Lecture 4 - Elctrical And Optical Properties Flashcards
At RT (300k) what is one of the least resistive materials
Copper
How can resistivity be engineered in some materials
By adding small amounts of dopants
When does resistivity increase
If the number of mobile charge carriers in a metal reduces
What are dielectrics
An insulator
In insulators (dielectrics) electrons can’t move at all, why is this
Bc there resistivity is so large
When do material conduct
All materials conduct when a high enough voltage is applied
What does a Fermi energy level describe
How the bands are filled with electrons
What is a Fermi energy level
Amount of energy required to add another electron to a solid
What is the probability in a Fermi energy level to already be occupied
1/2
What do solid particles all have
Thermal energy
How to tell if something is an insulator or semi conductor
Size of band gap-
Smaller band gap is a semi conductor
Larger band gap is an insulator
At absolute zero T=0 what does the semiconductor become
An insulator
What happens to a semi conductor when temperature increases
Semiconductor becomes more conducting
In a semiconductors when T>0 conduction band and valance band are equally filled, what does this mean in terms of electrons and holes
When temperature is greater than 0 the bands are equally filled therefore the elctrons and hole can both carry charge
What do you do to a semiconductor to make the charge carriers increase
Add dopants that make the charge carriers increase
What is it called when the absorption coefficient depends on the wavelength of light
Absorption spectrum - unique for every material
What happens when a photon when the energy is greater than a band gap energy
Photons are strongly absorbed
What does a photon energy promote
Promotes an electron from a valance band to the conduction band and leaves a hole behind in the valance band
What does the electrons in the conduction band have a lot of
Available energy levels
What do metals strongly absorb
Absorb light over a wide range of spectrum
What do electrons strongly reflect
Electrons strongly reflect light over a wide range of spectrum
In terms of light what does it mean when electrons have large absorption coefficients
Light interacts with the metal very close to the surface and the surface quality of the metal strongly influences the optical appearance
Property of pure insulators (opaque?)
Completely transparent in the visible spectrum
What does impurities do to pure insulators
Impurities can change their optical properties and can make them coloured in the visible
