Classification of semiconductors

Semiconductors are classified into two types.

a)      Intrinsic semiconductors.

b)      Extrinsic semiconductors.

a) Intrinsic semiconductors

·         A semiconductor in an extremely pure form is known as Intrinsic semiconductor.

                     Example: Silicon, germanium.

·         Both silicon and Germanium are tetravalent (having 4 valence electrons).

·         Each atom forms a covalent bond or electron pair bond with the electrons of neighboring atom. The structure is shown below.

At low temperature

• At low temperature, all the valence electrons are tightly bounded the nucleus hence no free electrons are available for conduction.

•  The semiconductor therefore behaves as an Insulator at absolute zero temperature.

At room temperature

• At room temperature, some of the valence electrons gain enough thermal energy to break up the covalent bonds.

• This breaking up of covalent bonds sets the electrons free and are available for conduction.

•  When an electron escapes from a covalent bond and becomes free electrons a vacancy is created in a covalent bond as shown in figure above. Such a vacancy is called Hole. It carries positive charge and moves under the influence of an electric field in the direction of the electric  field applied.

• Numbers of holes are equal to the number of electrons since, a hole is nothing but an absence of electrons.

Extrinsic Semiconductor

·         When an impurity is added to an Intrinsic semiconductor its conductivity changes.

                                

• This process of adding impurity to a semiconductor is called Doping and the impure semiconductor is called extrinsic semiconductor.

• Depending on the type of impurity added, extrinsic semiconductors are further classified as n-type and p-type semiconductor.

n-type semiconductor

• When a small current of Pentavalent impurity is added to a pure semiconductor it is called as n-type semiconductor.

• Addition of Pentavalent impurity provides a large number of free electrons in a semiconductor crystal.

• Typical example for pentavalent impurities are Arsenic, Antimony and Phosphorus etc. Such impurities which produce n-type semiconductors are known as Donor impurities because they donate or provide free electrons to the semiconductor crystal.

• To understand the formation of n-type semiconductor, consider a pure silicon crystal with an impurity say arsenic added to it as shown in figure 1.5.
•  We know that a silicon atom has 4 valence electrons and Arsenic has 5 valence electrons. When Arsenic is added as impurity to silicon, the 4 valence electrons of silicon make co-valent bond  with 4 valence electrons of Arsenic.

• The 5^th Valence electrons finds no place in the covalent bond thus, it becomes free and travels to the conduction band as shown in figure. Therefore, for each arsenic atom added, one free electron will be available in the silicon crystal. Though each arsenic atom provides one free electrons yet an extremely small amount of arsenic impurity provides enough atoms to supply millions of free electrons.

Due to thermal energy, still hole election pairs are generated but the number of free electrons are very large in number when compared to holes. So in an n-type semiconductor  electrons are majority charge carriers and holes are minority charge carriers . Since the current conduction is pre-dominantly by free electrons( -vely charges) it is called as n-type  semiconductor( n- means –ve).

p-type semiconductor

• When a small amount of trivalent impurity is added to a pure semiconductor it is called p-type semiconductor.

•  The addition of trivalent impurity provides large number of holes in the semiconductor crystals.

•  Example: Gallium, Indium or Boron etc. Such impurities which produce p-type semiconductors are known as acceptor impurities because the holes created can accept the electrons in the semi conductor crystal.

To understand the formation of p-type semiconductor, consider a pure silicon crystal with an impurity say gallium added to it as shown  in figure 1.7.

• We know that silicon atom has 4 valence electrons and Gallium has 3 electrons. When Gallium  is added as impurity to silicon, the 3 valence electrons of gallium make 3 covalent bonds with 3 valence electrons of silicon.

• The 4^th valence electrons of silicon cannot make a covalent bond with that of Gallium because of short of one electron as shown above. This absence of electron is called a hole. Therefore for each gallium atom added one hole is created, a small amount of Gallium provides millions of holes.

Due to thermal energy, still hole-electron pairs are generated but the number of holes are very large compared to the number of electrons. Therefore, in a p-type semiconductor holes are majority carriers and electrons are minority carriers. Since the current conduction is predominantly by hole( + charges) it is called as p-type semiconductor( p means +ve)