Extrinsic Semiconductors

EXTRINSIC SEMICONDUCTORS

In an intrinsic semiconductors at room temperature current conduction is very less. To increase the conductivity of the semiconductor some suitable impurity (or) doping agent is added. The process of adding impurities to a semiconductor is called as doping. This impure form of semiconductor is called as extrinsic semiconductor. Generally for 10⁶ to 10¹⁰ atom of semiconductor material one impurity atom is added.

The purpose of adding impurity is to increase either the number of free electrons (or) holes in a semiconductor. Generally, two types of impurity atoms are added to the semiconductor namely the impurity atoms containing 5 valence electron (Pentavalent) and the impurity atoms containing 3 valence electron (Trivalent). Depending upon the type of impurity atoms added to the semiconductor, the resulting semiconductor may be of the following two types.

(i) N-type semiconductors

(ii) P-type semiconductors

N-Type Semiconductors

The semiconductors, which are obtained by introducing pentavalent impurity atoms are known as N-type semiconductors. The pentavalent impurity is an element from group V of the periodic table. The elements in this group contain 5 valence electrons. The typical examples of such elements are phosphorus (P), antimony (Sb), Arsenic (As) and bismuth (Bi). These elements donate excess electron carriers.

It has been observed that when a pentavalent impurity is added to a pure semiconductor, it displaces some of its atoms. Fig 3.14 shows the structure of silicon crystal lattice containing as antimony atom at the central position.

It may be noted that out of 5 valence electrons, 4 electrons will form covalent bonds by sharing one electron each will the electrons of the neighbouring atoms. The 5^th electron is an extra electron, and is loosely bound with the antimony atom. This extra electron, if detached from the antimony atom, will be available as a carrier of the current. The energy required to detach this electron is of the order of 0.05 ev for silicon and 0.01 ev for germanium.

After donation, the impurity atom becomes a positively charged ion and is known as a donor ion. N-type semiconductor, the current flows due to the movement of electrons and holes. But a major part of the current flows due to the movement of electrons. Therefore the electrons in an N-type semiconductor are known as majority carriers and holes as minority carriers.

P-Type Semiconductors

The semiconductors, which are obtained by adding a trivalent impurity atoms are known as P-type semiconductors. The trivalent impurity is an element group of III of the periodic table. The elements in this group contain 3 valence electrons. Examples of such elements are Gallium (Ga), Indium (In), Aluminium (Al), Boron (B) etc. These elements make available positive charge carriers because they create holes, which can accept electrons. Therefore, such elements are known as acceptor (or) P-type impurities.

It has been observed that when trivalent impurity is added to a pure semiconductor, it displaces some of its atoms fig.3.16 shows the structure of a silicon crystal containing an indium atom at the central position. The 3 valence electrons, of an indium atom, forms 3 covalent bonds by sharing one electron with the electrons of neighbouring atoms. However, the fourth covalent bond is incomplete. A vacancy, which exists in the incomplete covalent bond constitutes a hole.

Now, the indium atom seeks its surrounding atoms so as to acquire the 4th electron, to complete the covalent bond. It does so by taking an advantage of thermal motion, which brings one electron form the surrounding atom.

Thus, an electron, which is in a favourable position is captured by an indium atom. After doing this, the indium atom becomes an immobile ion. The energy, involved in capturing the electron to 0.05 ev for silicon and 0.01 ev for germanium.

The greater number of holes in the valence band than that of electrons in the conduction band, the Fermi level shifting downwards towards the top of the valence band as shown in fig.3.17 The addition of a trivalent impurity atom takes (or) accepts its fourth electrons. After accepting, the impurity atom becomes a negatively charged ion and is known as an acceptor ion. The P-type semiconductor the current flows due to the movement of holes and electrons. But a major part of the current flows due to the movement of holes. Therefore, holes in a P-type semiconductor are known as majority carriers and electrons as minority carriers.