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)