1. Introduction to Semiconductors
ECE 2204

2. Periodic Table
Column IV

3. Column IV Atoms
Have 4 unpaired electrons in the outer shell

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

5. Periodic Table
Column V
Column III

6. 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.

7. 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.

8. 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.

9. 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]

10. Diamond or Zinc Blende Unit Cell
a is the lattice constant.
a = nm for Si

11. Bandgap Energy The amount of energy required to break a covalent bond.
Material
EG (eV) C ~6
GaN
3.4 Si
1.1
GaAs
1.4 Ge
0.66
GaSb
0.73

12. 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: X X

14. 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.

15. Si Boule Fabrication

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

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

18. Wafers
The Si boule ground into a smooth cylinder and then sliced into wafers using a diamond edged saw.
Each wafer is between 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.