4.5

Question 1

compare electrical conductivity and thermal conductivity

Answer 1

Electrical cond (1/resistivity) is abt equal to thermal cond

Wiedemann-Franz law:
kt=L(sigmae)T
(pekt=LT)
L = WF constant
kt = electronic conduction to thermal conductivity
sigmae = electrical conductivity
T = abs temp

Question 2

what part of Fermi distribution controls semiconductors conductivity

Answer 2

sigma = nqmu

n controls
n can vary strongly and n increase with T

Question 3

what part of Fermi distribution controls metal conductivity

Answer 3

sigma = nqmu
mu controls
bc n = # valence e- which is roughly constant and mu decrease with T

Question 4

F(E) is what ?

Answer 4

probability ot observe an electron at a given energy, if a state exists in that energy

F(E) proportional to n/N
n= charge carriers in semiconductors is based on prob distribution F(e)
N = total number e-
see fermi Dirac distribution

Question 5

how does fermi energy affect state probability

Answer 5

further from Fermi energy states are populated, take too much energy to cross conduction band
if ur way above, states are 0
around Fermi energy there’s a zone where e- can be excited from valence to conduction band

Question 6

describe shape of fermi distribution for conductors (cond region, valence band rectangle)

Answer 6

tan

Question 7

fermi Dirac distribution for conductors, what does area mean

Answer 7

on top of Ef, area is number of electrons in conduction, under Ef area is the number of missing electrons in valence band below Ef

Question 8

semiconductors cond level

Answer 8

has a conductivity between metals and insulators

Question 9

Fermi energy percent for semiconductors, gap energy

Answer 9

Ef = energy where the probability of occupancy is 50% for an T>0K
1.1eV for Si to 6eV for diamond

Question 10

types of semiconductors

Answer 10

intrinsic (pure), Ef in middle of band gap
extrinsic (doping), Ef position changes according to doping

Question 11

for intrinsic semiconductors, fermi distribution looks like what ? area represents what

Answer 11

tan line with rectangle (Eg) in the middle
area top = number e- in conduction
rectangle middle = area number missing e- in valence (holes in cond in valence band )

Question 12

how do intrinsic semi-conductors work

Answer 12

if u have thermal energy, you can have an electron that gets excited to conduction band, jumping from atom to atom, creating conduction.
but era e- that left created a hole where it was. this can be filled by an e- in the next atom. now the hole has moved again.
electrons move in one direction, holes are being filled in another direction. filling holes is a different e- each time and in the valence band. free e- has moved to the conduction band
mobility of e- is always higher than in holes, bc they’re free to move around than the e- that are filling the holes.

Question 13

mue > meh means

Answer 13

more difficult to move a hole than an electron

Question 14

sigma e- (electrical conductivity) relationship to T for metals and for semiconductors

Answer 14

for metals: when T inc, sigma e dec (because mu dec)

for semiconductors: when T inc, sigma e inc, because n inc

Question 15

n type semiconductor works how

Answer 15

replace form host Si (3s23p2) atoms with P (3s23p3) impurity.

a surplus of 1 electron for each P atom is added. the electron goes easily in the conduction band. deltaE needed is very small. the donor energy is very close to donor conduction band, Ed &laquo_space;Eg/2

Question 16

p-type semiconductor

Answer 16

replace some host Si (3s23p2) atoms with B (2s22p1) impurity.

missing 1 electron for every B atom added: hole created.
this hole goes easily in “conduction” in the valence band. deltaE needed is very small.

the acceptor state is right above the valence band. an electron will then jump to the acceptor state, leaving a hole in the valence band.

Ea (acceptor E) &laquo_space;Eg/2

Question 17

do n or p types have higher fermi levels

Answer 17

n-type have higher fermi levels than p-type
(ie, the Ef of n-type is close to conduction band horizontally, while the Ef of p-type is close to valence band horizontally)

Question 18

for intrinsic semi conductors, what is conductivity to temp relationship

Answer 18

high temp = high conductivity

Question 19

for extrinsic semi-conductors what is relationship between temp and conductivity

Answer 19

conductivity is controlled by the number of dopants in extrinsic region, so it doesn’t really change with temp in this region. it only affects conductivity if temp is very high (intrinsic region) or low (freeze out region)

Question 20

what is a diode ? causes what ?

Answer 20

p-n rectifier junction. current can only flow in one direction

Question 21

study diodes, forward vs reverse bias etc

Question 22

describe transistor

Answer 22

constituted of 3 semiconductor sections (pnp or npn). current can flow between the emitter (also called source) and the collector (also called drain) ONLY if a potential is applied at the base (also called gate).

Question 23

transistor - amplifiers

Answer 23

signal sent to base is amplified between emitter and collector

Question 24

transistor - electronic switches

Answer 24

(for logic gates)
current can only flow between the collector and the emitter when a voltage is applied at the base.

Question 25

logic gates are made with what

Answer 25

transistors

Question 26

AND logic gate

Answer 26

need two true to give a true output

Question 27

OR logic gate

Answer 27

true output if either input is true

Question 28

NAND logic gate

Answer 28

negative of AND
it gives the opposite of with you’d get with AND

Question 29

review what logic gates look like

Question 30

review binary additions

Question 31

microfabrication def + main steps

Answer 31

a series of methods developed for the fabrication of well defined structures on micro and nano scale

successive steps of
i. deposition (plasma)
ii. patterning (not plasma)
iii. etching (plasma)

Question 32

photolithography

Answer 32

technology developed to make patterns on the microscale
- at the basis of most microfabrication processes
- uses UV light to transfer a pattern in a metal mask to a photo reactive material (photoresist polymer) on a surface
- a photoresist is a polymer that is cross-linked by UV light, making it insoluble
- the pattern is created by dissolving the non-cross-linked part.

Question 33

thermoelectric effect

Answer 33

for a metallic conductor, at a high temp, the fermi E is less close to walls of diagram (more electrons)
at a cold temp, the fermi E is closer to walls of diagram
Seeback potential etc

Question 34

see slide on measuring thermal emf + thermocouples

Question 35

peltier effect study slides

Question 36

peltier effect: electrons flowing from p-type to n-type semi-conductor results in what

Answer 36

energy transfer or cooling depending on the circuit and where the hot/cold element is
(p type = lower fermi, n type = higher fermi)

Question 37

study superconductivity