Transcript Class16review

Review for Exam 2
Spring, 2002
Charges in Conductors
 Electric fields are created when positive charges
and negative charges are separated
 A uniform electric field existing over a region sets
up a potential difference between points in that
region: DV=EDx, where Dx is the distance along
a field line.
 If I apply a potential difference across a
conducting object (including semiconductors),
charges experience a force, and charge carriers
will flow until the potential difference is removed.
What Have We Learned About
Electrical Storage
• The electric force FE on a charge q0 can be considered due
to an electric field which is produced by other charges in
the area
FE = q0 E
• If moving a charge between two points requires work (or
does work), the charge gains (or loses) potential energy:
DV = –  E  dx = (for a constant field) EDx
• Capacitors store charge Q in proportion to the voltage V
between the plates:
C = Q/V = C = e0 A/d
• Capacitors are used in RAM
What Have We Learned About
Magnetic Storage?
• Two domains magnetized in same direction is a 0
• Two domains magnetized in opposite directions is
a1
• Direction of magnetization changes at start of new
bit.
• Magnetic data is written by running a current
through a loop of wire near the disk
• As magnetic data passes by coil of wire, changing
field induces currents according to Faraday’s Law:
e
d B
dB
 iR  
 A
dt
dt
What Have We Learned About
Magnetoresistance?
• Charges traveling through magnetic field experience
magnetic force (provided velocity and field are not
aligned):
FB = qv x B = (if v perpendicular to B) qvB
• In a current-carrying wire, this force results in more
frequent collisions and thus an increased resistance:
Magnetoresistance
• Electrons traveling through magnetized material undergo
spin-dependent scattering
• When magnetic field is present in magnetic superlattice,
scattering of electrons is cut dramatically, greatly
decreasing resistance: Giant magnetoresistanced
Stuff to remember about GMR
• Electrons (and other elementary “particles”) have
intrinsic magnetic fields, identified by spin
• The scattering of electrons in a ferromagnetic
material depends on the spin of the electrons
• Layers of ferromagnetic material with alternating
directions of magnetization exhibit maximum
resistance
• In presence of magnetic field, all layers align and
resistance is minimized
What Have We Learned About
Spectra?
• ENERGY LEVELS ARE QUANTIZED
• Different elements have different allowed energies (since
different numbers of protons and electrons provide
different structure of attraction
• Light emitted when electrons move from a high energy
level to a lower energy level in an atom will have only
certain, QUANTIZED, allowed energies and wavelengths.
• Those wavelengths depend solely on the element emitting
the light and compose the characteristic emission spectrum
for that element
Our Model of the Atom
• If the atom is in the “ground state” of lowest energy, electrons fill the
states in the lowest available energy levels. The first shell has two
possible states, and the second shell has eight possible states. Higher
shells have more states, but we’ll represent them with the eight states
in the first two sub-shells.
• Electrons in the outermost shell are called “valence” electrons. We’ll
make them green to distinguish from e- in filled shells
E=0 (unbound)
n=4
n=3
n=2
n=1
Really eight distinct states with
closely spaced energies, since two
electrons cannot occupy the same
state.
Electrons in Solids
• The shifted energies in adjacent atoms combine to create a continuous
“band” of allowed energies for each original energy level; each band,
however, has a finite number of states equal to the number in original
atoms
• Electrons can move from the locality of one atom to the next only if an
energy state is available within the same band
Conductors & Semiconductors
• In conductors, the valence band is only partiallyfull, so electrons can easily move from being near
one atom to being near another
• In semiconductors and insulators, the valence
band is completely full, so electrons must gain
extra energy to move
• In semiconductors, extra electrons (or holes) can
be introduced in a “controlled” way.
What Have We Learned About
Solids?
• In conductors, the valence band is only partially-full, so
electrons can easily move
• In semiconductors and insulators, the valence band is
completely full, so electrons must gain extra energy to
move
– semiconductors have smaller band gap, insulators have larger band
gap
• Conductors have a partially-filled valence band
– The primary effect of higher temperature on resistance is to
increase R due to more collisions at higher temperatures
• Semiconductors have a completely-filled valence band
– The primary effect of temperature on resistance is due to this
requirement: the higher the temperature, the more conduction
electrons
What have we learned about
Resistance?
• In many, ohmic, materials, current is proportional
to voltage:
V = iR
• Resistance is proportional to the length of an
object and inversely proportional to crosssectional area:
R = rL/A
• The constant of proportionality here is called the
resistivity. It is a function of material and
temperature.
V EL
i  Anevd  
R
R
eE
vd 
m
Ane 
1/ R 
Lm
2
A Good Analogy to Remember
p-n junction
Energy
+
+
+
+
+
+
+
Vo
+
--
-
-
p-type
n-type
depleted region
(electric field)
-
-
-
-
Wide Bandgap
Semiconductors
What is a wide bandgap semiconductor?
Larger energy gap allows higher power and
temperature operation and the generation of more
energetic (i.e. blue) photons
Traditional wide bandgap semiconductors include
AlAs/GaAs, SiC
The III-nitrides (AlN, GaN and InN) have recently
become feasible.
Impact
Automotive industry
Avionics and defense Information technology Displays
(data storage)
Solid state lighting
Traffic lights
Wireless
communications
Electric power industry
Health care
Heterojunctions