p n junction
TRANSCRIPT
Valence band
Conduction band
p-n Junction When a p-type semiconductor and an n-type semiconductor come into contact with each other they form what is called a p-n junction
N type P typeJunction
Free electrons Free holes
p-n Junction
N type P type
Conduction band
Valence band
Junction
Free electrons Free holes
Free electrons in the conduction band in the n-type semiconductor
are initially able to move across the junction and “drop into” a free hole
in the p-type semiconductor
Similarly, holes from the p-type semiconductor are able to move across the junction allowing for an electron to
fall into it
This means that in terms of charges, the hole and electron cancel each other out and disappear
p-n Junction
N type P type
Conduction band
Valence band
Junction
Free electrons Free holes
As a result electrons and holes near the junction tend to cancel each other out, producing a region depleted of moving charges – this creates what is known as a the depletion zone
Depletion zone
p-n JunctionThe movement of electrons into the positive charge carriers (or holes) in the p-type material leaves the n-type semiconductor positively charged and the p-type material negatively charged
N type P type
Conduction band
Valence band
Junction
Free electrons Free holes
Depletion zone
+ -
p-n JunctionAs holes try to diffuse into the
depletion zone, they are repelled by the build up of positive charge
in the n-type conductor
N type P type
Conduction band
Valence band
Junction
Free electrons Free holes
Depletion zone
+ -
Similarly, electrons trying to enter the depletion zone are repelled by the build up of negative charge in
the p-type conductor
p-n Junction
N type P type
Conduction band
Valence band
Junction
Free electrons Free holes
Depletion zone
This buildup of positive charge in the n-type semiconductor and negative charge in the p-type semiconductor results in the formation of an electric field across the depletion zone
+ -
This electric field comes to completely prevent the movement of electrons
and holes across the depletion zone
p-n Junction
N type P type
Conduction band
Valence band
Junction
Free electrons Free holes
Depletion zone
+ -
When the positive terminal of a battery is connected to the p-type semiconductor side and the negative terminal to the n-type semiconductor side, a repulsive force occurs at the
terminals. + -
p-n Junction
N type P type
Conduction band
Valence band
Junction
Free electrons Free holes
Depletion zone
+ -+ -
This in conjunction with the attractive force from the opposite terminal, causes electrons in the n-type semiconductor to move
towards the opposite positive terminal and causes the positive charge carriers or holes to move towards the n-type semi conductor.
This constitutes a current in same direction
p-n Junction
N type P type
Conduction band
Valence band
Junction
Free electrons Free holes
Depletion zone
+ -
- +
If connected oppositely, electrons are holes are attracted to their own terminals and thus no current is produced. Therefore, a p-n junction
only allows current carriers to travel in one direction.
This is known as a diode
p-n JunctionThis concept can also be understood by looking at the energy bands
The electric field across the depletion zone also causes the band energies to bend
This is the energy bands when no
potential difference is applied
Depletion zone
Depletion zone
Depletion zone
N-type P-type N-type P-typeN-type P-type
When a potential difference in the wrong direction is applied, the energy difference increases
and no electrons or holes are able to cross
When a potential difference in the right direction is applied,
the energy difference decreases so that electrons and holes are able to cross
Conduction band
Valence band
Conduction band Conduction band
Valence bandValence band
p-n Junction – Summary
• When a p-type and an n-type semiconductor come into contact with each other they form a p-n junction
• Initially electrons and holes are able to travel across the junction and cancel each other out
• This results in the formation of a depletion zone • It also results in the n-type material becoming
positively charged and the p-type material becoming negatively charged
p-n Junction – Summary • This results in the formation of an electric field
across the junction which prevents further movement of holes and electrons across the junction
• This electric field also causes the energy levels to ‘bend’
• When a potential voltage is set up across the junction in the right direction, an electric current is created
• A p-n junction only allows the flow of electrons in one direction