Semiconductors Flashcards
Atoms
Smallest Particle of an element that retains characteristics of that element
Every element has unique atomic structure
Atom has a planetary type of structure.
- Central nucleus
- Orbiting electrons
Nucleus
Positively charged particles (Protons) Uncharged particles (Neutrons)
Electrons
Basic particles of negative charge
Free Electrons
Valence Electron gains sufficient energy from surrounding medium
Valence electrons are in outermost shell
Can break away from parent atom
Becomes free electron
Free electrons capable of moving from one atom to another in material - drift randomly from atom to atom within copper
- Free electrons make electric current possible
Materials used in Electronics
Conductors
Semiconductors
Insulators
Conductors
Materials in which current can be established with relative ease
Large number of free electrons
One to three valence electrons in atomic structure
Most metals are good conductors
Silver is best but expensive
Copper most widely used
Periodic Table Groups based on Valence Electrons
Group 3, B (Boron), AI (Aluminium)
Group 4, Si (Silicon), Ge (Germanium)
Group 5, P (Phosphorus), Sb (Antimony)
Semiconductors
Classed below conductors in ability to carry current
Fewer free electrons
Four valence electrons
Unique Characteristics
Basis for semiconductor devices
- Diode, Transistor, Integrated Circuit (IC)
Examples, Silicon & Germanium
Insulators
Poor conductors of electric current
Used to prevent current where it is not wanted
Very few free electrons
More than four valence electrons in atomic structure
Covalent Bonds
When atoms combine to form a solid crystalline material they arrange themselves in a symmetrical pattern
Atoms within crystal structure held together by covalent bonds
Covalent bonds created by interaction of valence electrons in atoms
Silicon is a crystalline material
Important terms - Valence Shell, Nucleus, Core
Covalent Bonds - Silicon
Silicon Atom with 4 valence electrons
Shares electrons with each of it’s four neighbours
Effectively creates 8 valence electrons for each atom
Produces state of chemical stability
Sharing produces covalent bonds that hold atoms together
Each shared atom attracted equally by two adjacent atoms which share it
Intrinsic crystal; crystal with no impurities
Conduction Electrons and Holes
Intrinsic silicon crystal at room temperature
Sufficent heat (thermal energy) for some valence electrons to jump into conduction band
Become free electrons (conduction electrons)
Vacancy in valence band called a hole
For every conduction electron there is a hole left in valence band
Called electron-hole pair
Recombination
Occurs when conduction-band electron loses energy
Electron falls back into hole in valence band
At room temp, at any instant, intrinsic Si has a number of ‘free’ electrons (conduction band) drifting randomly through the material.
Electron Current
Apply voltage across piece of intrinsic silicon
Free electrons in conduction band easily attracted towards +
Movement of free electrons called electron current
Hole Current
At valence level holes created by free electrons
Valence electron can move into nearby hole with little change in energy level leaving another hole where it came from
Called hole current
N-type and P-type Semiconductors
Intrinsic semiconductors don’t conduct well
Modified to increase conductivity
Increase number of free electrons
Addition of impurities
Two types of extrinsic semiconductive materials
P-type
N-type
Key building blocks of most electronic devices
Doping
Controlled addition of impurities to intrinsic semiconductive materials produces an extrinsic semiconductor.
Identify intrinsic materials
Doping increases number of current carriers (increases conductivity)
Electrons
Holes
Two types of impurities
P-type
N-type
N-Type Semiconductor
Pentavalent impurity atoms added to intrinsic silicon
Increases number of conduction band electrons
Atoms have 5 valence electrons Arsenic (As) Phosphorus (P) Bismuth (Bi) Antimony (Sb)
Find pentavalent elements on periodic table
N-Type: Doping Silicon
Pentavalent atom forms covalent bonds with 4 adjacent Si atoms
4 of Sb atom’s valence electrons used for covalent bonds
Extra electron becomes conductive electron
Pentavelent atom gives up electron- called a donor atom
Number of conductive electrons controlled by number of added impurity atoms
Conductive electrons created by doping does not leave a hole because its is excess of the no. reuired to fill the valence band.
P-Type Semiconductor
Increase number of holes in intrinsic Silicon by adding trivalent impurity atoms
Aluminium (Al), Boron (B), Gallium (Ga)
P-Type: Doping Silicon
Each trivalent atom forms covalent bonds with 4 adjacent Si atoms
All three of the Boron atom’s valence electrons used for covalent bonds
Four electrons required
Hole formed with each trivalent atom
Number of holes controlled by amount of added trivalent impurity
Majority and Minority Carriers
Silicon or germanium doped with pentavalent atoms
Most current carriers are electrons
N-type semiconductor
N stands for Negative charge on electron
Electrons called majority carriers
Some holes created when electron-hole pairs are thermally generated
Holes in n-type material called minority carriers
