2. 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.
3. 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.
4. 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.
5. 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.
6. 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.
7. Types of Semiconductor:
Semiconductors are mainly classified into twoSemiconductors are mainly classified into two
categories:categories:
ii.. IntrinsicIntrinsic
ii. Extrinsicii. Extrinsic
i.i. IntrinsicIntrinsic : chemically very pure and possesses: chemically very pure and possesses
poor conductivity.poor conductivity.
- It has equal numbers of negativeIt has equal numbers of negative
carriers (electrons) and positivecarriers (electrons) and positive
carriers (holes).carriers (holes).
- Impurities do not affect its electricalImpurities do not affect its electrical
behavior.behavior.
8. 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
9. Extrinsic semiconductor :
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.
10. 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
11. 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.
12. 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.
13. 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 (added
negative carriers)
+
14. N– type materialN– type material
Antimony (Sb) impurity in n-type material
- Diffused impurities with
5 valence electrons are
called donor atoms.
15. 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)
16. P-type materialP-type material
Boron (B) impurity in p-type material
-The diffused impurities
with 3 valence electrons
are called acceptor
atoms.
17. 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
19. 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).
