Download Introduction to Semiconductors

Introduction to Semiconductors
ECE 2204
Periodic Table
Column IV
Column IV Atoms
Have 4 unpaired electrons in the outer shell
Covalently Bonds
Because all the atoms are Si and they all are equidistance from
one another, there is equal sharing of electrons.
Periodic Table
Column V
Column III
Polar Covalently Bonds
As atoms are more electronegative than the Ga atoms. They keep the
electrons longer than the Ga atoms. Thus, As atoms have a slight
negative charge while the Ga atoms have a slight positive charge.
This is why GaAs is a piezoelectric material. When GaAs is squeezed, a
voltage is generated.
Other Semiconductors
• There are semiconductors that are composed of an
equal number of atoms that have two outer shell
electrons and atoms with six outer shell electrons –
called II-VI semiconductors
▫ Examples:
 CdTe – absorber in solar cells fabricated by First Solar
 ZnSe – used to fabricate blue-green lasers
 HgCdTe –detectors to measure your blackbody radiation
• Organic molecules are used because they are less
expensive, though long-term reliability has been a
big issue.
▫ OLEDs are used in large area displays, but it has taken
25 years to refine the technology to the stage where
they can be used without concerns about moisture, UV
exposure, etc.
Other Types of Bonding
• In ionic semiconductors, the electronegative
atoms take the outer shell electrons from the
electropositive atoms.
▫ The ionic semiconductors are used for specialized
applications such as piezo-optical devices.
• Organic semiconductors , DNA, carbon
nanotubes (CNTs), and graphine are considered
to be mixtures of molecules rather than as
mixtures of atoms.
▫ Many of these materials are being investigated as
an alternative to Si.
Crystalline Materials
• There is a unit cell, the smallest pattern of
atoms, that is replicated in all directions to form
the crystal.
▫ Common unit cells





Simple cubic [NaCl (salt) crystals]
Body-centered cubic [iron, tungsten]
Face-centered cubic [aluminum, gold]
Diamond or Zinc Blende [Si, Ge, GaAs, InP]
Hexagonal Closest Packaged (HCP) [SiC]
Diamond or Zinc Blende Unit Cell
a
a
a
a is the lattice constant.
a = 0.545 nm for Si
Bandgap Energy
• The amount of energy required to break a
covalent bond.
Material EG (eV)
Material EG (eV)
C
~6
GaN
3.4
Si
1.1
GaAs
1.4
Ge
0.66
GaSb
0.73
electron-Volt (eV)
• 1eV = 1.6x10-19 J
• It is a strange unit as the e can cancel the
variable (e or q) in an equation.
• For example:
qkT  25.9meV
qkT
 25.9meV
X
X
kT  25.9mV
Amorphous Si (a-Si)
• There is no regular pattern of atoms in an
amorphous material.
▫ The random arrangement influences the bandgap
of the semiconductor. It also changes the
mechanical, electrical, and optical properties of
the material when compared to the properties of
the same material in crystalline form.
 a-Si is used to fabricate inexpensive solar cells as it is
much easier to produce than c-Si (crystalline Si).
 a-Si is also used as transparent conductive contacts
on CMOS and CCD images for digital camera.
Si Boule Fabrication
Seed Down
Seed Pulling
Shoulder
Si Crystal Growth
Czochralski
www.qahill.com/tz/sil
icon/silicon.html
http://www.tf.uni-kiel.de/matwis/amat/elmat_en/kap_6/illustr/i6_1_1.html
Si is mechanically very strong.
This boule probably weighs 12 kg. The thin seed crystal is
all that is holding the boule off
the ground.
The person in the cleanroom
gown (also known as a bunny
suit) is measuring the
temperature of the boule using
an infrared (IR) sensor.
Wafers
• The Si boule ground into a smooth cylinder and
then sliced into wafers using a diamond edged
saw.
• Each wafer is between 300-800 mm in thickness.
▫ A human hair or a piece of white paper is
approximately 100 mm thick.
▫ Diameters of wafers are specified in millimeters.
A 300 mm wafer is about 12” in diameter.
 This is the size wafer that is used by Micron
Technology in their fab in Manassas, VA.