ELECTRONIC PROPERTIES OF MATTER

1. ELECTRONIC PROPERTIES OF MATTER - Semi-conductors and the p-n junction -

2. 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

3. 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.

4. 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)

5. 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

6. Two categories of semi-conductors • 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

7. 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

8. 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

9. 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)

10. 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

11. 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]