Semiconductor and it's types.

 In this content, you can learn about introduction of semiconductor and it's types in basic electronics.

SEMICONDUCTOR

       Semiconductor is a type of solid material whose electrical properties lies between conductor and insulator.It is nor a conductor neither a insulator.

   At a room temperature it becomes conductor and at absolute zero temperature (0 k), it becomes an insulator. The most commonly used semiconductor materials are silicon (Si), germanium (Ge), gallium arsenide (GaAs), indium arsenide (In As), cadmium sulphide (CDs) etc.

                         The most widely used semiconductor materials are silicon and germanium.The atomic number of silicon is 14 and it's electronics configuration is 1s^2,2s^2,2p^6,3s^2,3p^2. The atomic structure of silicon is shown in figure:-

 

There are four electrons in the outer most orbit or valance shell which is known as valance electrons.

Classification of Semiconductor

 Semiconductor materials are classified as:-

      1. Intrinsic or Pure Semiconductor

     2. Extrinsic or Impure Semiconductor

1. Intrinsic or Pure Semiconductor:-

 A semiconductor is an extremely pure form is known as intrinsic semiconductor.It's electrical conductivity is poor than extrinsic (impure) Semiconductor.

The intrinsic semiconductor, at room temperature behave as a conductor.The intrinsic semiconductor, at absolute zero (0k) temperature behave as an insulator.

(A). Intrinsic semiconductor at 0k temperature:- 

              At 0k temperature each of the four valence electrons forming bond with the four neighbouring atoms. As shown in following figure.A covalent bond consists of two electrons and these covalent bond are in the valence band (lower energy level) of energy band.

The conduction band is fully empty i.e no free electrons in the conduction band.Thus, the intrinsic semiconductor, at absolute zero temperature (0k) behave as an insulator. At (0k) temperature the energy gap is high i.e for (Si) is 1.21 ev and for Ge is 0.78 ev. 

(B). Intrinsic semiconductor at room temperature (300k):-

              At room temperature (300k), some covalent bonds break and the electrons become free. The electrons are called free electrons and they move from valence band to conduction band. A vacant space is from when electron is move from valence band and that vacant space is called hole. Hence, when a covalent bond break than a electron-hole pair are produced.

                     In intrinsic semiconductor, the concentration of free electrons will always be equal to the concentration of holes. Hence, at room temperature intrinsic semiconductor has some conductivity. Since, an electron is a negative charged particle, the vacancy (or hole) created by this electron will be assumed as positive charged.These electrons and holes are also called free charge carrier because they are free to move through out crystal.

  At room temperature the valence band is partially empty and conduction band is partially filled.The free holes are existing in valence band and free electrons are existing in conduction band. At room temperature the value of energy gap is decreased i.e it is for Si is 1.1 ev and for Ge is 0.72 ev.

2. Extrinsic or Impure Semiconductor:-

        An extrinsic semiconductor is impure form of semiconductor. An extrinsic semiconductor is obtained by adding some amount of impurity atoms to a intrinsic or pure semiconductor.Thus process of adding impurity to a pure semiconductor is called doping.

Hence, the conductivity of extrinsic semiconductor is increased by the doping process.The impurities atoms are called dopants. The impurity (dopants) can be of two types.
  

 (a) Donor impurity or Pentavalent impurity.

 (b) Acceptor impurity or Trivalent impurity.

(a) Donor impurity and Pentavalent impurity:-

  The donor or Pentavalent impurity atom containing five valence electrons and the doping is called as donor doping.The donor impurities added with pure semiconductor materials to obtained N-type semiconductor.

 The example of Pentavalent or donor impurities are Arsenic, Antimony and Phosphorus.The Pentavalent impurity atom had positive charge.

(b) Acceptor impurity or Trivalent impurity:-

             The Acceptor or Trivalent impurity atom only consists of three valance electrons. The Trivalent impurity atoms added to pure semiconductor to obtain the P-type semiconductor. 

The example of trivalent impurity or Acceptor impurities are Boron, Gallium, and Indium.The trivalent impurity atom has negative charge.

Type of Extrinsic Semiconductor 

There are two types of extrinsic semiconductor:-

    (1) N-type semiconductor

    (2) P-type semiconductor

(1) N-type Semiconductor:-

               If Pentavalent impurity atoms are added to intrinsic semiconductor, N-type semiconductor is obtained.The Pentavalent impurity atom have five electrons in the outer most orbit. Four, out of these five electrons form covalent bond with four neighbouring silicon atoms. Remaining one electron is loosely bound to the parent atom, which is called free electron. 

A very little amount of energy is required to de-attach this electron from the nucleus of its parent atom. In this way, each Pentavalent atom donates one electrons, because of this concentration of electrons is becomes higher than concentration of holes.

                 Hence, in N-type semiconductor, electrons are majority carriers and holes are minority carriers. The Pentavalent impurity atom donates one electron and after donating this electron, the impurity atom becomes a positively charged ion. This is why it is called donor impurities.

      The N-type semiconductor substrate is denoted as shown in figure.

(2) P-type Semiconductor:-

    If trivalent impurity atoms are added to intrinsic semiconductor, P-type semiconductor obtained. The Trivalent impurity atom have three electrons in the outer most orbit. All three electrons form covalent bond with three neighbouring silicon atoms. The Trivalent impurity atom have one vacant space which is called holes, so each trivalent atom have one hole.

 In this way, each trivalent impurity atom donates one hole, because of this concentration of holes is becomes higher than concentration of electrons. Hence, in P-type semiconductor holes are majority carriers and electrons are minority carriers.

 The trivalent impurity atom have one hole (vacancy) so it can accept one electron. There after receiving this hole the impurity atom becomes a negatively charged ion. This is why it is called Acceptor impurities.

       The P-type semiconductor substrate is denoted by as shown in figure.

Conclusion:-

You have learnt about introduction of semiconductor and it's types in basic electronics.