Majority and Minority Carriers
We know that due to the effect of impurity, n-type material has a large number of free electrons wheres p-type material has a large number of holes. However, it maybe recalled that even at room temperature, some of the co-valent bonds break, thus releasing an equal number of free electrons and holes. An n-type material has its share of electron-hole pair (released due to breaking of bonds at room temperature) but in addition has a much larger quantity of free electrons due to the effect of impurity. These impurity-caused the free electrons are not associated with holes. The free electrons in this case are considered majority carriers since the majority portion of current in n-type material is by the flow of free electrons and holes are the minority carriers. similarly, in a p-type material, hole number the free electrons. therefore, holes are the majority carriers and free electrons are minority carriers.
PN juction
When we p-type semiconductor is suitably joined to n-type semiconductor,the contact surface is called pn junction. Most of the semiconductor devices contain one or more pn junctions. The pn junction is very importance because it is, in effect, the control element for semiconductor devices. A thorough knowledge of the formation and properties of pn junction can enable the reader to understand the operation of semi conductor devices,when pn junction is formed, the free electrons near to the junction in the n region begin to diffuse across the junction into the p region. As the free electrons diffuse across the junction,they combine with holes and leave a layer of positive charges (pentavalent ions) in the n-region and a layer of negative charges (trivalent ions) in the p-region. This forms a barrier potential. This action continues until the voltage of the barrier stops further diffusion. For silicon, the barrier potential is about 0.7V and for germanium, it is about 0.3V.Note that depletion region (the two layers of positive and negative charge form the depletion region) is formed very quickly and is very thin compared to the n region and p region. The term deplation refers to the fact that the region near the pn junction is depleted.
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