Advanced materials

Question 1

how has doped semiconductors benefited society?

Answer 1

• manufacture and miniaturization

revolutionized communications, information, entertainment industries

Question 2

how can the conductivity of semiconductors be increased?

Answer 2

by controlling the number of valence electrons in introduced impurities

Question 3

why does pure silicon conduct poorly at RT?

Answer 3

because the energy gap between the valence and the conduction bands is large

Question 4

how can the conductivity of Si be increased?

Answer 4

by adding small amounts of other elements (as dopants) to increase or decrease the number of electrons in the bands

Question 5

how to make n-type semiconductor?

Answer 5

adding P (G5, five electron present; negative charge)

Question 6

how to make p-type semiconductor

Answer 6

adding Ga (G3, 3 valence electrons; positive holes present)

Question 7

what happens when n-type and p-type semiconductors come into contact?

Answer 7

make a p-n junction

Question 8

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

Answer 8

current will flow through the p-n junction

Question 9

what happens when polarity is reversed? (n-type to positve; p-type to negative)

Answer 9

no current will flow; the p-n junction therefore acts as a rectifier (convert AC to DC)

Question 10

what type of junctions do transistors have?

Answer 10

n-p-n junctions

Question 11

how are n-p-n junctions made?

Answer 11

by sandwiching a p-type semiconductor between two n-type semiconductors

Question 12

true or false; the current flowing through one junction controls the current flowing through the other junction; what is the significance of this answer

Answer 12

true

- can thus be used to amplify a signal

Question 13

true or false; modern computer chips have millions of transistors

Answer 13

true (42 million Pentium 4) on a chip of silicon 1-2cm^2

Question 14

how are p-n junctions manufactured.. steps 1-3

Answer 14

1. ) SiO2 is generated on the surface of n-type silicon chip by heating in oxygen
2. ) surface covered by light sensitive polymer (photoresist)
3. ) a template with spaces having desired shape of the p-n junction is added

Question 15

how are p-n junctions manufactured 4-6

Answer 15

4. ) light shining on the template alters the exposed polymer making it easy to selectively dissolve. Template removed
5. ) 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

Question 16

What are liquid crystals used for?

Answer 16

displays of digital watches, calculators, laptops, etc..

Question 17

what makes liquid crystals unique

Answer 17

they flow like liquid but pack at the molecular level with a high degree of order

Question 18

true or false; molecules that form liquid crystals have long rod like structures

Answer 18

true

Question 19

what is the purpose of the rod like structure of liquid crystals

Answer 19

allow intermolecular attractions but inhibit perfect crystalline packing

Question 20

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

Answer 20

true

Question 21

what are the 3 common types of order that liquid crystals can exhibit?

Answer 21

nematic
cholesteric
smectic

Question 22

how are these orders in liquid crystals changed?

Answer 22

electric fields

Question 23

how does a liquid crystal display work? (1-3)

Answer 23

1. ) two polarizers sandwich two glass plates which in turn sandwich a liquid crystal layer
2. ) the whole assembly sits on a mirror
3. ) ambient light passes through the first polarizer and polarized light enters the LC layer

Question 24

how do liquid crystal displays work (4-5)

Answer 24

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