Semiconductors Flashcards
What are semiconductors?
materials with a resistance somewhere in between the resistance of a conductor and an insulator
A solid in which the gap between the valence band and the conduction band is small
Band gap
A range of energies that are NOT ALLOWED
Valence band
The highest band filled with electrons in a solid (not in conductors though)
Next highest energy band after conduction band
(There will be filled energy levels below the valence band but these are not important to us)
Conduction band
The first unfilled band above the valence band in a solid
Highest energy band
Electrons can move freely here as the band is not full
What are Insulators?
A solid in with the valence band is full. Large gap between valence and conduction bands
How do we decrease the resistance in the semiconductor?
Heating the semiconductor (giving the electrons enough energy to jump into the conduction band)
Doping the semiconductor with an impurity atom
What are Intrinsic semiconductors
(temp up, resistance down eg thermisors)
A ‘pure’ semiconductor material (eg germanium and silicon) with some free electrons in the conduction band at room temperature
What is Doping (extrinsic conductors)?
adding an impurity atom into the semiconductor (that has too many or too few (1) outer electrons)
How do intrinsic semiconductors work?
-most electrons in the valence band)
-band gap small (e- excited to conduction band when heated)
-charge can flow when e- are in the conduction band
How does Doping work?
Doping will either create an extra ‘free’ electron, or a ‘hole’ where an electron will normally be, increasing conductivity
group IV elements can be doped by adding elements from group I or VIII
n type semiconductors
‘free’ electron
extra electron free to move in conduction band
p type semiconductors
‘hole’ is introduced in valence band
thought of as +
creates a flow of e-
e- can move into hole
How is a p-n junction formed?
When a semiconductor contains 2 types of doping which are next to each other, a p-n junction is formed
How can a p-n junction be represented?
by considering the energy difference between the bands of the p-type and the n-type
draw a p-n junction diagram
(check jotter)
What exists inside the p-n junction
an electric field
What is biasing
In order for the semiconductor to conduct, a voltage is required to allow the electrons and holes to overcome the electric field.
This is knows as biasing
What is applied to bias the p-n junction?
an external voltage
Forward-biased?
(jotter for diagram)
Cell connected-positive end to p-type
negative end to n-type
electric field is smaller
reverse biased?
cell connected-negative end to p-type positive end to n-type
the electric field is bigger
This does not conduct
Are LEDs forward or reverse biased?
forward
How do LEDs work?
Voltage applied caused electrons to move away from the n-type conduction band towards the conduction band of p-type
electrons drop from conduction bad to valence brand EITHER side of the junction
Photon emitted when electron drops
(hole moves up)
For LEDs what do different band gaps (different materials change)
band gap size—>energy lost —>different wavelength of photon —>different colour LED
What formulas are used in colour of LEDs questions?
E=hf
and maybe v=fxlamda
Draw a diagram of an LED
Jotter
Draw a diagram of a photodiode
jotter
draw symbol for photodiode
jotter
What is the photovoltaic effect?
At the junction, photons provide energy to raise the electrons from the valence band of the semiconductor to the conduction band
This is known as the photovoltaic effect
Draw a diagram of a solar cell
jotter
What are solar cells
p-n junctions that are designed so that a potential difference is produced when photons are absorbed
how do solar cells work?
The electric field in the p-n junction caused the electrons in the conduction band to move towards the n-type semiconductor and a potential difference is produced across the solar cell
What are conductors?
A solid which the conduction and valence band overlap, or the valence band is only partially filled
