PN Junction Theory

PN JUNCTION THEORY

Energy bands in solids: The range of energies possessed by an electron in a solid is known as energ band.

The important energy bands in solids are 1. Conduction band

2. Forbidden band

3. Valence band

1. Conduction band:

In figure the uppermost band is the conduction band. All electrons in the conduction band are the free electrons and can be easily removed by the application of external voltage. If the substance has empty conduction band, it means current conduction is not possible in that substance. Generally insulators have empty conduction band.

2. Forbidden band:

Below the conduction band is series of energy levels that collectively from the forbidden band. Electrons are never found in this band. Electrons may jump back and forth from the bottom valence band to the top conduction band but they never come to rest in the forbidden band.

3. Valence band:

The range of energies ( i.e. band ) possessed by valence electron in the valance band. The valence electrons are more or less bound to the individual atoms. Electrons can be moved from the valence band to the conduction band by the application of external energy.

Classification of solids based on energy band theory:

The extent of forbidden band ( i.e. separation between conduction band and valence bands) will determine whether a substance is an insulator, a conductor or a semiconductor.

1.Insulator

In an insulator the energy has between valence band and conduction band is very large =15eV. Therefore a very high electric field required to push the valence electrons to a conduction band. For this reason the electrical conductivity of insulator is extremely small and may be regarded a nil under ordinary conditions.

The resistance of the insulator decreases with the increase in the temperature (i.e.) an insulator has negative temperature coefficient of resistance.Insulator is a material that offers very low level of conductivity under pressure from an applied voltage source.

Examples of insulators are glass, wood, plastic.

2. Conductor:

In a conductor the valence band and conduction band overlap. Due to this overlapping a slight potential difference across a conductor causes the free electrons to constitute electric current.

The term conductor is applied to any material that will support a generous flow of charge when a voltage source of limited magnitude is applied across its terminal.

Examples of Conductors are copper, aluminium.

3. Semiconductor:

In a semiconductor the energy gap between valence band and conduction band is very small =1eV . Therefore comparatively smaller electric field ( smaller than insulators but greater than conductors) is required to push the electrons from the valence band to the conduction band.

At low temperature the valence band of a semiconductor is completely full and conduction band is completely empty. Therefore a semiconductor virtually behaves as an insulator at low temperature. As temperature is increased valence electrons cross over to the conduction band and the conductivity increases. The electrical conductivity of a semiconductor increase with the rise of temperature (i.e.) a semiconductor has negative temperature coefficient of resistance.

A semiconductor is a material that has a conductivity level between the conductor and an insulator. Germanium and silicon are the examples of semiconductor.

The semiconductor materials are neither conductors nor insulators. In semiconductor materials the atoms are arranged in an orderly pattern called as crystal. The atoms in the crystal structure are held by covalent bond. The union of atoms sharing the valence electrons is called as covalent bond. That means a valence electron being shared by two adjacent atoms.

Intrinsic semiconductor:

A pure semiconductor is called as intrinsic semiconductor. For example a silicon crystal is an intrinsic semiconductor because every atom in the crystal is a silicon atom. The residual heat at room temperature is sufficient to make a valence electron of an intrinsic semiconductor to move away from the covalent bond. Hence the covalent bond is broken. The broken electron becomes a free electron to move in the crystal. This is shown in figure. When the electron breaks a covalent bond and moves away a vacancy is created in the covalent bond. This vacancy is called as Hole. A hole has a positive charge. When a free electron generated a hole is created. If the temperature of the semiconductor increases the number of holes electron pair increases and the current through the specimen rises. This form of intrinsic semiconductor has very little conduction. Hence it has no practical usage.

Extrinsic semiconductor:

The intrinsic semiconductor has little conductivity can be increased by the addition of a small amount of suitable impurity. The process of adding impurities to the semiconductor is called as dopping. A doped semiconductor is called as an extrinsic semiconductor. Extrinsic semiconductor is very useful to fabricate any kind of electronic devices.

Depending upon the type of impurity added extrinsic semiconductors are classified in to

1. N-type semiconductor

2. P-type semiconductor

N-type semiconductor:

When a small amount of pentavalent impurity is added to a pure semiconductor it is known as N-type semiconductor. Typical examples of pentavalent impurities are Arsenic and Antimony. Such impurities which produce N-type semiconductors are known as donor impurities. Because they donate or provide free electrons to the semiconductor crystal.

Figure shows the formation of N-type semiconductor. Four valence electrons of the Arsenic atom form covalent bonds with four germanium atoms. The fifth valence electron of arsenic atom finds no place in the covalent bonds and is thus free. Therefore for each arsenic atom added one free electron will be available in the germanium crystal. Though each arsenic atom provides one free electron get an extremely small amount of impurity provides enough to supply millions of free electrons. Since the material has large number of free electrons it is called N-type semiconductor. N stands for negative.

In N-type the free electrons are called as majority carriers and some times few holes are also created. This holes are minority carriers.

P-type semiconductor:

When a small amount of trivalent impurity is added to a pure semiconductor it is called P-type semiconductor. Typical examples of trivalent impurities are aluminium, boron and indium. Such impurities which produce P-type semiconductor are known as acceptor impurities because the holes created can accept the electrons.

Figure shows the formation of P-type semiconductor. Three valance electrons of the aluminium atoms form covalent bonds with three germanium atoms. In the fourth covalent bond only germanium atoms contributes one valence electronwhile aluminium has no valence electron to contribute as all its three valence electrons are already engaged in the covalent bonds with neighbouring germanium atoms. (i.e.) fourth bond is incomplete being short of one electron. This missing electron is called hole. Therefore each aluminum atom added one hole is created. A small amount of aluminium provides millions of holes. Since the material now has a large number of holes it is called P-type semiconductor. P stands for positive.

In P-type the holes are the majority carriers and the free electrons are the nminority carriers.

PN junction:

Pure P-type or N-type materials taken separately are of very limited use. If we join a piece of P-type material to a piece of N-type material such that the crystal structure remains continues at the boundary a PN junction is formed.

When a P type semiconductor is successfully joined to N-type semiconductor the contact surface is called PN junction. PN junction can not be made by simply pushing the two pieces together which would not form a single crystal structure. Special fabrication techniques are needed to form a PN junction.

Now suppose the two pieces are suitably treated to form PN junction there is a tendency for the free electrons from the N-type to diffuse over to the P side and the holes from the P side to the N side. Since both the materials are originally electrically neutral a positive charge is built up on tha N side of the junction and negative charge on the P side of the junction. This situation soon prevents further diffusion. It is because now positive charge on the N side repels holes to cross from P type to N type and negative charge on the P side repels free electrons to enter from N type to P type.

This is a layer of positively charged ions on the N side and on P side of the junction there was a layer of negatively charged ions. An electric field is created across the junction between the oppositely charged ions. This is called as junction field or barrier.

Once the junction field established no carries can move through the junction. Hence the junction field is called as depletion region. It creates potential or voltage across the junction. This potential is known as barrier potential. The barrier potential for a silicon PN junction is 0.7V and for a germanium PN junction it is 0.3V. The barrier voltage is more for silicon because its lower atomic number allows more stability in the covalent bonds. The junction potential decreases at higher temperature.