Semiconductors material such as germanium, silicon, Gallium arsenide, have electrical properties lies between the conductors and insulators varies according to the applied external electric field. In a crystalline pattern i.e. crystal lattice, semiconductors have very electrons because their atoms are closely grouped together. They have negative temperature coefficient because its resistivity decrease with the increase of temperature or vice versa. Here we are going to explain you basics of semiconductors, working, types etc.
Energy band Theory:
When atoms come together to form a compound, their atoms orbital energies mix to form molecular orbital energies. When molecular orbits are formed then many of the energy levels will start to be close to or even completely degenerate in energy. These energy levels are known as bands of energy. At absolute 0K, the semiconductors act as insulators, conducts no current and above this temperature they act as a conductor. In energy band theory basic terminologies are:
(i) Valence band: Valence band are those bands where all of the valence electrons reside and are involved in the highest molecular orbit. These are fully filled or partially filled.
(ii) Conduction band: These bands are empty or partially filled where positive or negative charge carriers exist. Negative charge carriers are the electrons while the positive charge carriers are holes.
(iii) Forbidden gap: The gap between the valence band and the conduction band is known as the forbidden gap.
When we apply electric field, due to excitation electrons starts moving from valence band to conduction band and constitute current. As shown in figure below, in semiconductors there is small forbidden gap between valence and conduction band indicates that more electrons move and constitutes current. In insulators, there is large gap between the conduction band and valence band of approx 15ev indicates that insulators do not conduct current. While in conductors, there is no forbidden gap between the valence band and conduction band. From this theory, it is clear that the resistivity of semiconductor is always lies between conductor and insulator.
Types of Semiconductors:
In a semiconductor, current can be carried out by the flow of electrons or by the flow of positively charged holes in the electron structure of the material. Depending on this semiconductors are classified into two categories:
(i) Intrinsic semiconductors: An intrinsic semiconductor is made up of a very pure semiconductor material i.e. it is the one where the number of holes is equal to the number of electrons in the conduction band. The main characteristic of this type of semiconductor is the Fermi level which lies somewhere in between the valence band and the conduction band.
(ii) Extrinsic semiconductor: Extrinsic semiconductors are those where a small amount of impurity has been added to the basic intrinsic material. The phenomenon of adding an impurity to the material is known as doping. The materials chosen for doping are deliberately chosen in such a manner that either they have 3 electrons in their valence band or they have just 5 electrons in their valence band. Therefore such dopants are known as Trivalent and pentavalent.
On the basis of this extrinsic semiconductors are of two types:
(i) P-type semiconductor: In this type of semiconductor we add trivalent impurities such as boron, aluminium, gallium etc to an intrinsic semiconductor creates deficiencies of valence electrons called holes.
Following points to be noted in p-type semiconductor are:
Large number of holes
Acceptors are negatively charged
Small number of electrons in relation to number of holes
Doping gives negatively charged acceptors and positively charged holes
(ii) N-type Semiconductors: In this type of semiconductor we add pentavalent impurities such as antimony, phosphorus and arsenic contribute free electrons, greatly increasing the conductivity of the intrinsic semiconductor.
Following points to be noted in n-type semiconductor are:
Large number of Free electrons
Donors are positively charged
Small number of holes in relation to number of electrons
Doping gives positively charged donors and negatively charged free electrons
Application of semiconductor devices
Semiconductor devices are the key element of all electronic circuits. In every commercial product, they are used from the family car to the pocket calculator. Following of the applications are:
- Voltage regulator circuits
- Rectifiers which are used in Dc power supplies
- Solid state devices, computers and data processing systems
- Portable radios and Tv receivers
- Military equipment
- Wave shaping circuits such as clippers and clampers
- Data display systems and data processing units
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