Transcript Chapter 1 - Introduction to Semiconductor Material

INTRODUCTION TO
SEMICONDUCTORS MATERIAL
Chapter 1 (Week 2)
1.3 Covalent Bonding
1.
Covalent bonding occurs when pairs of electrons are shared by non-metal
atoms.
2.
When atoms combine into molecules to form a solid material, they arrange
themselves in a fixed pattern called a crystal – atoms within the crystal
structure are held together by covalent bonds (atoms share valence
electrons) .
3.
Atoms are electrically stable when their valence shells are complete or fully
occupied.
4.
An intrinsic crystal is one that has no impurities.
EKT 102: Basic Electronic Engineering
1.3 Covalent Bonding (cont.)
• The number of covalent bonds is equal to eight minus the group number in
the periodic table.
• Group number = number of electrons in the valence shell.
EKT 102: Basic Electronic Engineering
1.3 Covalent Bonding (cont.)
Covalent bonding – holding atoms together by sharing valence electrons
Semiconductor atoms bond together to form a solid material = crystal
Sharing of
valence
electron
produces the
covalent bond
To form Si crystal
(c) Covalent bonds in a silicon crystal
Figure 12: Illustration of covalent bonds in silicon crystal
EKT 102: Basic Electronic Engineering
1.4 Conduction in Semiconductor
• The electrons of an atom can exist
only within prescribed energy
bands.
• Each shell corresponds to a certain
energy band and is separated from
adjacent shells by band gaps - no
electrons can exist.
EKT 102: Basic Electronic Engineering
Figure 13: Energy band diagram for an
unexcited (no external energy) atom in a
pure (intrinsic) Si crystal.
1.4 Conduction in Semiconductor (cont.)
 When an electron jumps to the conduction band, a vacancy (called a hole) is
left in the valence band within the crystal.
• Recombination occurs when a conduction-band electron loses energy and
falls back into a hole in the valence band.
Figure 14: Creation of
electron-hole pairs in a
silicon crystal
EKT 102: Basic Electronic Engineering
1.4 Conduction in Semiconductor
•
The number of free electrons (also known as conduction electrons) is equals to the
number of holes in the valence band.
Figure 15: Electron-hole pairs in a silicon
crystal. Free electrons are being generated
continuously while some recombine
with holes.
EKT 102: Basic Electronic Engineering
1.4 Conduction in Semiconductor (cont.)
Electron Current
•
When a voltage is applied across a piece of intrinsic silicon, the thermally generated
free electrons in the conduction band, which are free to move, are now easily
attracted toward the positive end.
•
The movement of free electrons in a semiconductive material is called electron
current.
Figure 16: Electron current
in intrinsic silicon is
produced by the movement
of thermally generated free
electrons.
EKT 102: Basic Electronic Engineering
1.4 Conduction in Semiconductor (cont.)
Hole Current

Electron remaining in the valence band are still attached to the atom – not free to move like free
electron.

However, valence electron can move into nearby hole – leaving another hole it comes from.

Thus, hole has moved from one place to another.

The movement of electrons in a valence band is called hole current.
Figure 17: Hole current in
intrinsic silicon.
EKT 102: Basic Electronic Engineering
1.5 N-Type and P-Type Semiconductors
Doping
•
Semiconductive materials do not conduct current well because of the limited
number of free electrons in the conduction band and holes in the valence band.
•
Intrinsic semiconductive materials must be modified by increasing the free electrons
and holes to increase its conductivity and make it useful for electronic devices
– by adding impurities.
•
Doping is the process of adding impurity atoms to intrinsic semiconductors improve
its conductivity.
•
Tw types of doping – N-Type and P-Type.
EKT 102: Basic Electronic Engineering
1.5 N-Type and P-Type Semiconductors (cont.)
N-Type Semiconductor
•
Is formed by adding pentavalent
(5 valence 𝒆) impurity atoms.
•
To increase the number of free
electrons.
•
1 extra electrons becomes a
conduction electrons because it is
not attached to any atom.
•
Pentavalent atom gives up
(donate) an electron –
call a donor atom.
Figure 18: Pentavalent impurity atom in a silicon crystal structure.
An antimony (Sb) impurity atom is shown in the center. The extra
electron from the Sb atom becomes a free electron.
EKT 102: Basic Electronic Engineering
1.5 N-Type and P-Type Semiconductors (cont.)
N-Type Semiconductor
•
No. of conduction electrons can be controlled by the no. of impurity atoms.
•
Since most of the current carriers are electrons, semiconductor doped with
pentavalent atoms is an n-type semiconductor.
•
The electrons are called the majority carriers, while the holes is minority carriers.
EKT 102: Basic Electronic Engineering
1.5 N-Type and P-Type Semiconductors (cont.)
P-Type Semiconductor
•
Is formed by adding trivalent (3 valence 𝒆) impurity atoms.
•
To increase the number of hole.
•
A hole is created when each trivalent atom is added.
•
Because the trivalent atom can take an electron, it is often
referred to as an acceptor atom.
•
No. of holes can be controlled by the no. of trivalent impurity
atoms.
•
Since most of the current carriers are holes, semiconductor
doped with trivalent atoms is an p-type semiconductor.
•
The holes are called the majority carriers, while the
conduction electrons is minority carriers.
EKT 102: Basic Electronic Engineering
Figure 19: Trivalent impurity atom in a silicon
crystal structure. A boron (B) impurity atom is
shown in the center.