Putting Electrons to Work
Doping and Semiconductor Devices
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:

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
• 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
What Have We Learned About
Atoms?
• ENERGY IS QUANTIZED
• Electrons can absorb energy and move to a higher level;
they can emit light and move to a lower level
• In hydrogen the emitted light will have energy
E = (13.6 ev)(1/nf2 – 1/ ni2)
• The wavelength is given by l = hc/E = 1240(nm eV)/E
• Energy levels of nearby atoms are slightly shifted from
each other, producing bands of allowed energies
• Electrons move from the locality of one atom to the next
only if an energy state is available within the same band
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.
i  nevd
eE
vd 
m
2
ne 
1/ r 
m
A Good Analogy to Remember
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
Crystal (Perfect)
Crystal (Excited)
Crystal (Excited)
Band Gap
Energy
Conduction Band
Band Gap Energy Eg
(Minimum Energy needed to
break the chemical bonds)
Valence Band
Position
Band Gap
Energy
Conduction Band
h  Eg
photon
in
Valence Band
Position
Band Gap
Energy
Conduction Band
photon out
Valence Band
Position
Band Gap
Energy
Conduction Band
photon out
Valence Band
Position
Crystal (Doped n-type)
+5
Plus a
little
energy,
d.
+5
Crystal (Doped p-type)
+3
N-type semiconductors
• N-type semiconductor is doped with a material having
extra valance electrons
• Result is filled energy states in the band gap just below the
conduction band
• These electrons can easily gain energy to jump to the
conduction band and move through the material
Crystal (Doped p-type)
+3
P-type semiconductors
• P-type semiconductor is doped with a material having
fewer valance electrons
• Result is “holes”, or empty energy states in the band gap
just above the valance band
• Since no single electron travels through the material, we
describe the charge carrier as a positive hole moving the
other way
Doped Semiconductors
Energy
donor level
acceptor level
n-type
p-type
•Put them together?
p-n junction
Energy
+
+
+
+
+
+
+
+
--
-
-
p-type
n-type
depleted region
(electric field)
-
-
-
-
P-n junction
• As more electrons from the n-side combine with holes from the
p-side, each additional combination adds to the potential
difference across junction
• This can be envisioned as shifting the energy bands, making it
harder for electrons to travel across the barrier
p-n junction
Energy
+
+
+
+
+
+
+
Vo
+
--
-
-
p-type
n-type
depleted region
(electric field)
-
-
-
-
What happens if a bias is
applied?
Biased junction
Negative
bias
positive
bias
p-type
n-type
depleted region
(electric field)
Biased junction
Negative
bias
photon out
p-type
n-type
depleted region
(electric field)
P-n junction
• Originally both p and n sides are electrically neutral
• Electrons in n side see holes in p side and combine
Second electron needs add’l
energy to get over charge
barrier – can represent as rise in
energy levels of p section
Forward Biasing
• Eventually, the potential difference is so large, electrons cannot travel
across it without gaining energy
• Applying a forward bias decreases the potential difference so current
can flow
Reverse Biasing
• Applying a reverse bias will increase the barrier rather than
decreasing it, so no current flows
Light-emitting Diode
• When an electron loses energy to recombine with a hole, it can emit
that lost energy in the form of light.
• This light always has roughly same E, so LEDs emit small range of
wavelengths
 This light-emitting property of p-n junctions can be utilized to create a
laser
 Be sure to come to class to hear Dr. Schowalter say . . .
Do Today’s Activity
• How is an incandescent light bulb different
from an LED?
• What is the difference between the different
colors of LED?
• Why might these differences occur?
Before the next class, . . .
• Finish Homework 16
• Do Activity 15 Evaluation by Midnight
Monday
• Read Chapter 6 in Turton.