I. Magnetic Memory Question and Refresher
a. DRAM: Dense (one transistor and capacitor can be fit in a very small
amount of space), but volatile. When you unplug DRAM from a power
source, the information is lost.
b. Non-volatile memory is crucial for information storage. Magnets don’t
require electricity to maintain their magnetic polarization, so they can
store information even when they are unplugged.
c. In homework question #1, we ask about a new technology (not the one
that is used in current hard disks).
II. Doping and Transistor Function
You may remember the concept of valence electrons from high school. Valence
electrons are responsible for much of the reactivity of an atom because they are
generally farthest from the nucleus and the most likely to interact with other
atoms. As a general rule, most atoms “prefer” to have 8 valence electrons.
Silicon atoms have 4 valence electrons. In solid silicon, individual silicon atoms
are arranged in a lattice where each atom has 4 neighboring atoms. The silicon
atoms share one valence electron with each of their neighbors (in the form of a
covalent bond) so that they can fill their octet (have 8 valence electrons).
(Insert drawing of Si lattice here.)
Since all of silicon’s valence electrons are occupied in bonds, they cannot carry
electric current easily. Pure silicon is a semiconductor.
When a minute amount of phosphorus or arsenic atoms are introduced into a
silicon lattice, each impure atom will bond with 4 silicon neighbors. There will be
one excess electron that is not part of a bond because phosphorus and arsenic
atoms have one more electron (and one more proton) than silicon. This excess
electron is mobile and can carry charge.
By introducing an atom with one less electron than silicon (like Boron) into a Si
lattice, you can also increase the conductivity of the material. In this type of
doping, the impure atom will not be able contribute electrons to 4 covalent bonds
because it only has 3 valence electrons. This will leave silicon with a 7, not 8,
valence electrons. Since silicon wants 1 more electron to fill its octet, this type of
doping is said to leave a “hole” – a positively charged space for one more
electron. A hole increases the conductivity of silicon because it creates a space
into which a neighboring electron can move.
A type of transistor that contains two regions of doped n-type silicon (it could
also be p-type) one at the source, the other at the drain, separated by a region of
doped p-type silicon. Even though both n-type and p-type doped silicon are
conductive, current cannot pass through a system where a piece of p-type silicon
is sandwiched between two n-type pieces. When a positive voltage is applied (or
negative in the case of an p-type doping) electrons migrate toward the positive
voltage source from both the doped regions and the p-type region. The migration
of electrons creates a channel of mobile electrons between the two N-type doped
regions. These mobile electrons can carry current from the source to the drain.
III. Light: Brief Questions to Confirm Understanding
a. Brief Discussion of wavelength as an indicator for type of radiation.
i. Visible Light: 400-700nm – within this range certain wavelengths
correspond to different colors that we see. (Other animals may
have different visible ranges)
ii. Infrared Radiation: heat! …. Longer wavelength, lower energy
iii. Ultraviolet Light: invisible, damaging high energy rays.
b. Easy Questions:
i. You went to the pumpkin patch last weekend. Explain how you
were able to identify the pumpkins from the vast amount of
surrounding vegetation. (Specifically: why do you see a pumpkin?
How is light involved?)
ii. Now you are busy carving pumpkins for Halloween. Why do the
seeds appear white?
iii. Your jack-o-lantern is complete. After putting a lit candle inside it,
you have no trouble seeing the openings you have carved, even in
the absence of light. What mechanism is responsible for enabling