Introduction to metal

Types of metal:

Insulator:  The metal which is bad conductor of electricity is called insulator. Some examples of insulators include plastic, rubber, glass, porcelain, air, paper, cork, mica, ceramics and certain oils.

Conductor: The metal which is good conductor of electricity is called conductor. All metals are conductors and some examples include copper, aluminum, brass, platinum, silver, gold and carbon.

 Semiconductor: The metal which is neither good conductors nor good insulator is called semiconductor. Some examples of semiconductors include Ge, Si, GaAs, CdS, GaN.

There are two classes of semiconductor materials.

                                      1. Single crystal:----------- Ge , Si.

                                      2. Compound crystal:-----  GaAs , CdS .

                 

The three semiconductors used most frequently are Ge , Si, GaAs .

Type of semiconductor:

 1.  Intrinsic semiconductor

 2. Extrinsic semiconductor

Intrinsic semiconductor :

A semiconductor in on extremely pure form is called intrinsic semiconductor.In an intrinsic 

semiconductor, even at room temperature, hole- electron pairs are created.When electric

field is applied across an semiconductor, the current conduction takes place by two process,

namely: by free electrons and holes.

Extrinsic semiconductor:

Doping:

The intrinsic semiconductor has little current conduction capability at room temperature. To 

be  useful on electronics devices , the pure semiconductor must be altered so as to significantly

increase its conducting properties. This is achieved by adding a small amount of suitable impurity

to a semiconductor . It is then called impurity or extrinsic semiconductor . The process of adding 

impurities to a semiconductor is known as doping.

Depending  upon the type of impurity added , extrinsic semiconductors are classified into :

 i) n-type semiconductor      

 ii) p-type semiconductor

 n-type semiconductor:

When a small amount of pentavalent impurity

is added to a pure semiconductor , it is know

 as n-type  semiconductor.To produce a n-

type semiconductor a small amount of pent-

avalent impurity is added to a pure semico-

nductor . The addition of pentavalent impurity

provides a large number of free electrons  in 

the semiconductor.

p-type semiconductor:


When a small amount of trivalent impurity is added to a pure semiconductor it is know as p-type

 semiconductor.

                                                                            
 





                












To produce a p-type semiconductor a small amount of trivalent impurity is added to a pure 

semiconductor. The addition of trivalent impurity provides a large number of free electrons 

in the semiconductor.

Majority and Minority Carriers:


n-type material :  


When a  tetravalent semiconductor material and a pentavalent material is doped , there is a free

electron which don’t make bond. So , in an n-type material the electron is called the majority 

carrier and the hole is called the minority carrier.

Example: 

p-type material :  


When a tetravalent semiconductor material and a trivalent material is doped , there is a lack of

electron which cannot make bond and make a hole. So , In a p-type material the hole is the 

majority carrier and the electron is the minority carrier.


PN junction:


When a p-type semiconductor is suitably joined to n-type semiconductor ,the contact 

surface is called PN junction. Most semiconductor devices contain  one or more PN 

junction .The PN junction is of great importance because it is in effect, The control 

element for semiconductor devices.


Properties of PN junction:

To explain the properties of a PN junction,

consider two types of materials; one P-

type and other N type as shown in figure. 

In this figure, left side material  is a N-type 

semiconductor having negative acceptor 

ions and positive charged holes. The right 

side material is P-type semiconductor having

positive donor ions and free electrons.

 
 

                  

                     •distribution of the charge carriers before 

         

                      the diffusion   

                    • distribution of the charge carriers after   

                      the diffusion of the charge carriers 

   

                    • junction barrier

                    • charge distribution in the junction barrier