Electrical and optical properties Flashcards
What is Ohm’s law?
As the variable resistance is increased the current in the circuit is reduced and the voltage across the specimen
decreases linearly
What is voltage drop measured in?
C (coulomb)
What is one of the most least resistive materials at room temperature (300K)?
copper p= 1x10⁻⁸ Ohm.m
How can resistivity of some materials be engineered?
by adding small amounts of dopants
How does resistivity increase?
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.
Is resistivity high in insulators?
yes as electrons cannot move at all
What is between the conduction band and the valence band?
the band-gap (Eg)
Energy is point up is the conduction band below or above the valence band?
above the valence band
What is the fermi energy level?
describes how the bands are filled with electrons and the amount of energy required to add another electron to the solid
How can electrons fill over valence bands?
they acquire enough energy to jump the energy gap
What happens when you add thermal energy to valence bands?
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.
In an insulator when T=0, what does the valence electrons look like? (3)
- empty conduction band
- band gap (>2eV)
- filled valence band
All the valence bands are filled after the energy gap
What do valence electrons look like in a semi-conductor? (3)
- Empty conduction band
- Band gap (<2eV)
- filled valence band
Some electrons have crossed the electron gap.
What do valence electrons look like in a metal? Like Cu (5)
- empty band
- band gap
- empty states
- Ef (fermi energy)
- Filled states
partially filled band (conduction band) so the electrons can move.
What do valence electrons look like in a metal? Like Mg (4)
- Empty band
- Ef
- empty band and filled band intersect each other except at one corner
- filled band
What is Fermi energy?
The boundary between the filled and unfilled energy levels. For electrons to conduct, they must move to where there are unfilled energy levels.
What is resistivity in a metal made up of?
- thermal resistivity
- impurity
- deformation
More scattering event…..
less carrier mobility
where does the number of scattering come from?
fermi level consideration but mobility can vary a lot between samples
At T=0, absolute zero what does a semiconductor become?
becomes an insulator
At T>0 temperature increases, what does a semiconductor become?
becomes less resistive
Why does a semiconductor become less resistive as T>0?
both conduction band and valence band are partially filled so both electrons and holes can carry charge
How can you increase the charge carriers in a semiconductor?
add dopants that make the charge carries increase – and we can increase either the electrons or holes (depends on the dopant)
What is a photon?
a quantum unit of light
What is the absorption spectrum?
How the absorption coefficient depends on the wavelength of light
Why is there a whole in semiconductors when photons are absorbed?
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
How do metals absorb light?
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
What do insulators look like?
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
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?
The voltage, V, for 2cm will be 2 times the value of the voltage for 1cm.
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?
The voltage, V, at 1000 Ohms will be twice the value of the voltage at 2000 Ohms
What is the band gap?
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.
On a graph, how would you show an element with a large band gap?
A smaller wave length and the fastest one to begin decreasing
