Presentation on theme: "Caroline Chisholm College Physics"— Presentation transcript:
1 Caroline Chisholm College Physics Identify that some electrons in solids are shared between atoms and move freelyCompare qualitatively the relative number of free electrons that can drift from atom to atom in conductors, semiconductors and insulatorsThe conductivity of a material depends on how easilyelectrons can move through the crystal latticeIf electrons are bonded strongly, the material is an insulatorIn conductors, valence electrons are free to move like a cloudThe presence of an electric field causes the derandomisation of this ‘cloud’ motionE fieldAn atom that doesn’t have the outer energy level filled will try to fill it by bonding -IONIC (gaining or giving electrons) or COVALENT (sharing electrons)
2 Caroline Chisholm College Describe the difference between conductors, insulators and semiconductors in terms of band structures and relative electrical resistanceCaroline Chisholm CollegePhysicsTableJacaranda p.226Atoms in a solid are close enough together for their outer energy levels (valence bands) to overlap. In a conductor, these valence bands are only partly filled and are the conduction bands. In an insulator, the valence bands are FULL and energy is needed to be input for an electron to reach the next level (conduction band).A semiconductor has a smaller energy gap than a conductor and a valence band which is almost fullPerform an investigation to model the difference between conductors, insulators and semiconductors in terms of band structuresPractical 12.1Jacaranda p.241Click here to do it now
3 Caroline Chisholm College Physics Identify absences of electrons in a nearly full band as holes, and recognise that both electrons and holes help to carry currentCaroline Chisholm CollegePhysicsIn a semiconductor, resistivity decreases with temperatureAt absolute zero, a semiconductor is an insulatorThis is because increasing temperature means that thermal energy causes electrons to jump the ‘gap’into the conduction band.This leaves holes in the valence band, which move opposite to, and slower than, the electron flow.Electrons are negative charge carriers in the conduction bandHoles are postive charge carriers in the valence bandPerform an investigation to demonstrate a model for explaining the behaviour of semiconductors, including the creation of a hole or positive charge on the atom that has lost the electron and the movement of electrons and holes in opposite directions when an electric field is applied across the semiconductorTry 'moving a hole' withsome Chinese Checkers!DO try thisat home!
4 Caroline Chisholm College Physics Identify that the use of germanium in early transistors is related to lack of ability to produce other materials of suitable purityMaking a semiconductorGroup 4 elements are the most widely used - they have 4 electrons in the valence band.The valence band is filled by sharing an electron with 4 adjacent atoms (covalent bonding)Germanium was used in early transistors because it was easy to purify.It is a good semiconductor but it conducts TOO well when hot. It is also rareExplain why silicon became the preferred raw material for transistorsSilicon has good semiconducting properties - thermal energy causes some of the valence electrons tojump the gap to the conduction band, leaving holes in the lattice.Although it is more difficult to purify, it is less affected by high temperatures than Germanium because it forms a protective oxide layer when heated, so is more suitable in electronics.It is very common - found in sand.
5 Caroline Chisholm College Physics Describe how ‘doping’ a semiconductor can change its electrical properties‘Dopant’ atoms in the semiconductor lattice can form extra energy levels in the gap - aiding conductionINTRINSIC SEMICONDUCTORS - natural semiconductor, doping not necessary. e.g. Si, GeEXTRINSIC SEMICONDUCTORS - modified semiconductor, doped to change conductivity. Dopants include P, BDoping is placing impurity atoms into the semiconductor crystal lattice to change the conductivity propertiesDESIGNER SEMICONDUCTORSIdentify differences in p and n-type semiconductors in terms of the relative number of negative charge carriers and positive holesp-type semiconductors have group 3 impurity atoms substitutedin the lattice of group 4 atomsn-type semiconductors have group 5 impurity atoms substitutedin the lattice of group 4 atomsFor each impurity atom, oneelectron moves to the nextlevel - - the conduction band. Thisarrangement has excess negativecharge so is called ‘n-type’For each impurity atom, oneelectron hole, or positive chargecarrier, is formed. Thisarrangement has positivecharge so is called ‘p-type’For each impurity atom, oneelectron moves to the nextlevel - - the conduction band. Thisarrangement has excess negativecharge so is called ‘n-type’ELECTRON HOLE!EXTRA ELECTRON!Electrons move into the holes, creating new holes that other electronsmove into, which makes other new holes etc.etc.....M. Edwards 15/7/02c