N-type semiconductors:

When pentavalent impurity such as arsenic is added to a pure silicon crystal during crystal growth, the resulting crystal is called n-type semiconductor.  Let us consider the case when pentavalent arsenic (antimony, bismuth or phosphorus) is added to pure silicon   (or germanium) crystal.

When a pentavalent impurity is added to tetravalent silicon, the arsenic atom will occupy one site of the silicon atom.  Thus, out of five electrons in arsenic four electrons make covalent bonds with neighbouring four silicon atoms and the fifth is loosely bound to the silicon atom as shown in Fig. 2. Now by increasing the energy or applying electric filed, this electron can easily be excited from valence band to conduction band.  Thus every arsenic atom contributes one conduction electron without creating a positive hole.  Hence arsenic is called as donor impurity.  The crystal as a whole has excess of electrons which are negatively charged particle; this type of semiconductor is called a n-type semiconductor.