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 oppositedirection. 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

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

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 Addapted from: - 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 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]