Advanced materials Flashcards
how has doped semiconductors benefited society?
- manufacture and miniaturization
revolutionized communications, information, entertainment industries
how can the conductivity of semiconductors be increased?
by controlling the number of valence electrons in introduced impurities
why does pure silicon conduct poorly at RT?
because the energy gap between the valence and the conduction bands is large
how can the conductivity of Si be increased?
by adding small amounts of other elements (as dopants) to increase or decrease the number of electrons in the bands
how to make n-type semiconductor?
adding P (G5, five electron present; negative charge)
how to make p-type semiconductor
adding Ga (G3, 3 valence electrons; positive holes present)
what happens when n-type and p-type semiconductors come into contact?
make a p-n junction
what happens when the negative terminal of a battery is connected to the n-type semiconductor and the p-type is connected to the positive terminal
current will flow through the p-n junction
what happens when polarity is reversed? (n-type to positve; p-type to negative)
no current will flow; the p-n junction therefore acts as a rectifier (convert AC to DC)
what type of junctions do transistors have?
n-p-n junctions
how are n-p-n junctions made?
by sandwiching a p-type semiconductor between two n-type semiconductors
true or false; the current flowing through one junction controls the current flowing through the other junction; what is the significance of this answer
true
- can thus be used to amplify a signal
true or false; modern computer chips have millions of transistors
true (42 million Pentium 4) on a chip of silicon 1-2cm^2
how are p-n junctions manufactured.. steps 1-3
- ) SiO2 is generated on the surface of n-type silicon chip by heating in oxygen
- ) surface covered by light sensitive polymer (photoresist)
- ) a template with spaces having desired shape of the p-n junction is added
how are p-n junctions manufactured 4-6
- ) light shining on the template alters the exposed polymer making it easy to selectively dissolve. Template removed
- ) the exposed SiO2 surface is etched with HF revealing Si surface below. Remaining photoresist is removed
6.) Wafer is exposed to high temperature vapour of G13 element (eg. Ga). This diffuses into the bare areas to generate p-type silicon adjacent to n-type silicon
remaining SiO2 is removed
What are liquid crystals used for?
displays of digital watches, calculators, laptops, etc..
what makes liquid crystals unique
they flow like liquid but pack at the molecular level with a high degree of order
true or false; molecules that form liquid crystals have long rod like structures
true
what is the purpose of the rod like structure of liquid crystals
allow intermolecular attractions but inhibit perfect crystalline packing
true or false; molecules forming liquid crystals have a dipole associated with them and thus sufficiently strong electric fields can orient large numbers of these molecules in approx same direction
true
what are the 3 common types of order that liquid crystals can exhibit?
nematic
cholesteric
smectic
how are these orders in liquid crystals changed?
electric fields
how does a liquid crystal display work? (1-3)
- ) two polarizers sandwich two glass plates which in turn sandwich a liquid crystal layer
- ) the whole assembly sits on a mirror
- ) ambient light passes through the first polarizer and polarized light enters the LC layer
how do liquid crystal displays work (4-5)
4.) with the current on, the LC molecules are aligned and light cannot pass through the other polarizer (that is oriented 90 degrees) to the mirror
so the viewer sees no light (appears black)
5.) with the current OFF, the LC molecules lie in the twisted nematic arrangement which rotates polarized light and allows it to pass through the other polarizer to the mirror.
the reflected light traces this path and reaches the viewer
