ELECTRONIC PROPERTIES OF MATTER

- Semi-conductors and the p-n junction -

Recap of previous years:

Insulators - High resistance to current flow (no free charge carriers) Conductors - 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 animationshttp://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 junctionForward 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

Addapted from: http://www.rfcafe.com

- 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

- 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 1Explain, using labelled diagrams of the relevant band structures, why an insulatorcannot conduct electricity, while a conductor can. (5)

Question 2The conductivity of a semi-conductor increases as the temperature is increased. Draw a temperature versus conductivity graph to indicate this relationshipExplain this phenomenon using a fully labelled diagram (5)

Question 3Explain 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 5Explain, 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)

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