Semiconductor materials and Diode:
Various materials, like copper, aluminium and brass pass electric currents much easily. As they conduct electricity readily, their capability to oppose the flow of current is low. They thus have low resistivity and therefore are termed as conductors.
Other materials like plastics or glass have very high resistivity therefore do not conduct electricity. Such materials are termed as insulators.
Materials which have a resistivity mid-way among the conductors and insulators do conduct current, however very poorly at normal room temperatures, and therefore these are termed as semi-conductors.
How poorly or well any material conducts electricity based on the atomic structure of the material. Silicon and Germanium, and also a number of other materials and mixtures of materials in semiconductor group are broadly employed in the manufacture of transistors and diodes, and also integrated circuits like microprocessors.
To make these semiconductor materials appropriate for use in diodes and transistors the resistivity of material is altered in a controlled manner by first making very pure crystals of semiconductor material. Such crystals have only atoms of one type (for illustration silicon) arranged in a regular lattice formation. This much pure material is then "doped" by adding up tiny amounts of impurity atoms (around 1 impurity atom in every 10 million).
The thought is that pure semiconductors conduct poorly, since the electrons in their lattice structure are generally bound very tightly to their atoms, leaving only a small number of electrons free to move, from atom to atom, via the material, thus forming a very weak electric current. By adding up impurities with various atomic structures either more, or in other situations, less free electrons are added. Such controls the capability of semiconductor to pass current, by efficiently changing the resistivity of material.
Various impurities like Phosphorus and Arsenic add additional free electrons (that is, negative charge carriers) to the material. This is termed as N type semiconductor.
Other impurities like Boron and Aluminium can be added up in order to eliminate the free electrons, and hence the resultant material has less free electrons than prior to. Each missing electron in the crystal structure is termed as a "hole". Since free electrons are negative charge carriers, such holes in the structure are really positive charge carriers. The material doped in this way is termed as P type semiconductor. Putting N type and P type materials next to each other in a circuit makes a P-N junction, and makes a helpful device which is termed as a diode. Whenever a voltage is applied across a diode, a current will flow via the diode in one direction however not the other.
Forming a three-part sandwich (NPN or PNP) and carefully controlling the levels of doping in the three layers makes a transistor capable of amplification, and also many other helpful functions.
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