Conductors, Insulators and Semi-conductors, p-n Junction

 

Conductors, Insulators and Semi-conductors, p-n Junction

CLASSIFICATION OF MATERIALS:   

 On the basis of the ability of various materials to conduct current, the materials are classified as conductors, insulators and the semiconductors.

Conductors :

1. A substance which is very good carrier of electricity is called conductor.

2. 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 terminals.

3. The copper and aluminum are good examples of a conductor

Insulators:

1. A very poor conductor of electricity is called as insulator.

2.  An insulator is a material that offers a very low level of conductivity under       an applied voltage .

3.   Glass, Wood, Mica, Diamond are the examples of an insulator.

Semiconductors:

1. A substance  having conductivity between conductor and an insulator is called semiconductor.

2.  Silicon and Germanium are the examples of a semiconductor.

3.  Semiconductor does not conduct current at low temperatures but as      temperature   increases these materials  behave as good conductors.    

Band Theory: 

1. A bonding of atoms, due to the sharing of electrons, is called covalent bonding.

2.  In the crystal, closely spaced energy levels form a band called as the  energy band.   Each orbit has a separate energy band.

3.  A band of energy levels associated with valence electrons and uppermost filled band is called  valence band. Electrons from other bands cannot be removed but electrons from valence band can be removed by supplying a little energy.

4. The empty  band above  valence band lowest unfilled band is conduction band .

5.  The valence band and conduction band are separated by a gap called forbidden energy gap.

Band structure

a) The electrons in an isolated atom occupy discrete energy levels. When atoms are close to each other, these electrons can use the energy levels of their neighbors.

b) When the atoms are all regularly arranged is called the crystal lattice of a solid, the energy levels become grouped together in a band. This is a continuous range of allowed energies rather than a single level. There will also be groups of energies that are not allowed, is known as a band gap E_g .

c)  Similar to the energy levels of an individual atom, the electrons will fill the lower bands first.

d)  The Fermi level gives a rough idea of which levels electrons will generally fill up to that level.  

 Fig A

Conductors:

 1. In the metals there is no forbidden gap between valence band and conduction band. The conduction band and valence  band overlap.  ( Fig A)

2. Hence even at room temperature, a large number of electrons are available for conduction, these are also available in conduction band.
3. So without any additional energy, such metals contain a large number of free electrons and hence called good conductors.

INSULATORS:

1. In case of such insulating material, there exists a large forbidden gap in between the conduction band and the valence band. ( Fig A )

2.Practically it is impossible for an electron to jump from the valence band to the conduction band. Hence such materials cannot conduct and called insulators.

SEMICONDUCTORS:

1.The  substance in which forbidden gap is very narrow is called semiconductors. 

( Fig A )

2. The forbidden gap is of the order 1 eV.

3. In such substances, the energy provided by the heat at room temperature is sufficient to lift the electrons from the valence band to the conduction band.

4. Due to this semiconductor conducts small current.

Types of Semiconductors:
Semiconductor may be classified as under:

Intrinsic and Extrinsic semiconductors

1. The semiconductor are classified as an intrinsic and extrinsic semiconductor.

2. The intrinsic semiconductor is the pure form of the semiconductor.

3. The process of adding impurities to the intrinsic semiconductor is called doping.

4. After doping intrinsic semiconductor behaves as an extrinsic semiconductor and becomes good conductor of electricity due availability of more number of charge carriers.

Intrinsic Semiconductors:
1. Extremely pure form semiconductor is called an intrinsic semiconductor .

2. Examples of such semiconductors are: pure germanium and silicon which have forbidden energy gaps of 0.72 eV and 1.1 eV respectively.

3. The intrinsic semiconductor may be defined as one in which the number of conduction electrons is equal to the number of holes.

4. They have four valence electrons (tetravalent). They are bound to the atom by covalent bond at absolute zero temperature. ( Fig B )

5. When the temperature rises, due to collisions, few electrons are unbounded and become free to move through the lattice, thus creating an absence in its original position (is called hole).

6. The negative electrons  and positive holes are equal in number.

7. At absolute zero temperature the covalent bonds are very strong and there are no free electrons and the semiconductor behaves as a perfect insulator.

8.  Above absolute temperature: With the increase in temperature few valence electrons jump into the conduction band and hence it behaves like a poor conductor.

 Fig B

Extrinsic Semiconductor:

1. The conductivity of semiconductors can be greatly improved by introducing a small number of suitable atoms called impurities. The process of adding impurity atoms to the pure semiconductor is called doping.

2. Extrinsic (impure) semiconductor = intrinsic semiconductor

                                                                 +    impurities

3. An extrinsic semiconductor are  classified into:
           1.  n-type Semiconductor
           2.  p-type Semiconductor

1.  n-type Extrinsic Semiconductor:

 Fig C 

a.  This type of semiconductor is obtained when a penta-valent substance like prosperous (P) as donor  is added to pure germanium crystal. 

b.  As shown in FigC , each antimony atom forms four covalent bonds with the surrounding four germanium atoms with the help of four of its five electrons.

c.  The fifth electron is loosely bound to the antimony atom. This loosely bound electron become free due to  increase in thermal energy.

d.  In n-type semiconductors, electrons are the majority carriers while holes constitute the minority carriers.

e.  n- type semiconductor is electrically neutral.

p-type Extrinsic Semiconductor:

a.  p-type  of semiconductor is obtained when a trivalent element like boron (B) are added to a pure germanium crystal.

 Fig D

b.  In this case, the three valence electrons of boron atom form covalent bonds with four surrounding germanium atoms but one bond is left incomplete and gives rise to a hole as shown in Fig D.  

a.  Thus, boron which is called an acceptor impurity causes as many positive holes in a germanium crystal as there are boron atoms thereby producing a P-type (P for positive) extrinsic semiconductor.

b.  In this type of semiconductor, conduction is by the movement of holes in the valence band.

c.  In p-type semiconductors, holes are the majority carriers while electrons constitute the minority carriers.

d.  p- type semiconductor is electrically neutral.

Fermi Level: E_{F  :}

     The highest energy level that an electron can occupy at the absolute zero temperature is called  Fermi Level. The Fermi level lies between the valence band and conduction band in semiconductors, because at absolute zero temperature the electrons are all in the lowest energy state.

a ) Fermi level in Intrinsic semiconductor:

    In intrinsic semiconductor, the number of holes in valence band is equal to the number of electrons in the conduction band. Hence, the probability of occupation of energy levels in conduction band and valence band are equal. Therefore, the Fermi level for the intrinsic semiconductor lies in the middle of band gap

( Fig E ). 

Fig E

b )Fermi level in n- type semiconductor :

  At room temperature, in n- type semiconductor  the number of electrons in the conduction band is greater than the number of holes in the valence band. Hence, the probability of occupation of energy levels by the electrons in the conduction band is greater than the probability of occupation of energy levels by the holes in the valence band. This probability of occupation of energy levels is represented by Fermi level. Therefore, the Fermi level in the n-type semiconductor lies close to the conduction band and above donor level   (  Fig E ) .

c )Fermi level in p- type semiconductor:

At room temperature, in p- type semiconductor  the number of holes in the valence band is greater than the number of electrons in the conduction band. Hence, the probability of occupation of energy levels by the holes in the valence band is greater than the probability of occupation of energy levels by the electrons in the conduction band. This probability of occupation of energy levels is represented by Fermi level. Therefore, the Fermi level in the p-type semiconductor lies close to the valence band ( Fig E ).

p-n junction semiconductor diode:

      A p-n junction diode is formed when a p-type semiconductor is fused to a n-type semiconductor creating a potential barrier voltage across the diode junction. The word diode can be explained as ‘Di’ means two and ‘ode’ is obtained from electrode. It a semiconductor device, which conduct the current in one direction only. Two terminals: anode and cathode A p-n junction diode is formed either by using any semiconductor such as Si or Ge. The p and n type regions are referred to as anode and cathode respectively.

Construction of a Diode :

A diode is formed by joining two equivalently doped p-type and n-type semiconductor. A diode is basically constructed using a single piece of semiconductor, half of which is doped by p-type impurity and the other half by n-type. The plane dividing the two zones is called p-n junction. The electric symbol is as in Fig F.

 Fig F

Depletion layer or p-n junction :

At the dividing plane, the holes in the p-Type attract electrons from the n-Type material. Hence the electron diffuses and recombines with the holes in the p-Type material. It causes a small region of the n-type near the junction to lose electrons and behave like intrinsic semiconductor. Similarly, in the P-side also a small region behaves like an intrinsic semiconductor. This thin intrinsic region is called depletion layer or p-n junction, as it is depleted of charge.

This region offers high resistance and prevents the further diffusion of majority charge carriers. ( Fig G  ) 

 Fig G 

Biasing :
P-N junction is said to be biased when an external voltage is applied across it.  

There are two types of biasing;

1.  Forward bias: If p-region of the diode is connected with the positive terminal of the battery and n-region is connected with the negative terminal of battery then it is called forward bias.

2. Reverse bias : If p- region of the diode  is connected to negative terminal and n-region is connected to positive terminal of battery then it is called Reverse bias. 

1.  Forward bias:

 Fig H

a.  During the forward bias, the positive of the battery forces more holes into the p-region of the diode. The negative terminal forces electrons into the n-region.  ( Fig H )

b.  The excess of charge in p and n region  the depletion region to contract.

c.  As the voltage increases the depletion layer become thinner and thinner and hence diode offers lesser and lesser resistance. Since the resistance decreases the current increases.

d.  At one particular voltage level V_f  i.e. at threshold/cut-off voltage the depletion layer disappears  and hence from this point on wards the diode starts to conduct very easily. From this point on the diode current increases exponentially to the voltage applied.

2. Reverse bias :

 Fig I

a.  In reverse bias  the holes in p-type gets recombined by electrons from the battery. The electrons in n-type material are pulled out of the diode by the positive terminal of the battery. So the diode gets depleted of charge.   (Fig I )

b.  So initially the depletion layer widens  and it occupies the entire diode. The resistance offered by the diode is very high. The current in reverse bias is only due to minority charge which is in micro-ampere in high power Si and Ge diodes.

c.  The electric current, which is carried by the minority charge carriers in the p-n junction diode, is called reverse current.

d.  The voltage or point at which the electric current reaches its maximum level and further increase in voltage does not increase the electric current is called reverse saturation current.

e.  However, if the voltage applied on the diode is increased continuously, the p-n junction diode reaches to a state where junction breakdown occurs and reverse current increases rapidly.

V-I characteristics of p-n junction diode:
The graph between applied voltage and current in a p-n junction diode is called the V-I characteristics. Fig J.

 Fig J

Applications of a P-N junction Diode:
p-n  junction diode is used in various applications:
1. Rectification
2.  Laser diodes used in optical communications.
3.  Light Emitting Diodes (LEDs), used in digital displays
4.  In voltage multipliers.
5.  As switch in digital logic circuits used in computers.
6.  In voltage regulators.