It goes without saying that electrons rotate round the nucleus in specific circular orbits under the influence of nuclear forces which are the short range attractive forces. If the influence of electron is strong enough to conquer this force of attraction, it can be designated as conductors. If the energy of electrons, on the other hand, is least to overcome the nuclear short range forces, it is called as insulators. In simple words, the materials which reveal intermediate behavior in between insulators and conductors, and both are termed as semiconductors. Depending on their conducting properties, semiconductors can be classified into 2 types: Intrinsic semiconductors and Extrinsic semiconductors.
Basically electrons and holes are charge carriers present in semiconductors, holes as +vely charged and electrons being -vely charged. The unique characteristic property of intrinsic semiconductors is the possession neutralisation current. As a consequence of this current intrinsic semiconductor, they only have carrying capability of little current. This is to enhance its conducting properties that need to be modified in the case of intrinsic semiconductors. Doping is the property that enhances the conductivity of semiconductors. The amount of doping should be moderate. Extrinsic semiconductors are formed by intiating doping process.
Extrinsic semiconductors present in an impure form. The impurities added belongs to 3rd and 5th group in the periodic table, the trivalent impurities are 3rd group elements such as aluminium, boron, gallium, indium and pentavalent impurities. For the 5th group elements are arsenic, phosphorous, antimony, bismuth. Germanium and Silicon semiconductors are tetravalent in nature. When phosphorous, the pentavelent impurity, is added to silicon, take note that 4 valence electrons of silicon form a covalent bond with 4 valence electrons of silicon, but one electron will lie free without any participation.
Therefore, adding a small amount of impurity, electrons are generated, thus these semiconductors are n-type extrinsic semiconductors. By adding trivalent impurities, such as boron to a semiconductor 3 valence electrons of silicon form covalent bond with 3 valence electrons of impure atom. While one electron is electron deficient, the semiconductor completes its bonding process by stealing an electron from its neighboring atom. By adding trivalent impurity, there is a vacancy created. Thus, these impurities are known as acceptor impurities, while the semiconductors are called as p-type semiconductors.
Currents In Semiconductors
The current in semiconductors is usually carried by holes and electrons. The currents in semiconductors are of two types depending on the application of the external field. The first one is the drift current, and the second one is the diffusion current.
The first application is the crucial role of semiconducting materials to play a pivotal role in electronic field due to its peculiar property of temperature coefficient of resistivity. The second one is the semiconducting principle that can be utilised in the form of voltage regulator in zener diode where forward bias and reverse biasing action takes place. The third application is the fact that the semiconductors play a key role as an electronic device in the industry. Last but not the least, the semiconducting materials are used by Photodiodes and LED’s as display devices.