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Semi Conductors

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Germanium (Ge) & Silicon (Si) are examples of semi-conductors. Fig. 1 (A) shows a germanium atom. In the centre is a nucleus with 32 protons. The revolving electrons have distributed them selves in different orbits. There are 2 electrons in the first orbit & 8 electrons in the second orbit & 18 electrons in the third orbit. The four orbit is the outer or Valence orbit which contains 4 electrons.

Fig. 1(A) Germanium
Fig 1 (B) shows a silicon atom. It has 14 protons in the nucleus & 14 electrons in orbit. There are 2 electrons in the first orbit & 8 in second orbit. The remaining 4 electrons are in the outer or valence orbit.

fig. 1(B) Silicon

In semiconductor materials, the atoms are arranged in an orderly pattern called a crystal lattice structure. If a pure Silicon crystal is examined we find that the four electrons in the outer (Valence) shell of an atom is shared by the neighboring atoms.

Fig. 2 Sharing of Atom
The union of atoms sharing the Valence electron being shared by two adjacent atoms. Each atom appears to have a full outer shell of eight electrons.

Types of Semi Conductors -

A pure semiconductor is called an intrinsic semiconductor. For example, a silicon crystal is an intrinsic Semi conductor because every atom in the crystal is a silicon atom. One way to increase conductivity in a semiconductor is by ‘doping’. This means adding impurity atoms to an intrinsic semi conductor. The doped semi- conductor is known as an extrinsic semi conductor.
The residual heat at room temperature ( 300k) is sufficient to make a Valence electron of an intrinsic semi conductor to move away from the covalent bond & the electron becomes a free electron to move in the crystal.


Fig. 3 Extrinsic semi conductor structure
When an electron breaks a covalent bond and moves away, a vacancy is created in the broken covalent bond. This Vacancy is called a ‘hole’. A hole has a positive charge when a free electron is liberated, a hole is created.

N-type semi conductor-

A semi conductor with excess of electrons is called N-type. To obtain excess free electrons, the element doped with the semiconductor material is arsenic, or antimony or phosphorus. Each of these atoms has five electrons in its outer orbit. Because the outer orbits of the atoms can hold eight electrons, no hole is available for the fifth electron in the arsenic atoms to move into. It, therefore, becomes a free electron. The number of such free electrons is controlled by the amount of arsenic added to the crystal.In N-type, the free electrons are called majority carriers & the holes minority carriers.

P-type semi conductor-

To obtain more holes, a pure silicon crystal is doped with elements such as aluminum or boron or gallium. The atoms of each of these elements have three electrons only in their outer orbit. Adding gallium to pure silicon crystals allows the atoms of the two elements to share seven electrons.
A hole is created in the place of the eight electron. Now that the number of holes exceeds the number of free. P-type material. The holes in P-type are the majority carriers & the free electrons are the minority carriers.

PN Junction-

A PN Junction is formed by combining P & N type materials. The surface where they meet is called the PN Junction.


fig.-6 PN Junction
The free electrons in the N-regions diffuse across the junctions into the P-region. The free electrons lose energy & recombine with the holes in the P-region. This recombination when the electron moved from the N-region & diffused across the junction, it leaves the atom to be a positive ion. The positive ion is not balanced by a negative charge in the N-region. The hole is eliminated in the P-region by recombination. The elimination of the hole & its positive charge leaves the atom to be a negative ion in the P-region.
The ions in the crystal structure are fixed & cannot move. Thus, a layer of fixed charges is formed on the two sides of the junctions.
There is a layer of positively charged ions on the N-side & on the P-side of the junction there is a layer of negatively charged ions. An electric field is created across the junction between the oppositely charged ions. This is called a junction field. The junction field is also known as barrier. The distance between the sides of the barrier is the ‘width’ of the barrier.

Active Components

In electronic circuits, components other than resistors, capacitors & inductors are also used. Namely transistors, diodes, Vacuum tubes, SCRs, diacs, zener, diode etc. The application of electrical circuit laws ( Ohms law etc.) in the circuit containing the above components will not give correct results, i.e. these components do not obey. Ohm’s law, Kirchhoff’s law etc. These components are called active components.



Fig. 10(a) Physical appearance of transistors
There are two symbols to represent a transistor.

Pnpsymbol.png Npnsymbol.png

Fig. 10 (b) NPN or PNP transistor The selection of a symbol is based on either NPN or the PNP type of transistor.

SCR ( Silicon Controlled Rectifier)


Fig.11 shows physical appearance of SCR & its symbol SCRs are also called thyristors & used as switching devices.



Fig. 12 Diac & its sumbol A diac is a two lead device like a diode. It is a bidirectional switching device.



Fig. 13 Triac A triac is also a semi-conductor device with three leads like two SCRs in parallel. The triac can control the circuit in either direction.

Bridge rectifier or diode bridge

It is a single package of four semi-conductor diodes connected in bridge circuit. The input AC & the output DC leads are marked & terminated. It has two doped regions with three leads & has one emitter & two bases.

Half wave rectifier

This simplest form of AC to DC converter is by using one diode such an AC to DC Converter is known as half-wave rectifier as shown in. A diode D, & a load resistance RH in series are connected across the secondary of a step down transformer ( fig.1 a ). The transformer steps up or steps down the supply voltage as needed. Further the transformer isolates the power line & reduces the risk of electrical shock. During the positive half-cycle of the input line frequency, ( fig b) the diode anode is made positive with respect to the cathode. The diode D, Conducts because it is forward-biased. Current flows from the positive end of the supply through diode D, & RL. During the negative half cycle of AC input line frequency, the diode is reverse biased. Practially no current flows through the diode & the load RH & there is no

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