of the atomic lattice causes the motion of the conduction charges to be slowed down. This represents a loss of energy which is dissipated as heat.This is the main physical reason why your laptop gets hot and in general, why switching information by using electrons generates heat as a by product.

Most of the basics are well explained in this video along with an example calculation.



The Key concepts are:

• In most all material the movement of the physical electrons is really very small, about 1 mm/sec
  
  
• The smaller the cross sectional area of the wire, the bigger the drift velocity - makes sense since your trying to squeeze electrons along a wire length through a smaller and smaller "pipe"
  
  
• The bigger the charge carrier density (n) slower the drift velocity as there are simply more charges trying to diffuse. This means the drift velocity is material dependent.
  
  
• Since the amount of charge carrier density (n) increses with temperature, the drift velocity also depends on temperature.

Although your light turns on very quickly when you flip the switch, the actual drift velocity of electrons through copper wires is very slow. It is the "signal" which propagates along wires at essentially the speed of light. in other words, electricity propagates at the speed of light, but not the individual electrons through the conductor. Electricity is a signal more so than the movement of actual individual electrons.