Electronic Devices Test -9

A p-type semiconductor is formed by doping pure germanium or silicon crystal with suitable trivalent impurity atom like Al, B, In, etc. When impurity atom take the position of germanium atoms in germanium crystal, its three electrons form covalent bonds by sharing electrons with the neighbouring three atoms of germanium whereas the fourth covalent bond is left incomplete or has a vacancy of one electron. This vacancy is called a hole.

The addition of pentavalent impurities such as antimony, arsenic or phosphorous contributes free electrons, greatly increasing the conductivity of the intrinsic semiconductor.

Doping means the introduction of impurities into a semiconductor crystal to the defined modification of conductivity.

In P-Type Semiconductor, The addition of trivalent impurities such as boron, aluminum or gallium to an intrinsic semiconductor creates deficiencies of valence electrons,called "holes".

The more abundant charge carriers are called majority carriers, which are primarily responsible for current transport in a piece of semiconductor. In n-type semiconductors they are electrons, while in p-type semiconductors they are holes.

Each electron in an atom has a different energy level. These different energy levels with continuous energy variation form what are called energy bands. The valence band includes the energy levels of the valence electrons and the conduction band is the energy band above valence band.

In conductors, there are two situations. One where the conduction band overlaps the valence band and electrons move freely into the conduction band. In the second situation, the conduction band is partially filled. This allows for high conductivity or low resistance.

The electron theory states that all matter is composed of atoms and the atoms are composed of smaller particles called protons, electrons, and neutrons. The electrons orbit the nucleus which contains the protons and neutrons. It is the valence electrons that we are most concerned with in electricity. These are the electrons which are easiest to break loose from their parent atom. Normally, conductors have three or less valence electrons; insulators have five or more valence electrons; and semiconductors usually have four valence electrons.

A region of values of energy that electrons in an ideal crystal (without defects) cannot have. In semiconductors the forbidden band separating the valence band and the conduction band is usually considered. In this case the energy difference between the lower level (bottom) of the conduction band and the upper level (ceiling) of the valence band is called the width of the forbidden band.

Valence band and conduction band are separated by small energy gap. At room temperature, some electrons from valence band can cross this energy gap and enter the conduction band.

The resistance of p-n junction diode becomes low when forward biased and becomes high when reverse biased. Therefore it allows the current to pass through it when it is forward biased and does not allow when reverse biased. Hence it can rectify AC into DC.

If We introduce a "Trivalent" (3-electron) impurity into the crystalline structure, such as Aluminium, Boron or Indium, which have only three valence electrons available in their outermost orbital, the fourth closed bond cannot be formed. Therefore, a complete connection is not possible, giving the semiconductor material an abundance of positively charged carriers known as "holes" in the structure of the crystal where electrons are effectively missing.