Lecture 1 - Semiconductors

Valence Electrons

are those electrons that occupy the outer shell.

Gold, Silver, Aluminum, Copper

What are examples of conductors

Copper

It has only one electron in its valence band, which can easily escape to the conduction band, leaving behind a positive ion (the core).

Insulators

have tightly bound electrons with few electrons available for conduction.

Eight valence electrons

Best insulators have ___________.

Semiconductors

are between conductors and insulators in their ability to conduct electricity.

Silicon and Germanium

Examples of semiconductors

Silicon

is the most commonly used semiconductor.

4

How many valence electrons does silicon and germanium have?

True

True or False:

In a Silicon the valence electrons are at the third shell and for Germanium it is at the fourth shell, which means that Silicon has a stronger pull, it requires high amount of energy to escape from the atom.

Strong Covalent Bonds (shared electrons)

Unlike metals, silicon forms ______________ with its neighboring atoms making a crystalline structure.

Covalent Bonds, Chemical Stability

This effectively creates eight valence electrons for each atom and produces a state of _________.

Intrinsic silicon

is a poor conductor because most of the electrons are bound in the crystal and take part in forming the bonds between atoms

Semiconductor

is different from an insulator because at any temperature above absolute zero temperature, there is a small number of electrons in the lattice which will become loose from their atoms, leaving behind an empty space called a "hole".

Hole

there is a small number of electrons in the lattice (Crystalline structure) which will become loose from their atoms, leaving behind an empty space called a "____".

When heat energy is applied to an intrinsic silicon which is purely 100% silicon that is chemically stable. The electrons in the valence band moves onto the conduction band which creates a hole on the valence band. Heat energy is kinetic energy which disturbs the electrons on the valence band.

Explain what is going on here

Situation: An intrinsic silicon is receiving heat energy

Electron and hole current

The current which will flow in an intrinsic semiconductor consists of both _____________.

Electron Current

The conduction band electrons are free to move under the influence of an electric field.

Hole Current

The bound (valence) electrons move between atoms, effectively moving “holes” from one atom to another as illustrated. Holes act like positive charges, with their own mobility

decreases, increases

In a conductor when heat is applied, the electric current ________ as the temperature increases.

Which means the resistivity of the conductor also _______.

increases, decreases

In a semiconductor when heat is applied, the electric current ________ as the temperature increases.

Which means the resistivity of the conductor also _______.

In conductors when the temperature increases this leads to vibrating atoms which opposes the flow of electrons.

Explain the temperature effect on resistivity for conductors.

In semiconductors when the temperature increases this leads to vibrating atoms which opposes only a few electrons and remaining large number of electrons flow freely from one place to another because of the hole.

Explain the temperature effect on resistivity for semiconductors.

extrinsic material

A semiconductor material that has been subjected to doping is called an ______.

n-type and p-type

What are the two types of extrinsic material

Extrinsic materials

Since semiconductors are generally poor conductors, their conductivity can be drastically increased by the controlled addition of impurities to the intrinsic (pure) semi-conductive material. This process, called doping.

doping

he controlled addition of impurities to the intrinsic (pure) semi-conductive material. This process, called ______.

N-type material

Created by introducing impurity atoms (doping) that have 5 valence electrons (pentavalent)

N-type

This allows four out of the five orbital electrons bonds with its neighboring silicon atoms leaving one “free electron” to become mobile when an electrical voltage is applied (electron flow).

Majority Carriers, Minority Carriers

N-type material electrons is called “________” while the resulting holes are called “_______”.

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