Caroline Chisholm College Physics Identify that some electrons in solids are shared between atoms and move freely Compare qualitatively the relative number of free electrons that can drift from atom to atom in conductors, semiconductors and insulators The conductivity of a material depends on how easily electrons can move through the crystal lattice If electrons are bonded strongly, the material is an insulator In conductors, valence electrons are free to move like a cloud The presence of an electric field causes the derandomisation of this ‘cloud’ motion E field An 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)

Caroline Chisholm College Describe the difference between conductors, insulators and semiconductors in terms of band structures and relative electrical resistance Caroline Chisholm College Physics Table Jacaranda p.226 Atoms 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 full Perform an investigation to model the difference between conductors, insulators and semiconductors in terms of band structures Practical 12.1 Jacaranda p.241 Click here to do it now

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 current Caroline Chisholm College Physics In a semiconductor, resistivity decreases with temperature At absolute zero, a semiconductor is an insulator This 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 band Holes are postive charge carriers in the valence band Perform 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 semiconductor Try 'moving a hole' with some Chinese Checkers! DO try this at home!

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 purity Making a semiconductor Group 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 rare Explain why silicon became the preferred raw material for transistors Silicon has good semiconducting properties - thermal energy causes some of the valence electrons to jump 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.

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 conduction INTRINSIC SEMICONDUCTORS - natural semiconductor, doping not necessary. e.g. Si, Ge EXTRINSIC SEMICONDUCTORS - modified semiconductor, doped to change conductivity. Dopants include P, B Doping is placing impurity atoms into the semiconductor crystal lattice to change the conductivity properties DESIGNER SEMICONDUCTORS Identify differences in p and n-type semiconductors in terms of the relative number of negative charge carriers and positive holes p-type semiconductors have group 3 impurity atoms substituted in the lattice of group 4 atoms n-type semiconductors have group 5 impurity atoms substituted in the lattice of group 4 atoms For each impurity atom, one electron moves to the next level - - the conduction band. This arrangement has excess negative charge so is called ‘n-type’ For each impurity atom, one electron hole, or positive charge carrier, is formed. This arrangement has positive charge so is called ‘p-type’ For each impurity atom, one electron moves to the next level - - the conduction band. This arrangement has excess negative charge so is called ‘n-type’ ELECTRON HOLE! EXTRA ELECTRON! Electrons move into the holes, creating new holes that other electrons move into, which makes other new holes etc.etc..... M. Edwards 15/7/02 c