Introduction to SemiconductorsECE 2204

Periodic TableColumn IV

Column IV AtomsHave 4 unpaired electrons in the outer

shell

Covalently BondsBecause all the atoms are Si and they all are equidistance from one another, there is equal sharing of electrons.

Periodic TableColumn III

Column V

Polar Covalently BondsAs 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 Cella

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:

mVkTmeVqkTmeVqkT

9.259.259.25

X X

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

http://www.tf.uni-kiel.de/matwis/amat/elmat_en/kap_6/illustr/i6_1_1.html

www.qahill.com/tz/silicon/silicon.html

Si is mechanically very strong. This boule probably weighs 1-2 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.