9.4.3 2 (i) Identify that some electrons in solids are shared between atoms and move freely There are three main ways in which matter is held together. They all involve the valence or outer shell electrons. These are:- IONIC BONDING This is where atoms are held together by electrons transferring from one atom to another. This leaves two ions appositively charged that are attracted to another. COVALENT BONDING This is where sets of electrons are shared between two atoms. This sharing relationship keeps the two atoms joined together. In both IONIC and COVALENT bonding the electrons are locked up in the boding and are localized to a single or a couple of atoms. They cannot move around the solid. METALLIC BONDING In the third main type of bonding the electrons are delocalized and are free to wander through a lattice of positive ions. As the delocalized electrons move through the positive lattice they are attracted to it and hold the lattice together like a glue. The valence electrons in metals are known as FREE ELECTRONS or delocalized electrons and are able to move freely. 9.4.3-2 (ii) Compare qualitatively the relative number of free electrons that can drift from atom to atom in conductors, semiconductors and insulators Type of material Numbers of FREE electrons Resitsivity/ resitance Insulator LOW HIGH Semi-Conductor MEDIUM MEDIUM Conductor HIGH LOW Because of the bonding in metals they are excellent conductors as the boding allows the valence electrons to be mobile and as a result they have a large number of free electrons wehen compared to semi conductors and insulators.
9.4.3-2 (iii) Drscribe the difference between conductors, insulators and semiconductors in terms of band structures and relative electrical resistance From this data it can be seen that conductors have resistances typically in the order of 10 4 10 5 (10000 100000) times smaller than semiconductors and around 10 20 times less than insulators. Band Theory A useful way to visualise the difference between conductors, insulators and semiconductors is to plot the available energies for electrons in the materials. Instead of having certain energies as in the case of free atoms, the electrons have a range of energies described as energy bands. The valence band is the highest range of electron energies preset in a solid. The conduction band is the range of electron energies enough to free an electron from binding with its atom to move freely within the atomic lattice of the material. Conductors The conduction band and valence band overlap. This means that there are some electrons in a conductor that already have enough energy to free themselves from their parent atom and move freely through the solid Semiconductors There is a small energy gap between the valence band and conduction band. This means that the valence electron have to absorb a certain amount of energy before they are able to break free from their parent atom and move freely. Insulators There is a large energy gap between the valence band and conduction band. This means that the valence electron have to absorb a large amount of energy before they are able to break free from their parent atom and move freely. NB. Band theory also states that an electron cannot absorb an amount of energy that would place it in the energy gap.
9.4.3-2(iv) identify absences of electrons in a nearly full band as holes, and recognise that both electrons and holes help to carry current Semiconductors & Conductivity Semiconductors such as Germanium & Silicon have ½ full outer shell. This means that they have 4 out of a possible 8 electrons in their outer shell. These 4 electrons are shared with 4 neighbouring atoms in covalent bonding. This bonding results in very few electrons in the conduction band at room temperature If the semiconductor is heated more electrons are boosted up into the conduction band and are free to move to another atom. When an electron is boosted up into the conduction band it leaves a hole behind. A positive charge is associated with this hole. Another free electron traveling in the conduction band may fall into this hole if it happens to loose sufficient energy. If a potential difference is applied across the semiconductor energetic free electrons are encouraged to leave their parent atoms and migrate towards the positive terminal until they fall into a hole left by a migrating electron in front of it. The net result of this movement is o The movement of electrons towards the positive terminal o The movement of positive holes towards the negative terminal. This mechanism of electrical conductance is outlined below
Electron movement Positive Hole Movement
9.4.2-2(vi) describe how doping a semiconductor can change its electrical properties 9.4.2-2(vii) identify differences in p and n-type semiconductors in terms of the relative number of negative charge carriers and positive holes Doping The electrical conductivity of semiconductors can be manipulated by introducing other atoms into its crystalline structure. This vastly increases the number of free electrons or holes in a semiconductor. n-type Semiconductors In this type Silicon is doped with an element from group 5 (on the periodic table) Group 5 elements have 5 electrons in the outer shell. When the doped element covalently bonds with neighbouring Si atoms it uses 4 of the 5 electrons in the bonding & the one left over is free to carry current. This creates a free eelctron for every doped atom. Current is facilitated by mobile negative charge carriers (electrons) and hence this type is called n-type.
p-type Semiconductors In this type Silicon is doped with an element from group 3 Group 3 elements have 3 electrons in the outer shell. When the doped element covalently bonds with Si it only has 3 of the required 4 electrons to bond with the neighbouring Si atoms. This creates a hole for every doped atom. Current is facilitated by mobile positive holes and hence this type is called p-type. NB The conductivity of a semiconductor can be manipulated by controlling the amount of group 3/5 element allowed to be doped into the semiconductor.
The diagram to the right identifies the important elements for semiconductors. Silicon and Germanium, from group IV, being the materials used for producing semiconductors. The Group III elements of Boron and Gallium are used as Dopants to produce p- type semiconductors where the Group V elements of Arsenic and Phosphorous are used as dopants to produce n-type semiconductors. 9.4.2-2(v) identify that the use of germanium in early transistors is related to lack of ability to produce other materials of suitable purity Germanium was the first Group 4 element that could be sufficiently purified to behave as a semiconductor. Germanium as an element is relatively rare in the Earth s crust. It is never found in an uncombined form in nature, existing only as a compound. Early diodes and transistors were made from germanium because suitable industrial techniques were developed to purify the germanium to the ultrapure level required for semiconductors during World War II. Germanium has one major problem when used in electronic components: it becomes a relatively good conductor when it gets too hot. The conductivity level means that hot germanium electronic components allow too much electric current to pass through them. This can damage the electronic equipment and cause it to fail to perform the task for which it was designed. The problem is that the resistance to electric current flow that makes the semiconductor useful in electronic components also generates heat. Silicon was the other element with semiconducting properties that was predicted to be ideal for the production of electronic components. Unlike germanium, silicon is very common in the Earth s crust. Like germanium, silicon never appears as a free element in nature. Silicon is always combined into chemical compounds so it has to be purified before it can be used in the production of semiconductors. Almost every grain of sand you see is made of silicon dioxide, so silicon as a raw material is far more plentiful than the rare germanium. The problem with using silicon in electronic components is that it is more difficult to purify. However, silicon makes the most useful semiconductors for electronics. It is affected less by higher temperatures in terms of maintaining its performance level.
The first silicon transistors were made in 1957. After the production of those first silicon transistors, the germanium transistors were largely phased out of production, except for specialised applications. From the 1960s onwards, silicon became the material of choice for making solid state devices. It is much more abundant than germanium and retains its semiconducting properties at higher temperatures. Silicon is the preferred material for manufacturing solid-state devices such as transistors, integrated circuits, solid-state memories and so on because: It is more abundant than germanium, being the second most abundant element in the earth s crust; It continues working as a semiconductor at higher temperatures than does germanium. When they are heated up, germanium transistors produce too many free electrons. It develops a glassy, insulating oxide layer when heated to high temperatures in an oxygen-rich atmosphere. This is part of the manufacturing process of integrated circuits. 9.4.2-2(viii) describe differences between solid state and thermionic devices and discuss why solid state devices replaced thermionic devices Function Thermionic Device. Solid State Equivalent Consist of an evacuated glass tube containing electrodes. Thermionic emission (ejection of electrons through heating) occurs at one of the electrodes. There is a limit to how small they can be made. Rectification of current. Allow current flow in one direction An arrangement of p and type semiconductors. There is no space inside. Can be made extremely small. Amplification of Current
Thermionic devices, such as radio valves and amplifiers, cannot match the efficiency, cost or reliability of solid state devices. Appliances utilising valves had a number of disadvantages compared with devices utilising solid state devices. Thermionic devices and appliances were bulky. Even radios advertised as portable could be lifted only with difficulty by a child. So much power was required that batteries had to be large or numerous. A twelve, D-sized battery radio that was considered portable was not unusual. A large amount of heat was developed by the valves. This required engineering solutions to protect surrounding electronics. Valves are fragile. Like a light globe, there is a seal between the evacuated glass tube and the Bakelite (an early plastic) base through which the leads pass from internal connections to the pins in the base. This meant radios or tape recorders could not be carried as easily, or treated as roughly as modern tape recorders or portable CD players. The cathode was coated with a metal that released large numbers of electrons. The heat that was produced slowly boiled off the metal coating and the coating reacted with the traces of gas present in the tube. Valves had a relatively short lifetime. Technicians would start testing a malfunctioning appliance by testing the valves, even replacing them to see if the fault disappeared. Solid state devices are now among the least likely faults. One of the original uses for the valve was in telephone exchanges. As telephone networks began to expand rapidly in the late 1940s and 1950s, the unreliability of the valve began to be intolerable. Individual sockets and valves were mounted on a metal chassis. Components were connected by insulated wires to other discrete components. There was often movement between the chassis and sockets, leading to broken solder joints. The glass envelopes of the valve were fragile and the seals frequently broke, allowing air into the valve and destroying it. High voltages were required to correctly bias the triodes to amplify signals. This is in contrast to a silicon transistor that requires around 0.6 V to do the same job.