Electrical and optical properties

Question 1

What is Ohm’s law?

Answer 1

As the variable resistance is increased the current in the circuit is reduced and the voltage across the specimen
decreases linearly

Question 2

What is voltage drop measured in?

Answer 2

C (coulomb)

Question 3

What is one of the most least resistive materials at room temperature (300K)?

Answer 3

copper p= 1x10⁻⁸ Ohm.m

Question 4

How can resistivity of some materials be engineered?

Answer 4

by adding small amounts of dopants

Question 5

How does resistivity increase?

Answer 5

If the number of mobile carriers of charge in a material reduces – in a metal such as Cu there are lots of mobile electrons to carry charge and so the resistivity is low.

Question 6

Is resistivity high in insulators?

Answer 6

yes as electrons cannot move at all

Question 7

What is between the conduction band and the valence band?

Answer 7

the band-gap (Eg)

Question 8

Energy is point up is the conduction band below or above the valence band?

Answer 8

above the valence band

Question 9

What is the fermi energy level?

Answer 9

describes how the bands are filled with electrons and the amount of energy required to add another electron to the solid

Question 10

How can electrons fill over valence bands?

Answer 10

they acquire enough energy to jump the energy gap

Question 11

What happens when you add thermal energy to valence bands?

Answer 11

We now have
partially filled bands where the electrons can move.
In the conduction band the electrons can carry charge and in the valence band the hole they leave behind can carry charge.

Question 12

In an insulator when T=0, what does the valence electrons look like? (3)

Answer 12

• empty conduction band
• band gap (>2eV)
• filled valence band
  All the valence bands are filled after the energy gap

Question 13

What do valence electrons look like in a semi-conductor? (3)

Answer 13

• Empty conduction band
• Band gap (<2eV)
• filled valence band

Some electrons have crossed the electron gap.

Question 14

What do valence electrons look like in a metal? Like Cu (5)

Answer 14

• empty band
• band gap
• empty states
• Ef (fermi energy)
• Filled states

partially filled band (conduction band) so the electrons can move.

Question 15

What do valence electrons look like in a metal? Like Mg (4)

Answer 15

• Empty band
• Ef
• empty band and filled band intersect each other except at one corner
• filled band

Question 16

What is Fermi energy?

Answer 16

The boundary between the filled and unfilled energy levels. For electrons to conduct, they must move to where there are unfilled energy levels.

Question 17

What is resistivity in a metal made up of?

Answer 17

• thermal resistivity
• impurity
• deformation

Question 18

More scattering event…..

Answer 18

less carrier mobility

Question 19

where does the number of scattering come from?

Answer 19

fermi level consideration but mobility can vary a lot between samples

Question 20

At T=0, absolute zero what does a semiconductor become?

Answer 20

becomes an insulator

Question 21

At T>0 temperature increases, what does a semiconductor become?

Answer 21

becomes less resistive

Question 22

Why does a semiconductor become less resistive as T>0?

Answer 22

both conduction band and valence band are partially filled so both electrons and holes can carry charge

Question 23

How can you increase the charge carriers in a semiconductor?

Answer 23

add dopants that make the charge carries increase – and we can increase either the electrons or holes (depends on the dopant)

Question 24

What is a photon?

Answer 24

a quantum unit of light

Question 25

What is the absorption spectrum?

Answer 25

How the absorption coefficient depends on the wavelength of light

Question 26

Why is there a whole in semiconductors when photons are absorbed?

Answer 26

Photons with an energy greater than the band gap energy are strongly absorbed.
The photon energy promotes an electron from the valence band to the conduction band an leaves a hole behind in the valence band

Question 27

How do metals absorb light?

Answer 27

The electrons in the conduction band have a lot of available energy levels and thus metals strongly absorb light over a wide range of the spectrum

Question 28

What do insulators look like?

Answer 28

Pure insulators such as diamond (Eg~3eV) and SiO2(Eg~9eV) are completely transparent in the visible
Sometimes they have impurities which can change their optical properties and make them coloured in the visible

Question 29

The circuit diagram shows the circuit for measuring the current voltage curve (often called the I-V curve) of a specimen under test. The voltage, V, measured across the specimen is measured for the same material but for 2 values of length, 1cm and 2cm. How many times more will the value of V be at 2 and 1 cm?

Answer 29

The voltage, V, for 2cm will be 2 times the value of the voltage for 1cm.

Question 30

The circuit diagram shows the circuit for measuring the current voltage curve (often called the I-V curve) of a specimen under test. The specimen under test has a resistance of 1 Ohm. The voltage, V, measured across the specimen is measured for 2 values of the variable resistance, 1000 Ohms and 2000 Ohms. How will the value of V change as this occurs?

Answer 30

The voltage, V, at 1000 Ohms will be twice the value of the voltage at 2000 Ohms

Question 31

What is the band gap?

Answer 31

represents the minimum energy that is required to excite an electron up to a state in the conduction band where it can participate in conduction.

Question 32

On a graph, how would you show an element with a large band gap?

Answer 32

A smaller wave length and the fastest one to begin decreasing