2. SEMI CONDUCTORS
DONE BY :
DILEEP KUMAR .D
HAREESH KUMAR .M
RAVI TEJA .G
UPENDRA .M
VENKATA SAI KUMAR
.D
3. Energy Band
■ In any material, there are 2 energy band:
1. Valence band : the outermost shell that determines the conductivity
2. Conduction band : the band outside the valence shell.
The 2 bands are separated by one energy gap called – forbidden gap.
■ The valence band contains with electrons.
■ The electrons can move to the conduction band if it have enough
energy ( eg: light or heat).
■ When the electron absorbs enough energy to jump from valence
band to the conduction band, the electron is said to be in excited
state.
4. ■ The concept of energy bands is particularly important in classifying materials
as conductors, semiconductors, and insulators.
• Semiconductor : has a smaller forbidden band and requires less energy to
move an electron from the valence band to the conduction band.
•Therefore, for a certain amount of applied voltage, more current will flow in the
semiconductor than in the insulator.
5. ■ semiconducting elements:
– low electrical conductivity at room temperature
– Electrical conductivity increases with temp.
■ Gap between valence and conduction band is intermediate in size.
■Semiconducting elements form the basis of solid state electronic
devices.
■Metalloids (such as silicon or germanium) are semiconducting
elements whose electrical conductivity increases as temperature
increases.
■A striking property of these elements is that their conductivities
increase markedly when they are doped with small quantities of other
elements.
6. ■ Made from materials that have four valence electrons in
their outer orbitals.
■ Germanium and silicon are the most common.
■ Silicon is preferred due to its ability to withstand heat.
■ A pure semiconductor material such as silicon or
germanium has no special properties and will make a
poor conductive material.
7. ■When silicon is doped with phosphorus, it becomes an n-type
semiconductor, in which electrical current is carried by negatively
charged electrons.
■When silicon is doped with boron, it becomes a p-type
semiconductor, in which an electrical current is carried by positively
charged holes.
■Joining a p-type semiconductor to an n-type semiconductor produces
a p-n junction, which can function as a rectifier.
■A rectifier is a device that allows current to flow in one direction, but
not the other.
8. :Types of
Semiconductor
■ Semiconductors are mainly classified into two
categories:
i. Intrinsic
ii. Extrinsic
i. Intrinsic : chemically very pure and possesses
poor conductivity.
-It has equal numbers of negative
carriers (electrons) and positive
carriers (holes).
- Impurities do not affect its electrical
behavior.
9. Intrinsic Semiconductor
Silicon has 4 outer shell
valence electrons
Forms into a lattice
structure to share electrons
The pure semiconductor material without impurities atoms.
example: Silicon and Germanium
10. Extrinsic
semicondu
ctor :
■ improved intrinsic semiconductor with a small
amount of impurities added by a process, known
as doping process, which alters the electrical
properties of the semiconductor and improves its
conductivity.
■ Introducing impurities into the semiconductor
materials (doping process) can control their
conductivity.
11. ■Adding impurities atom into intrinsic
semiconductor = extrinsic semiconductor.
■The process of adding specific types of
atoms to a semiconductor to favorably alter
electric characteristics – Doping
■2 types of extrinsic (impure)
semiconductor;
❖ N-type
❖ P-type
12. ■When an impurity increases the number of
free electrons, the doped semiconductor is
negative or n-type.
■An impurity that reduces the number of free
electrons, causing more holes, creates a
positive or p-type semiconductor.
13. Doping
■Doping : Adding impurities to the silicon
crystal lattice to increase the number of
carriers.
■Add a small number of atoms to increase
either the number of electrons or holes.
14. Donors n-Type Material
Donors
-Add atoms with 5 valence-band
electrons
-ex. Phosphorous (P)
-“Donates” an extra e- that can
freely travel around
-Leaves behind a positively
charged nucleus (cannot move)
-Overall, the crystal is still
electrically neutral
-Called “n-type” material
negative carriers)
(added
+
15. N– type material
Antimony (Sb) impurity in n-type material
- Diffused impurities with
5 valence electrons are
called donor atoms.
16. Acceptors Make p-Type Material
–
–
h+
Acceptors
• Add atoms with only 3 valence-
band electrons
• ex. Boron (B)
• “Accepts” e– and provides extra
h+
to freely travel around
• Leaves behind a negatively
charged nucleus (cannot move)
• Overall, the crystal is still
electrically neutral
• Called “p-type” silicon (added
positive carriers)
17. P-type material
Boron (B) impurity in p-type material
-
The diffused impurities
with 3 valence electrons
are called acceptor
atoms
18. PN Junction Formation
■
■
■
■ A PN junction is fabricated from a single slice of
semiconductor.
One side doped with acceptor impurity atoms – p region
One side doped with donor impurity atoms – n region
The interface separating the n and p regions is referred
as the metallurgical junction.
The PN junction
20. Semiconductor Properties
For T > 0K
Electron shaken free and can
cause current to flow
e–
h+
-Generation – Creation of an electron (e-)
and hole (h+) pair.
-h+ is simply a missing electron, which
leaves an excess positive charge (due to
an extra proton).
-Recombination – if an e- and an h+ come
in contact, they annihilate each other
-Electrons and holes are called “carriers”.
because they are charged particles – when
they move, they carry current.
-Therefore, semiconductors can conduct
electricity for T > 0K … but not much
current (at room temperature (300K), pure
silicon has only 1 free electron per 3 trillion
atoms).
