Part 4 (Semiconductors) Flashcards
What are semiconductors?
Materials with a band gap that is small enough compared to kBT (a gap energy Eg = 1eV or less at 300K)
How many valence bonds do semiconductors have?
4
Which elements are most semi conductors made of?
Si or Ge
Where is the valence band located?
Below the band gap (completely filled band if right valence)
Where is the conduction band located?
Above the band gap (thermally excite carriers into it)
What happens at 0K?
Valence band is completely filled
Conduction band is completely empty
What happens at finite T?
Electron-hole pairs thermally excited
What happens when both bands are populated?
Both bands contain carriers that can conduct a current of electricity
What is mobility?
Drift velocity per unit E-field
contains information about scattering
What can both electrons and holes do?
They can conduct
What is an intrinsic semiconductor?
A pure semiconductor
What happens to carriers at a gap?
They are thermally excited over the gap
What does carrier density depend on?
DoS and Fermi-Dirac stats
What do carrier densities have?
Strong exponential temperature dependences
What are extrinsic semiconductors?
Semiconductors with dopants (deliberate impurities)
Small amounts of impurity are substituted that have a different valence.
What are donors?
Dopants with higher valence (e.g P or As in Si) provide one extra electron per dopant atom
The semi conductor is referred to as being n-type
What are acceptors?
Dopants with lower valence (e.g B or Al in Si) provide one fewer electron per dopant atom
The semiconductor is referred to as being p-type
When can we estimate the intrinsic electron density from the conductivity of intrinsic?
From the conductivity of intrinsic element at 300K
What is inside a semiconductor?
An effective mass and a dielectric constant
How big is the ionisation energy of donors and acceptor compared to the gap energy?
They are only a small fraction of gap energy and impurity levels are close to the band edge
Where is the ionisation energy found for n-type semiconductors?
Below the conduction band
Where is the ionisation energy found for p-type semiconductors?
Above the conduction band
What are the similarities with replacing Eg (gap energy) with Ed (donor energy) ?
The Fermi energy is in the middle of the ionisation energy (at least for low T) The conductivity (ignoring the weak T dependence of the mobility) has the same form
What are the three regimes, corresponding to the different energy scales?
1: Dopants are ionised, since dopant ionisation energy is the smallest energy (EXTRINSIC)
2. The carrier density saturates when all of the dopants become ionised (SATURATION)
3. Intrinsic carriers start to appear since gap energy is the largest energy (INTRINSIC)
