The semiconductor (diode) mentioned in the basic electronics tutorial on covalent bonding and conduction is essentially neutral.
It contains no free electrons in its conduction bands. Even with the application of thermal energy, only a few covalent bonds are broken, yielding a relatively small current flow.
A much more efficient method of increasing
current flow insemiconductors is by adding very small amounts of
selected additives to them, generally no more than a few parts per
million. These additives are called impurities and the process of adding
them to crystals is referred to as the doping process.
The purpose of semiconductor doping is to increase the number of free charges that can be moved by an external applied voltage.
When an impurity increases the number of free electrons, the doped semiconductor is NEGATIVE or N TYPE, and the impurity that is added is known as an N-type impurity. However, an impurity that reduces the number of free electrons, causing moreholes, creates a POSITIVE or P-TYPE semiconductor, and the impurity that was added to it is known as a P-type impurity.
Semiconductors which are doped in this manner — either with N- or P-type impurities — are referred to as EXTRINSIC semiconductors.
P and N type semiconductors will be presented in detail in following tutorials.