Before we look at examples of doped semiconductors, let's look at how the silicon atoms in pure silicon interact to form the crystal structure of the material. In pure silicon, each atom has four valence electrons and these are shared with four neighboring silicon atoms to make four double bonds. Now each atom will have a completely filled valence shell of eight electrons. At low temperature this bond is very stable, completely filling the valence band and thus making conduction impossible. Here is a model of the structure of pure silicon:

p-doping is when you add atoms with less valence electrons to the semiconductor so that the material gets a shortage of electrons in the crystal bonds. This way positive holes that can transport current are formed. The materials that add holes are called acceptors because they accept electrons from the surrounding atoms. In a p-type semiconductor the major carrier of current are the holes, not the electrons.

This is a model of silicon where one of the silicon atoms has been replaced by a boron atom. Boron has one electron less than silicon in its valence layer, making it an acceptor atom. The missing electron creates a hole that can be filled by any one of the neighboring electrons. By making the move to the hole, the moving electron creates a new hole, which of course can be filled by yet another electron.

In the process of n-doping you add atoms with one extra valence electron to the pure semiconducting material. This creates a situation where there are extra electrons that are just loosely bound in the crystal. The amount of energy needed to get these electrons to jump to the conduction band so that a current may pass is very small. The materials that add electrons are called donors. This is simply because they donate electrons to the semiconductor. In the n-type semiconductor the major carrier of current is the negative electrons.

This is a model of silicon where one of the silicon atoms has been replaced by an arsenic atom. The arsenic atom, which is a donor, has one more valence electron than silicon. The extra electron that is introduced doesn't participate strongly in any bond because there is no natural place left for it. Thus, it is essentially free to move around in the crystal and will contribute to any current through the material.