Three types substances are mainly we have viz solid, liquid and gases. Solids are classified in three types according to electrical conductivities. This substances through electricity or charges can pass easily facing no prevention is called conductors. Conductors of electricity are silver, gold, iron, copper etc from metal. From these substances electricity cannot pass easily called insulators. The examples of insulators are rubber, mica, quartz, ceramics, glass ebonite etc.
What are semiconductors? Some other neither substances we get which are not good conductor nor insulators their conductivity is between insulator and conductor. Such substances are called semiconductors. Silicon, gallium arsenide, germanium etc are the examples of semiconductors. We can get clear idea how far substances are good conductors or good insulators from substances specific resistance of glass is 1016 ohm-meter and the specific resistance of copper is 10-8 ohm-meter. The specific resistance of semiconductor is between 10-5 ohm-meter to 108 ohm-meter. Solid state physics deals with semiconductor and transistor. Silicon and germanium are hugely used as semiconductors. The devices of solid state are mainly made by germanium and silicon. Previously it is mentioned that semiconductors are those substances which conductivity lies between insulators and conductor. At lower temperature semiconductors become insulator, if a semiconductor crystal is heated, its conductivity increases and loses its resistance rapidly. Normally resistance of conductor increases with increasing temperature. Semiconductor has another special character is that a tiny fraction (about 1 part in 106) of a particular impurity is mixed with a pure semiconductor, its resistance decreases manifold. This type of mixing is known as doping. The semiconductors thus formed are called doped or impure semiconductors. Impure semiconductors or doped semiconductors are normally used in fabricating different electronic devices and components.
The structure of a conductor is such that valance electrons of the outer incomplete shell of an atom can move to outer incomplete shell of adjacent atoms to complete their incomplete shells.
Electrons can move in this way within the solid from one atom to another atom. As the valance electrons can move freely within atoms. So the conductor can pass charge or electricity easily and so the substance becomes good conductor. Insulator or non-conductor cannot pass electricity because they have no free electrons. Electrons remain strongly bound to the nucleus in insulators. In the outer shell silicon and germanium have 4 valance electrons of each atom. Outermost shell can accommodate 8 electrons we know that from atomic theory. The atom becomes stable if there are 8 electrons in the outer shell and it becomes insulator. Each atom wants to remain stable and each of them desires to have 8 electrons in the outer most shell. To complete the outer shell atoms of silicon, germanium etc collect necessary electrons from nearing atoms. It is to be noted that nearby atoms do not give their electrons once for all. In this situation two adjacent atoms mutually share their electrons and formed a special type of bond. This type bond is called covalent bond. Germanium crystal is shown in figure which has the formation of covalent bond. By covalent bonding each atom has 8 electrons and becomes stable. So, it is clear that covalent bonding silicon and germanium form intrinsic crystal. Germanium and silicon has no free electron exists in intrinsic crystal. They cannot pass electricity at absolute zero temperature. Some bonds are broken in these crystals due to thermal vibration or agitation if the temperature increased. As a result some electrons become free and carry current. In this process intrinsic semiconductors acquire small amount of electrical conductivity.
What is n-type semiconductor?
A pentavalent impurity like arsenic, phosphorous is added with pure semiconductor like silicon or germanium for making n type semiconductor. The most important requirement is that the size of the impurity atom should be nearly equal to the size of the atoms in the pure semiconductor silicon or germanium in this case. At a high temperature arsenic or antimony pentavalent atoms are added by special technology. The main structure of silicon or germanium should not be changed so the amount of impurity is controlled at the time of mixing rather these atoms are incorporated in the crystal lattice.
Arsenic or antimony has five valence electrons. Four electron of them makes bond with neighboring silicon or germanium. Fifth electron of them remains free. Every arsenic atom donates a free electron such way. The impurity atom called donor. Here some bonds break for the thermal energy. Which produces electron-hole pairs i.e. equal number of electrons and holes per cubic cm 107 free electrons contains in crystal formed. In n type semiconductor negatively charged electron plays the initial role for electrical conduction. So that electrons are majority carriers and the holes are minority carriers here. Because electrons are large in number, so this type of extrinsic semiconductor is called n type semiconductor.
The figure shows the energy band diagram of n type semiconductor. Donor impurity atom makes extra energy level ED. this energy level is very near to the conduction level. Difference between ED and EC is very tiny about 0.1eV. So electrons can go easily to the conduction from this new level. Some extra electrons come from the valence band by breaking covalent band due to thermal agitation. This make equal number of holes in the valence band.
P type semiconductor
Trivalent element like gallium or aluminum is mixed with pure semiconductor like silicon or germanium to make p type semiconductor. Three valence electrons of aluminum make bond with neighboring electrons of silicon. Here three covalent bonds are made. But the fourth bond is not completed. It is needed one electron to complete the bond. One electron of neibouring bond jumps into the vacancy and make a covalent bond with the vacancy. Now the bond is completed. A positively charge produces with covalent bond. In this way for each impurity atom accepts an election and a positive charge or hole produces in the semiconductor.
Thus each aluminum atom has one hole which is eager to accept an electron. For this cause trivalent aluminum impurity atom is called acceptor. In this way impurity atom gets one extra electron and it becomes a negatively charged ion. A large number of holes are produced by the impurity atom. Due to thermal energy some covalent bonds break and electron-hole pairs are made. The number of holes is larger than the electrons. So the holes are called majority carriers and the electrons are called minority carriers. The extrinsic semiconductor is called p type semiconductor because the holes are large in number here.
A figure is shown for energy band diagram for p type semiconductor. Additional energy level EA is produce by the acceptor impurity atom in the band gap. This level is very near to valence band EV . The different of energy gap between EA and EV is less than 0.1eV. So electron can go to new level from valence band easily. Thus hole or vacancy is created by going electron from valence band to new level show in the right figure. Electron-hole pair form due to thermal agitation. Some electrons go to conduction band from valence band and creates hole in valence band and thus free electrons in the conduction band.