Semiconductors and Band Theory Flashcards
What makes a solid a conductor?
The material must have free electron and empty states
Band (energy) gap in a conductor
Conduction band and the valence band overlap
Energy gap in an insulator
Massive
In a metal, both conduction and valance bands are
Partially filled
In an insulator what state is the valance band in
Full
For an insulator at room temperature the gap between the valance band and conduction band is large so electrons can’t
Move into the conduction band
What is the energy gap like in a semiconductor compared to an insulator
Smaller
What type of energy can move electrons from the valence band into the conduction band in a semiconductor
Heat energy
What is doping
Adding an impurity to a semiconductor to increase the conductivity
What is an n type semiconductor
A semiconductor doped with an atom that has extra electrons. These electrons end up free to move and so can more easily get into the conduction band
What is a p-type semiconductor
A semiconductor that has been doped with an atom with too few electrons. This creates a hole in the valence band (empty state)
What is a p-n junction semiconductor
It is just a layer of p type and n type semiconducting material
State in terms of band theory how an LED works
The forward bias potential difference across the junction causes electrons to move from the conduction band of the n-type semiconductor towards the conduction band of the ptype semiconductor. Photons are emitted when electrons ‘fall’ from the conduction band into the valence band either side of the junction.
Forward bias
N type to negative side of cell therefore electric field opposing conduction is reduced
Reverse bias
N type to positive side of supply, therefore electric field opposing conduction is increased
State in terms of band theory how a photovoltaic cell works (solar panel)
solar cells are p-n junctions designed so that a potential difference is produced when photons are absorbed. (This is known as the photovoltaic effect.) The
absorption of photons provides energy to ‘raise’ electrons from the valence band of the semiconductor to the conduction band. The p-n junction causes the electrons in the conduction band to move towards the n-type semiconductor and a potential difference
is produced across the solar cell
What is a semiconductor?
is like an insulator with a much smaller band gap. The gap is small enough that
room temperature is enough to give some electrons the energy to jump up from the valence to
conduction band and increase the conductivity of the solid. Conductivity increases (ie resistance
decreases) with increasing temperature.
What can you do to semi conductors?
Semiconductors can be doped by adding impurity atoms to reduce their resistance
