CONDUCTION IN SEMICONDUCTORS

Electrons and holes in an intrinsic semiconductors, conductivity of a semiconductor, carrier concentrations in an intrinsic semiconductor, donor and acceptor impurities, charge densities in a semiconductor, Fermi level in a semiconductor having impurities, diffusion, carrier life time, Hall effect.

The branch of engineering which deals with the flow of Electrons through vacuum, gas or semiconductor is called Electronics.

 Electronics essentially deals with electronic devices and their utilization.

Atomic Structure

·         Atom is the basic building block of all the elements. It consists of the central nucleus of positive charge around which small negatively charged particles called electrons revolve in different paths or orbits.

·          An Electrostatic force of attraction between electrons and the nucleus holds up electrons in different orbits.

• Nucleus is the central part of an atom and contains protons and neutrons. A proton is positively charged particle, while the neutron has the same mass as the proton, but has no charge. Therefore ,nucleus of an atom is positively charged.

• atomic weight = no. of protons + no. of neutrons

• An electron is a negatively charged particle having negligible mass. The charge on an electron is equal but opposite to that on a proton. Also the number of electrons is equal to the number of protons in an atom under ordinary conditions. Therefore an atom is neutral as a whole.

• atomic  number = no. of protons or electrons in an atom

• The number of electrons in any orbit is given by 2n² where n is the number of the orbit.

For example, I orbit contains 2x1² =2 electrons

                     II orbit contains 2x2² = 8 electrons

                   III orbit contains 2x3² = 18 electrons  and so on

• The last orbit cannot have more than 8 electrons.

• The last but one orbit cannot have more than 18 electrons.

Positive and negative ions

·         Protons and electrons are equal in number hence if an atom loses an electron it has lost negative charge therefore it becomes positively charged and is referred as positive ion.

·         If an atom gains an electron it becomes negatively charged and is referred to as negative ion.

Valence electrons

The electrons in the outermost orbit of an atom are known as valence electrons.

• The outermost orbit can have a maximum of 8 electrons.

• The valence electrons determine the physical and chemical properties of a material.

• When the number of valence electrons of an atom is less than 4, the material is usually a metal and a conductor. Examples are sodium, magnesium and aluminium, which have 1,2 and 3 valence electrons respectively.

• When the number of valence electrons of an atom is more than 4, the material is usually a non-metal and an insulator. Examples are nitrogen, sulphur and neon, which have 5,6 and 8 valence electrons respectively.

• When the number of valence electrons of an atom is 4 the material has both metal and non-metal properties and is usually a semi-conductor. Examples are carbon, silicon and germanium.

Free electrons

• The valence electrons of different material possess different energies. The greater the energy of a valence electron, the lesser it is bound to the nucleus.

• In certain substances, particularly metals, the valence electrons possess so much energy that they are very loosely attached to the nucleus.

• The loosely attached valence electrons move at random within the material and are called free electrons.

The valence electrons, which are loosely attached to the nucleus, are known as free electrons.

Energy bands

• In case of a single isolated atom an electron in any orbit has definite energy.

• When atoms are brought together as in solids, an atom is influenced by the forces from other atoms. Hence an electron in any orbit can have a range of energies rather than single energy. These range of energy levels are known as Energy bands.

• Within any material there are two distinct energy bands in which electrons may exist viz Valence band and conduction band.

• The range of energies possessed by valence electrons is called valence band.

• The range of energies possessed by free electrons is called conduction band.

• Valence band and conduction band are separated by an energy gap in which no electrons normally exist this gap is called forbidden gap.

Electrons in conduction band are either escaped from their atoms (free electrons) or only weakly held to the nucleus. Thereby by the electrons in conduction band may be easily moved around within the material by applying relatively small amount of energy. (either by increasing the temperature or by focusing light on the material etc. ) This is the reason why the conductivity of the material increases with increase in temperature.

But much larger amount of energy must be applied in order to extract an electron from the valence band because electrons in valence band are usually in the normal orbit around a nucleus. For any given material, the forbidden gap may be large, small or non-existent.

Classification of materials based on Energy band theory

Based on the width of the forbidden gap, materials are broadly classified as conductors, Insulators and semiconductors.

Conductors

• Conductors are those substances, which allow electric current to pass through them.

       Example: Copper, Al, salt solutions, etc.

• In terms of energy bands, conductors are those substances in which there is no forbidden gap. Valence and conduction band overlap as shown in fig (a).

• For this reason, very large number of electrons are available for conduction even at extremely low temperatures. Thus, conduction is possible even by a very weak electric field.

Insulators

• Insulators are those substances, which do not allow electric current to pass through them.

Example: Rubber, glass, wood etc.

• In terms of energy bands, insulators are those substances in which the forbidden gap is very large.

•  Thus valence and conduction band are widely separated as shown in fig (b). Therefore insulators do not conduct electricity even with the application of a large electric field or by heating or at very high temperatures.

Semiconductors

• Semiconductors are those substances whose conductivity lies in between that of a conductor and Insulator.

Example: Silicon, germanium, Cealenium, Gallium, arsenide etc.

• In terms of energy bands, semiconductors are those substances in which the forbidden gap is narrow.

• Thus valence and conduction bands are moderately separated as shown in fig(C).

• In semiconductors, the valence band is partially filled, the conduction band is also partially filled, and the energy gap between conduction band and valence band is narrow.

• Therefore, comparatively smaller electric field is required to push the electrons from valence band to conduction band . At low temperatures the valence band is completely filled and conduction band is completely empty. Therefore, at very low temperature a semi-conductor actually behaves as an insulator.

Conduction in solids

• Conduction in any given material occurs when a voltage of suitable magnitude is applied to it, which causes the charge carriers within the material to move in a desired direction.

•  This may be due to electron motion or hole transfer or both.

Electron motion

Free electrons in the conduction band are moved under the influence of the applied electric field. Since electrons have negative charge they are repelled by the negative terminal of the applied voltage and attracted towards the positive terminal.

Hole transfer

• Hole transfer involves the movement of holes.

• Holes may be thought of positive charged particles and as such they move through an electric field in a direction opposite to that of electrons.

• In a good conductor (metal) as shown in fig (a) the current flow is due to free electrons only.

• In a semiconductor as shown in fig (b). The current flow is due to both holes and electrons moving in opposite directions.

• The unit of electric current is Ampere (A) and since the flow of electric current is constituted by the movement of electrons in conduction band and holes in valence band, electrons and holes are referred as charge carriers.