Lecture 4 - Elctrical And Optical Properties

Question 1

At RT (300k) what is one of the least resistive materials

Answer 1

Copper

Question 2

How can resistivity be engineered in some materials

Answer 2

By adding small amounts of dopants

Question 3

When does resistivity increase

Answer 3

If the number of mobile charge carriers in a metal reduces

Question 4

What are dielectrics

Answer 4

An insulator

Question 5

In insulators (dielectrics) electrons can’t move at all, why is this

Answer 5

Bc there resistivity is so large

Question 6

When do material conduct

Answer 6

All materials conduct when a high enough voltage is applied

Question 7

What does a Fermi energy level describe

Answer 7

How the bands are filled with electrons

Question 8

What is a Fermi energy level

Answer 8

Amount of energy required to add another electron to a solid

Question 9

What is the probability in a Fermi energy level to already be occupied

Answer 9

1/2

Question 10

What do solid particles all have

Answer 10

Thermal energy

Question 11

How to tell if something is an insulator or semi conductor

Answer 11

Size of band gap-
Smaller band gap is a semi conductor
Larger band gap is an insulator

Question 12

At absolute zero T=0 what does the semiconductor become

Answer 12

An insulator

Question 13

What happens to a semi conductor when temperature increases

Answer 13

Semiconductor becomes more conducting

Question 14

In a semiconductors when T>0 conduction band and valance band are equally filled, what does this mean in terms of electrons and holes

Answer 14

When temperature is greater than 0 the bands are equally filled therefore the elctrons and hole can both carry charge

Question 15

What do you do to a semiconductor to make the charge carriers increase

Answer 15

Add dopants that make the charge carriers increase

Question 16

What is it called when the absorption coefficient depends on the wavelength of light

Answer 16

Absorption spectrum - unique for every material

Question 17

What happens when a photon when the energy is greater than a band gap energy

Answer 17

Photons are strongly absorbed

Question 18

What does a photon energy promote

Answer 18

Promotes an electron from a valance band to the conduction band and leaves a hole behind in the valance band

Question 19

What does the electrons in the conduction band have a lot of

Answer 19

Available energy levels

Question 20

What do metals strongly absorb

Answer 20

Absorb light over a wide range of spectrum

Question 21

What do electrons strongly reflect

Answer 21

Electrons strongly reflect light over a wide range of spectrum

Question 22

In terms of light what does it mean when electrons have large absorption coefficients

Answer 22

Light interacts with the metal very close to the surface and the surface quality of the metal strongly influences the optical appearance

Question 23

Property of pure insulators (opaque?)

Answer 23

Completely transparent in the visible spectrum

Question 24

What does impurities do to pure insulators

Answer 24

Impurities can change their optical properties and can make them coloured in the visible