ELECTRONIC PROPERTIES OF MATTER
Download
Report
Transcript 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]