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# Microelectronics Tutorial: Semiconductor Materials and Properties

Electronic devices such as transistors and diodes are fabricated using semiconductor materials.  Here, we briefly look at the properties and characteristics of semiconductors.  We’ll introduce some terminology associated with semiconductors.

Below is a short video on the crystal structure of silicon, carbon, and germanium

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The crystallization of silicon, carbon (in the modification diamond) and germanium is treated at this video. The procedure of hybridization of orbitals leads to the formation of face-centered cubic crystal structures.  Computer animations using the ball-and-stick model as well as the calotte model (also called space filling model) demonstrate the construction of the crystal lattice.

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Below is a short video on the PN junction of a semiconductor video.

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PN junction of a semiconductor diode

The working principle of a diode is treated at the 3D animated part of the video. The voltage-current characteristic of a silicon diode is recorded in forward and reverse direction at the second part. See how the depletion layer is created between a …

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Hopefully, the above video provided a basic understanding of a few semiconductor material properties.  As you heard there are two types of charged carriers (electrons and holes) that exist in a semiconductor.  In addition, there are two mechanisms that generate currents in a semiconductor.

The most common semiconductor found in integrated circuits is silicon.  There are other semiconductor material to provide high speed and optical detection or generation such as gallium arsenide.

## Intrinsic Semiconductors

In atoms, there are positively charged protons and neutral neutrons found in the nucleus.  You’ll also find negatively charged electrons orbiting the nucleus.  Electrons in the outermost shell or orbit are valence electrons.  In fact, the periodic table is organized according to the number of  valence electrons.

Below is a humorous video on the periodic table.

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Elements by Tom Lehrer

***********************Its a great honor to have our little cartoon featured on Wired Magazine. (www.wired.com) As the number 5 top video about science. We would be pleased if you saw the other 9 videos featured. www.wired.com **************** A Usel…

The Periodic Table of Irrational Nonsense by Crispian Jago

Author:dullhunk

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Germanium and silicon are usually known as elemental semiconductors while gallium arsenide is known as a compound semiconductor.

A silicon atom  has  four valence electrons.  When they silicon atoms are close to each other, they form a  crystal tetrahedral structure and lattice configuration. Each silicon atom has four nearest neighbors.   The shared valence electrons between atoms form covalent bonds.  These valence electrons in silicon atoms are available on the outer edge of the silicon crystal so larger single crystal structures are possible.

Image via Wikipedia

At low temperatures however, the valence silicon electrons will not move for a small voltage since it is not enough energy to overcome the covalent bonds.  In this case at low temperatures,  the silicon material acts an insulator.

On the other hand, at higher temperatures, electrons can break hold from the covalent bond to move away from its original position and gain some minimum energy, called the bandgap energy, $E_g$.   Exceeding the bandgap energy  means the valence electrons are now in the conduction band and are called free electrons.  An electric field will attract these free electrons to form a current moving through the conduction band in the silicon crystal.

For semiconductors , the bandgap energy is about 1 electron volt while for insulators it ranges from 3-6 electron volt.

When an electron breaks its covalent bond and moves away from its original position, an empty state or “hole” results.   More covalent bonds will be broken with increasing temperature resulting in an increase of  positive “holes”.   Also, the magnitude of the positive charge is the same as an electron charge.

The concentration of holes and electrons influence the magnitude  of the current.  When the electron and hole concentrations are equal (given as $n_i$, then the crystal structure is intrinsic.   The intrinsic concentration is given as

$n_i=BT^{frac{3}{2}}e^{frac{-E_g}{2kT}}$

where B is a constant related to a particular semiconductor,

$E_g$ is the bandgap energy in eV,

T is the Kelvin temperature (K),

k is Boltzmann’s constant in eV/K,

and e is the exponential function.

The parameters B and $E_g$ deal with semiconductor materials and are not strong functions of temperature in the case of silicon (Si), gallium arsenide (GeAs) and germanium (Ge).

The intrinsic carrier concentration $n_i$ frequently appears in current-voltage equations.

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