There are two main types of semiconductor materials: intrinsic - where the semiconducting properties of the material occur naturally i.e. they are intrinsic to the material's nature. extrinsic - they semiconducting properties of the material are manufactured, by us, to make the material behave in the manner which we require. 
Nearly all the semiconductors used in modern electronics are extrinsic. This means that they have been created by altering the electronic properties of the material. Several different semiconducting materials exist, but the most common semiconductor material is Silicon and the two most common methods of modifying the electronic properties are: Doping - the addition of 'foreign' atoms to the material. Junction effects - the things that happen when we join differing materials together. 
In the following the process of doping will be explained, but you can find material on junction effects in the section on diodes. Doping is the process by which engineers change an insulating material into a semiconductor. The basic process inserts a small 'population ' of a foreign element into the crystal lattice of the insulator. For example, we might insert some boron atoms into a lump of - otherwise very pure - silicon. It is conventional to call the main material the bulk and the small number of foreign atoms the dopant. The energy levels available for electrons orbiting in the dopant atom are different to those in the bulk material. The dopant also has a different number of electrons per atom since it is a different element. By choosing a suitable dopant we can achieve one or other of the two situations described below: we have chosen a dopant which has an energy level just below the bulk material's conduction band. Left to itself the dopant has an electron sitting in this level. Since this dopant level is very close to the bulk conduction band we only require a small amount of energy to free the dopant's electrons and they can move freely in the conduction band. The effect of doping is to provide the bulk material with a population of free electrons which have been 'borrowed' from - or donated by - the dopant. Semiconductors manufactured in this way are called n-type because the free charge carriers we have created are negative (electrons) and there are no corresponding holes in the valence band. The dopant atoms are called Donors because they donate their electrons for conduction. Each of the donor atoms end up with an overall positive charge because it has lost an electron. This positive charge is effectively unable to move since the atom is stuck between its neighbours. The alternative way to create a semiconductor is to add dopant which has a, normally empty, energy level just above the valence band of the bulk material. Electrons in the valence band find it relatively easy to hop into these new levels, opening up a population of holes in the valence band. For obvious reasons, dopant atoms which act like this are called Acceptors. They pick up and hold a fixed negative charge, creating a free population of holes to conduct through the valence band. Semiconductors made this way are called p-type because conduction can now take place when the positive hole moves in response to an applied field. 
By choosing the right doping atoms and the number of them that we inject per cubic centimetre, we can alter or engineer the electronic properties of the semiconductor. The ability to manufacture 'designer' materials is one of the reasons that semiconductor engineering is such a very useful technique. 


