Download electronic properties of matter

ELECTRONIC PROPERTIES
OF MATTER
- Semi-conductors and the p-n junction -
Recap of previous years:
Insulators
Conductors
- High resistance to current flow (no free charge carriers)
- Low resistance to current flow
- Resistance increases with increased temperature
- Contains many charge carriers that are free to move
- The charge carriers are ELECTRONS
Semi-conductor - more resistance to current flow than conductors
- Resistance DECREASES with increasing temperature
- Few free charge carriers (increases as temperature increases)
- The charge carriers are ELECTRONS and HOLES
Band Theory of Solids
Consider the energy levels for a single lithium atom
Consider 2 lithium atoms coming together and bonding (their orbitals will overlap)
Consider a number of lithium atoms bonding to form a lithium crystal
- There are a large number of overlapping orbitals
- The addition of a small amount of energy will see the
electrons being able to move between the orbitals
- This will create the effect of an electron
‘band’ being formed.
Terminology and Important features
- The outer-most band, containing electrons, is called the valence band
- The band in which electrons are able to move freely is called the conduction band
- The outer most band, in this situation, is only half-filled
- There is an energy ‘gap’ between the bands
Conductors (current flows easily)
- Conductors have a partly-filled conduction-valence band
- A small potential difference will lift electrons in the
conduction band to a higher level (within that band) where
they will be able to drift through the crystal
Insulators (current doesn’t flow)
- example: Carbon
- Insulators have an empty conduction band
- Insulators have a filled valence band
- There is a large energy gap between these bands
- Electrons will not be able to bridge this gap even with the
presence of an intense potential difference (no conduction)
Semi-conductors (The middle ground)
- Example: Silicon
- The energy gap is much smaller
- Thus, with the addition of heat (energy), some electrons
at the top end of the valence band will fill states in the
conduction band
Therefore we find…
- This leaves a number of ‘holes’ in the previously filled
valence band into which other electrons can move. This
movement of electrons constitutes a current
- As the electrons move in the one direction it looks as if the
gap from the missing electron moves in the opposite direction.
This is what we call a ‘hole’ and it can be considered a positive
charge carrier
- The excited electrons in the conduction band are free to move
and therefore also constitute a current
Two categories of semi-conductors
1) Intrinsic Semi-conductors
 Example: Si or Ge
 Very pure
 Charge carriers originate from the atoms
of the semi-conductor
2) Extrinsic Semi-conductors
 Material is ‘doped’ with small amounts of
impurities to increase the number of
charge carriers
 The majority of the charge carriers originate
from the atoms of the impurity
 Example: Si doped with P
Two types of Extrinsic Semi-conductors
n-type material
- This is made by doping the semi-conductor material with an element that has
1 more electron than the atoms of the semi-conductor
- The extra electron will not be present in a bond and will thus be able to drift
through the material
- Example: Si doped with P
- The impurity is known as the “Donor” because it donates an extra electron to the
crystal lattice
p-type material
- This is made by doping a semi-conductor with an element that has 1 less electron
than the atoms of the material
- This leaves a gap or a “hole” in the lattice thus increasing the number of positive
charge carriers
- Electrons from other bonds can fill this hole, but this will result in there being a new
hole. This process continues giving rise to what can be considered a movement of
positive charge carriers
semiconductor animations
http://www.youtube.com/watch?v=MCe1JXaLEwQ
The p-n Junction
- The p-n junction is created by combining an n-type material with a p-type
- Initially both materials are neutral
- When they come into contact, electrons from the n-type material will move into the
holes of the p-type material (This doesn’t happen the other way though, because the
electrons in the p-type material are at a lower energy than the holes of the n-type material)
- This creates a region known as the “depletion layer”
- An electric field is created, in 1 direction, as a result of the charge separation
http://www.wainet.ne.jp/~yuasa/flash/EngPnJunction.swf
Biasing of the p-n junction
Forward bias – Current can flow
 Connect the positive terminal of the battery to the .
p-type material and the negative terminal to the n-type.
 The free electrons in the n-type are repelled by the
negative terminal while the holes of the p-type are repelled
by the positive terminal.
 These meet at the junction, with electrons filling the holes
 Once the free electrons have been exhausted, the electrons
from the circuit fill the holes, while the holes that they create
are filled by others… current flows!
Reverse bias – No current flows
- Connect the positive terminal to the n-type and the negative
terminal to the p-type.
- The free electrons in the n-type are attracted towards the
positive terminal, while the holes of the p-type are attracted
towards the negative terminal.
- The electrons and holes thus move AWAY from the junction
and therefore away from each other… current cannot flow!
Everyday applications:
LED’s, solar cells, diodes on circuit boards (electronics)
Conduction in ionic solutions
- In liquids, many positively and negatively charged
particles (ions) are responsible for the conduction of
electricity
- An ionic solid (ie. A solid that is made up of ions,
regularly arranged in a crystal lattice) is placed
into solution. The solid dissolves, causing the ions
to be released into the solvent (eg. Water)
- The free moving ions act as charge carriers,
causing the solution to become a conductor
known as an electrolyte
Addapted from: http://www.rfcafe.com
- Rods (called electrodes) are attached to either
side of a cell/battery. The positive ions in solution
are attracted to the negative electrode and the
negative ions to the positive electrode
- The movement of these charges, between the
electrodes creates an electric current
Question 1
Explain, using labelled diagrams of the relevant band structures, why an insulator
cannot conduct electricity, while a conductor can.
(5)
Question 2
The conductivity of a semi-conductor increases as the temperature is increased.
Draw a temperature versus conductivity graph to indicate this relationship
Explain this phenomenon using a fully labelled diagram
(5)
Question 3
Explain the difference between an intrinsic and extrinsic semiconductor
(2)
Question 4
Classify the following semiconductors as p-type or n-type…
a) Silicon doped with aluminium
b) Germanium doped with antimony
c) Carbon doped with boron
(3)
Question 5
Explain, using a labelled diagram how one would forward bias a p-n junction.
Also explain how it is possible for current to flow when the junction has been
forward biased
(5)
[20]