4.5 Flashcards
compare electrical conductivity and thermal conductivity
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
what part of Fermi distribution controls semiconductors conductivity
sigma = nqmu
n controls
n can vary strongly and n increase with T
what part of Fermi distribution controls metal conductivity
sigma = nqmu
mu controls
bc n = # valence e- which is roughly constant and mu decrease with T
F(E) is what ?
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
how does fermi energy affect state probability
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
describe shape of fermi distribution for conductors (cond region, valence band rectangle)
tan
fermi Dirac distribution for conductors, what does area mean
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
semiconductors cond level
has a conductivity between metals and insulators
Fermi energy percent for semiconductors, gap energy
Ef = energy where the probability of occupancy is 50% for an T>0K
1.1eV for Si to 6eV for diamond
types of semiconductors
intrinsic (pure), Ef in middle of band gap
extrinsic (doping), Ef position changes according to doping
for intrinsic semiconductors, fermi distribution looks like what ? area represents what
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 )
how do intrinsic semi-conductors work
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.
mue > meh means
more difficult to move a hole than an electron
sigma e- (electrical conductivity) relationship to T for metals and for semiconductors
for metals: when T inc, sigma e dec (because mu dec)
for semiconductors: when T inc, sigma e inc, because n inc
n type semiconductor works how
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 «_space;Eg/2
p-type semiconductor
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) «_space;Eg/2
do n or p types have higher fermi levels
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)
for intrinsic semi conductors, what is conductivity to temp relationship
high temp = high conductivity
for extrinsic semi-conductors what is relationship between temp and conductivity
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)
what is a diode ? causes what ?
p-n rectifier junction. current can only flow in one direction
study diodes, forward vs reverse bias etc
describe transistor
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).
transistor - amplifiers
signal sent to base is amplified between emitter and collector
transistor - electronic switches
(for logic gates)
current can only flow between the collector and the emitter when a voltage is applied at the base.
logic gates are made with what
transistors
AND logic gate
need two true to give a true output
OR logic gate
true output if either input is true
NAND logic gate
negative of AND
it gives the opposite of with you’d get with AND
review what logic gates look like
review binary additions
microfabrication def + main steps
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)
photolithography
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.
thermoelectric effect
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
see slide on measuring thermal emf + thermocouples
peltier effect study slides
peltier effect: electrons flowing from p-type to n-type semi-conductor results in what
energy transfer or cooling depending on the circuit and where the hot/cold element is
(p type = lower fermi, n type = higher fermi)
study superconductivity
