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Question 1

What is a semi-conductor

Answer 1

At room temperature; conductivity w semi conductor is between metal and insulator

Completly filled bands by band gap is relatively small

Question 2

What is an intrinsic semiconductor

Answer 2

insulates at 0 temp but conducts ar room temp

This is due to excitation of electrons w large thermal energy across relatively small bandgao

increase un conductivity as temp increases so Energy thermal increases therefore excited electrons can cross band gap

Question 3

What is Valance band

Answer 3

Highest energy filled in superconcustor

Full at T=0

Question 4

What is conduction band

Answer 4

Lowest empty band

electrons in bottom have free-electron like dynamics

Question 5

What can doping cause

Answer 5

Ntype and Ptype superconductors

Question 6

What is ab N type superconductor

Answer 6

Excess of electrons by addition of donor impurities

Conductivity dominated by electrons occupying stated n ear bottom of conduction band

Question 7

P type superconductor

Answer 7

Excess of holes by addition of acceptor impurities

Conductibity dominated by holes occupying states near the top of valence band

Question 8

Properties of holes

Answer 8

• Zero energy at top of band
• Sum od wavevectors of hole is 0

The dynamical properties of a nearly filled band with n empty electron states is equivalent to that of
n hole states with a positive charge and a positive effective mass

Question 9

What is an indirect superconductor

Answer 9

Minimum energy of the conduction band occurs at a different k-vector value than max of the valance band

Question 10

Energy and wave vector of a phonon at Room temperature

Answer 10

𝜀𝑝ℎ𝑜𝑛𝑜𝑛 = k𝐵T ≈ 25 meV

𝒌𝑝ℎ𝑜𝑛𝑜𝑛 = 𝜀𝑝ℎ𝑜𝑛𝑜𝑛/ℏ𝑣𝑠 ~ 1010 m−1

Question 11

Conservation of energy and wavevector when absorbed phonon moved into state at bottom of conduction band from valance band

Answer 11

𝐸𝑔 = 𝜀𝑝ℎ𝑜𝑡𝑜𝑛 + 𝜀𝑝ℎ𝑜𝑛𝑜𝑛 ≈ 𝜀𝑝ℎ𝑜𝑡𝑜𝑛
and
Δ𝒌 = 𝒌𝑝ℎ𝑜𝑡𝑜𝑛 + 𝒌𝑝ℎ𝑜𝑛𝑜𝑛 ≈ 𝒌𝑝ℎ𝑜𝑛𝑜�

Question 12

What is a p-n semiconductor junction

Answer 12

region of a semiconductor that is selectively doped so that one part is
p-type and an adjacent part is n-type.

Question 13

How do you calculate the form of electrostatic potential

Answer 13

Poisson’s Equation

Partial 2nd derivation of phi/x = -rho(x)/ e0er

Question 14

Delpeion length equation

Answer 14

page 22

Question 15

What is generation current

Answer 15

flowing from the n-side to the p-side of the junction,
which is defined as a negative current.
Jgen = -A exp(-Eg/2kBT)

Question 16

Recombination Current

Answer 16

Small fraction of electrons have greater energy than electrostatic pot energy barrier and moved to p-type and a similar fraction have enouhj energy to move to n-type and recomnine w electron causing positive recombination current

Jrec = B exp(-e ΔΦ0/kBT)

Question 17

Applications of p-n junction

Answer 17

p-n diodes : p-n junctions are very asymmetric on
reversal of the sign of the applied voltage they make excellent diodes,

LEDs and Lasers:
. In direct bandgap semiconductors the dominant recombination
process involves the emission of photons and such p-n junctions can be used to make efficient light
emitting diode

For indirect semiconductors p-n junctions the recombination by photon emission also
requires absorption or emission of a phonon. - less prob and non radiative recom therefore low photon emission rate

Solar Cells:
In the depletion region of a p-n junction there is a strong electric field.
Electron-hole pairs which are produced in the depletion region are swept apart with the electron being
swept into the n-type material and holes being swept into the p-type material. A net current is created
which is to a first approximation proportional to the intensity of light falling on the junction. A large
fraction of the energy of the photons can be converted to electrical power in solar cells based on this
mechanism.