EXTRINSIC SEMICONDUCTORS

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Extrinsic Semiconductors:In intrinsic or pure semiconductors, the carrier concentration of both electrons and holes at normal temperatures very low, hence to get appreciable current density through the semiconductor, a large electric field should be applied. This problem can overcome by adding suitable impurities into the intrinsic semiconductors.The extrinsic semiconductors are those in which impurities of large quantity are present. In general, theimpurities can be either III group elements or V group elements. Based on the impurities present in the extrinsic semiconductors, they are classifies into two categories.1. n-type semiconductors and2. p-type semiconductors
n-typesemiconductors:In order for silicon crystal to conduct electricity, we need to introduce an impurity atom such as Arsenic, Antimony or phosphorus into the crystalline structure. These atoms have five outer electrons in their outermost co-valent bond to share with other atoms and are commonly called pentavalent impurities. Thisallows four of the five electrons tobond with its neighboring silicon atoms leaving one free electron to move about when electrical voltage is applied. As each impurity atom donates one electron, pentavalent atoms are generally known as donors. Antimoney (Sb) is frequently used as pentavalent additive as it has 51 electronsarranged in 5 shells around the nucleus. The resulting semiconductor material has an excess of current carrying electrons, each with a negative charge, and is therefore referred to as n-type material with theelectrons called majority carriers and the resultant holes minority carriers. The block diagram of n-type impurity doping and corresponding band diagram is shown in figure
1.p-type semiconductors:In contrast to n-type of semiconductor, if we introduce a trivalent (3 electron) impurity into the crystal structure, such as aluminum, Boron or indium , only three valence electrons are available in the outermost covalent bond meaning that the fourth 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. As there is a hole an adjoining free electron is attracted to it and will try move into the hole to fill it. However, the electron filling the hole leaves another hole behind, and is forth giving the appearance that the holes are moving as a positive charge through the crystal structure (conventional current flow). As each impurity atom generates a hole, trivalent impurities are generally known as acceptors as they are continually accepting extra electrons. Boron (B) is frequently used as trivalent additive as it has only 5 electrons arranged in 3 shells around the nucleus. Addition Boron causes conduction to consistsmainly of positive charge carriers results in a p-type material and the positive holes are called majority carriers while the free electrons are called minority carriers.