Part 4 (Semiconductors)

Question 1

What are semiconductors?

Answer 1

Materials with a band gap that is small enough compared to kBT (a gap energy Eg = 1eV or less at 300K)

Question 2

How many valence bonds do semiconductors have?

Answer 2

4

Question 3

Which elements are most semi conductors made of?

Answer 3

Si or Ge

Question 4

Where is the valence band located?

Answer 4

Below the band gap (completely filled band if right valence)

Question 5

Where is the conduction band located?

Answer 5

Above the band gap (thermally excite carriers into it)

Question 6

What happens at 0K?

Answer 6

Valence band is completely filled

Conduction band is completely empty

Question 7

What happens at finite T?

Answer 7

Electron-hole pairs thermally excited

Question 8

What happens when both bands are populated?

Answer 8

Both bands contain carriers that can conduct a current of electricity

Question 9

What is mobility?

Answer 9

Drift velocity per unit E-field

contains information about scattering

Question 10

What can both electrons and holes do?

Answer 10

They can conduct

Question 11

What is an intrinsic semiconductor?

Answer 11

A pure semiconductor

Question 12

What happens to carriers at a gap?

Answer 12

They are thermally excited over the gap

Question 13

What does carrier density depend on?

Answer 13

DoS and Fermi-Dirac stats

Question 14

What do carrier densities have?

Answer 14

Strong exponential temperature dependences

Question 15

What are extrinsic semiconductors?

Answer 15

Semiconductors with dopants (deliberate impurities)

Small amounts of impurity are substituted that have a different valence.

Question 16

What are donors?

Answer 16

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

Question 17

What are acceptors?

Answer 17

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

Question 18

When can we estimate the intrinsic electron density from the conductivity of intrinsic?

Answer 18

From the conductivity of intrinsic element at 300K

Question 19

What is inside a semiconductor?

Answer 19

An effective mass and a dielectric constant

Question 20

How big is the ionisation energy of donors and acceptor compared to the gap energy?

Answer 20

They are only a small fraction of gap energy and impurity levels are close to the band edge

Question 21

Where is the ionisation energy found for n-type semiconductors?

Answer 21

Below the conduction band

Question 22

Where is the ionisation energy found for p-type semiconductors?

Answer 22

Above the conduction band

Question 23

What are the similarities with replacing Eg (gap energy) with Ed (donor energy) ?

Answer 23

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

Question 24

What are the three regimes, corresponding to the different energy scales?

Answer 24

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)

Question 25

In phonons where does the temperature dependence come from?

Answer 25

Changes in the scattering rate

Question 26

What effect do higher temperatures have on conductivity?

Answer 26

Higher temperatures lead to more phonons, more scattering, lower mobility

The power law is weaker than exponential

Question 27

How are impurities different to metals?

Answer 27

They can ionised and so strong scattering centres

Electron velocity is temperature dependent
Faster electrons spend less time near the scatterer

Question 28

What is an indirect band gap semiconductor?

Answer 28

The top of the semiconductor’s valence band and the bottom of the conduction band are not at the same value of k

To cross the gap k must be changed

Question 29

What is a direct band gap semiconductor?

Answer 29

The top of the valence band and the bottom of the conduction band are at the same value of k

To cross the gap, no need to change k

Question 30

What are semi conductors used for?

Answer 30

To emit (LEDs, lasers) or absorb (solar cells) light

Question 31

In semi conductors, what must be conserved?

Answer 31

Energy and momentum as the photon is emitted and absorbed

Question 32

What is needed for the indirect band gap?

Answer 32

A phonon is needed to provide the momentum

Question 33

What do field effect transistors (FETs) rely on?

Answer 33

The presence of a band