Putting Electrons to Work Doping and Semiconductor Devices.
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Transcript of 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
a 1• 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:
dt
dBA
dt
diR B
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/ ni
2)
• The wavelength is given by = 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 cross-sectional area:
R = L/A• The constant of proportionality here is called the
resistivity. It is a function of material and temperature.
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
Band Gap
Valence Band
Conduction Band
Band Gap Energy Eg
(Minimum Energy needed tobreak the chemical bonds)
Energy
Position
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
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
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
• 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?