ULTIMATE guide to semiconductor: definition,types and more -ELECTRIGO

semiconductor-definition, types, meanings, examples, and energy band diagrams

Types of semiconductors

There are two basic groups or classifications which can be used to define the varied semiconductor types:

Intrinsic material: 

An intrinsic kind of semiconductor material made to be very pure chemically. As a result, it possesses a very low conductivity level having only a couple of the number of charge carriers, namely holes and electrons, which it possesses in equal quantities.

 Extrinsic semiconductor

• An intrinsic semiconductor has a limited number of free electrons at room temperature.
• Hence they do not conduct well at this temperature. To increase the conductivity, the number of free electrons in a semiconductor has to be increased. 
• This can be done by adding impurities to the intrinsic material.
•  The process of adding impurities to a semiconductor is known as doping.
•  Even the addition of l impurity atom for every 10 million semiconductor atoms changes the electrical property of the material. 
• When a semiconductor material is doped, it is called an extrinsic material. 
• Depending on the type of impurity added, extrinsic semiconductors are classified as 
• (1) n-type semiconductor 
• (2) p-type semiconductor

n-type semiconductor

    

The crystal lattice of a si atom displaced by an arsenic atom

• The number of free electrons in intrinsic silicon can be increased by adding a pentavalent atom to it. 
• These are atoms with five valence electrons. 
• Typical examples for pentavalent atoms are Arsenic, Phosphorous, Bismuth, and Antimony. 
• Fig shows the bonding between Si atoms with a pentavalent atom.
•  Four of the pentavalent atom's valence electrons form a covalent bond with the valence electrons of the Si atom, leaving an extra electron.
•  Since valence orbit cannot hold no more than eight electrons the extra electron becomes a conduction electron. 
• Since the pentavalent atom donates this extra conduction electron it is often called a donor atom.

       

Energy band diagram of n type semiconductor

• For each pentavalent atom added, one free electron exists in a silicon crystal.
•  A small amount of pentavalent impurity is enough to get more free electrons. 
• Since the number of free electrons is greater than the number of holes this extrinsic semiconductor is known as an n-type semiconductor. 
•  When a pentavalent atom is added a number of conduction band electrons are produced. 
• Only a few holes exist in the valence band, created by thermal energy.
•  Therefore in an n-type semiconductor, electrons are majority carriers and holes are minority carriers.

p-type semiconductor

     
        

Crystal lattice with a si atom displaced by a boron atom

• The number of holes in intrinsic silicon can be increased by adding a trivalent impurity to it. 
• These atoms have three valence electrons. Typical examples for trivalent atoms are boron (B); indium (In) and gallium (Ga). Fig shows the bonding between silicon crystal and trivalent atom.
•  Since each trivalent atom has only three valence electrons with it, only seven electrons exist in its valence orbit. 

        

Energy band diagram of p type semiconductor

• That is one hole results for each trivalent atom added.
•  Because a trivalent atom can take one electron, this often is referred to as an acceptor atom.
•  The number of holes can be controlled by the number of impurity atoms added to the silicon.
• The addition of a trivalent impurity produces a large number of holes in the valence band.
• However, there are very few thermally generated electrons in the conduction band.
•  Since the number of holes is more when compared to the number of electrons, this type of extrinsic semiconductor is known as a p-type semiconductor. 
• In a p-type semiconductor, the holes are majority carriers and electrons are minority carriers.