Semiconductor

Semiconductor

  

           A semiconductor is a material that imparts current but only partially.Its conductivity lies between a conductor which has fully conductivity and an insulator which has negligible conductivity.

            Semiconductors can be pure elements such as silicon or germanium or compounds such as gallium arsenide or cadmium selenide.In a process called doping small amount of impurities are added to pure semiconductors causing large changes in the conductivity of the material.

Doping Process

        The pure semiconductor is basically 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 in semiconductors 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 Doping.

N-Doping

        The 5-valent dopant has an outer electron more than the silicon atoms.Four outer electrons combine with ever one silicon atom, while the fifth electron is free to move and served as charge carrier.This free electron requires much less energy to be lifted from the valence band into the conduction band,than  the electrons which cause the intrinsic conductivity of silicon.The dopant which emits an electron is known as an electron donor.

         

        The dopants are positively charged by the loss of negative carriers and are built into the lattice, only the negative electrons can move.Doped semi metals whose conductivity is based on free (negative) electrons are n-type or n-doped.Due to the higher number of free electrons those are also named as majority charge carriers, which free mobile holes are named as the minority charge carriers.

P-Doping

         The 3-valent dopants can catch an additional electron thus leaving a hole in the valence band of silicon atoms.Therefore the electrons in the valence band become mobile.The holes move in the opposite direction to the movement of the electron into the energy level of indium as a dopant,is only 1% of the energy which is needed to raise a valence electron of silicon into the conduction band, with the inclusion of electron the dopant is negatively charged such dopants are called acceptors.The dopant is fixed in the crystal lattice only the positive charges can move.Due to positive holes these semiconductors are called P-conductive or P-Doped.The holes are the majority charge carriers and free electrons are the minority charge carriers.