All the industrial and domestic electronic devices are made of a semiconductor. As the name suggests, semiconductor materials are neither good conductor nor good insulator of electrical current like Silicon and Germanium. Because of the very few current carriers (free electrons) available at the conducting shell, the specific resistance of the materials is comparatively higher than conductor and lower than that of an insulator.
Semiconductors are the perfect insulator at 0oK temperature, at higher temperature the materials are neither conductor nor insulator. The energy gap of semiconductors is lower as almost 1ev. At normal temperature, when electrons at valance shell gain energy, the gap between the valence band and conduction band is reduced. Very few numbers of electrons become available at conduction band, so a very low current flow could be expected.
Germanium and Silicon are most widely used semiconductors and both belong to the 4 column of the periodic table which means they have 4 electrons in their valence shell. They are having single crystal structure each atom in a structure is connected other four atoms by a covalent bond, which means there will is no free electron in a crystal. But electrons can absorb external energy like light/heat to escape the bond and freely move.
Germanium and Silicon may have some impurities which can cause it loses its property and can conduct more current than expected. These semiconductors can be purified by the use of modern technology which is called Intrinsic Semiconductor. The intrinsic semiconductor has very few free current carriers at room temperature. As the temperature increase, electrons gain energy and become free electron and ultimately resistance of the material decreases which is negative temperature coefficient.
The electrical characteristics of an intrinsic semiconductor are not that beneficial. The characteristics can be changed by introducing some impurities in the intrinsic semiconductor like pentavalent and trivalent elements, then those material is called extrinsic semiconductor material. The operation of introducing impurities is called doping process.
By doping pentavalent element (Antimony), which has 5 electrons in the valence shell, four electrons of the shell make a covalent bond with four Silicon/Germanium atoms. The one electron remains un-bonded which is loosely connected to its parent atom can move freely. The pentavalent element is called a donor. A new type of material is achieved through the process and is called N-Type Material.
It’s worth noting that the number of electrons and proton remain same in the N-Type Material, so, it remains electrically neutral.
Similarly, the impurities of trivalent element (Boron, 3 electrons in valence shell), all the electrons combine with surrounding Silicon/Germanium atoms and vacancy for a covalent bond remains. There is a place for one covalent bond which is called Hole. The trivalent element is called accepter because it has a place for an electron. So, the new material form in the process is called P-Type Material.
Electron vs Hole current flow:
We have been familiar with electron flow but hole flow is something different. A hole is a vacant position where a covalent bond could be formed. If an electron from a covalent bond get energy and escape the bond, the electron can fill the hole and form a new bond here. Thus electron will in one direction and hole move in the opposite direction.
Majority Carrier vs Minority Carrier:
In an intrinsic semiconductor material, there are always some free electrons and some hole available at normal conditions. By doping process of pentavalent impurity, the number of electrons can be increased. Similarly, by using trivalent impurity, the number of holes can be increased. So, the N-type material has hole very few in number and P-type has very few free electrons. These few holes and free electrons are called minorities carriers. While free electrons in N-type and hole in P-type are called majority carrier.
What is semiconductors used for?
The semiconductor devices are used for all electronic devices nowadays, like a diode, transistor, thyristors. Every electrical appliance has these semiconductor devices in it, either if it is Television, Computer, Refrigerator for home and industrial machinery like CNC machines, Telecommunication equipment etc.