Direct current/Semiconductors

Semiconductors are materials which conduct electricity only in one direction, as a result of their chemical properties. In the chemical elements which comprised the semiconductors the gap between non-conducting and conducting band is less than of insulators, but much bigger than of conductors. That's why in their pure state their conductivity is relatively low. The examples of those elements are Silicon, Germanium, and some other elements with valence of 4.
 * The conductivity of those elements can be drastically increased by adding some impurities to them. This process is called doping. The results of this process are n-type and p-type semiconductors

N-type semiconductors
In order to increase the number of conductance band, or "free" electrons in the pure silicon (or any element of valence 4), a pentavalent atoms, such as arsenic (As), phosphorus (p), bismuth(Bi), and antimony (Sb) are added. Each pentavalent atom forms covalent bonds with 4-valent atom, and the 5th electron remains free. So the atom of the pentavalent element is called a donor atom.

The electrical conductivity of pure Germanium or Silicon can be drastically increased by the addition of impurity material in the ratio of 1 atom per 10 million atoms of germanium. The escaping of the electron from the atoms leaves a bound positive charge which cannot move. Recombination does not take place because of the shielding effect of the remaining electrons. Current flow through N type material is due primarily to the donated free electrons, so that they are called the majority current carriers. The holes are termed as the minority current carriers, since the current flow in the valence energy band is very small.

P-type semiconductors
To increase the number of "holes" in a pure silicon or another four-valent element, a trivalent impurity is added. A covalent bond is formed and one hole is expected to receive an electron. The atom of a trivalent material is called an acceptor atom.

The Diode
Both in n-type and p-type materials the number of protons and electrons are equal, so the materials by themselves are electrically neutral. But the piece of silicon (ar nay 4 valent element) can be doped so that on one side it is p type, and on other it is n type. P region has many free holes, and n region has many free electrons. But after a while when an energy is applied the electrons and holes start to drift between the regions, and some electric field forms. To get over it we need some potential. The barrier potential of a pn junction is approximately 0.7 v for Silicon, and 0.3 v for Germanium.

Biasing the diode
To make a diode work, we need to apply voltage to it. To bias a diode we need a DC voltage.

Forward Bias
Is a condition which lets the current through the pn junction. In this case, +voltage is applied to the P region of the diode, and -voltage is applied to the N region of the diode. Because the similar charges repel the negative side pushed free electrons so, that they are found in abundance on the N region. Such a flow of free electrons is called an electron current

Reverse Bias
Is an arrangement which does not allow the flow of electrons through the diode. It happens when -Voltage is applied to P, +Voltage is applied to N.

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