Putting Electrons to Work
Doping and Semiconductor Devices
What Have We Learned About
Optical and Electric Storage?


Laser light is focused through a (circular) lens
onto the surface of a CD
The central max of the diffraction pattern must be
no larger than one bit if data is to be resolved
d sin q = 1.22 l
tan q = y/D


Capacitors store charge Q in proportion to the
voltage V between the plates:
C = Q/V = 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
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 magnetoresistance
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.
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


Conductors have a partially-filled valence band


semiconductors have smaller band gap, insulators have
larger band gap
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
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


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
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
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?

npn junction


Put another n-type semiconductor on the other side of the p-type
semiconductor
No matter which way I apply potential difference, one p-n junction is
reverse biased, and electrons entering the p-type region quickly
combine with holes, creating more negative charge
MOSFET

If, however, I apply a positive potential to one side of the
p-type semiconductor, without allowing another path for
electrons to flow out of the device, I will create a channel
for e- to get from one n-side to the other.
n-type
n-type
p-type
MOSFET
Now, if I bias the device in either direction, current will
flow, electrons toward the positive potential, and
conventional positive current toward the negative potential
Gate
Drain
Source

n-type
n-type
p-type
MOSFET
The potential difference between drain and source is
continually applied
 When the gate potential difference is applied, current flows
Gate
Drain
Source

1
n-type
p-type
n-type
How do transistors fit in?
From How
Computers
Work, by Ron
White
How do transistors fit in?
 For
now, view transistor as switch:
 If switch is “on,” current can pass
 If switch is “off,” no current can pass
 We
can use this simple device to construct
complicated functions
NOT Gate - the simplest case

Put an alternate path (output) before a switch.
Output
Input
Switch



Dump
If the switch is off, the current goes through the
alternate path and is output.
If the switch is on, no current goes through the
alternate path.
So the gate output is on if the switch is off and off
if the switch is on.
NAND - a variation on a
theme


NAND gate returns a signal unless both of its two
inputs are on.
Put an extra switch after a NOT device
Output
Input
Switch


Input
Switch
Dump
If both switches are on, current is dumped.
Otherwise the current goes to the output.
AND - slightly more
complicated


AND gate returns a signal only if both of its two
inputs are on.
Use the NAND output as input for NOT
Output
Switch
Input
Switch


Input
Switch
Dump
If both inputs are on, the NOT input is off, so the
AND output is on.
Else the NOT input is on, so the output is off.