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Electrons at Work SECTION 2. ELECTRONS AT WORK 2a) Conductors, semi-conductors and insulators Learning content: State that solids can be categorised into conductors, semi-conductors or insulators by their ability to conduct electricity. Give examples of each type Classifying materials By considering their electrical properties, we can divide materials into three groups: Materials with many free electrons. These elections can Conductors easily be made to flow through the material. For example, all metals, semi-metals like graphite, antimony and arsenic. Materials which are insulators when pure, but will conduct Semiconductors when an impurity is added and/or in response to light, heat, voltage, etc. For example, elements like silicon (Si), germanium (Ge), selenium (Se); compounds like gallium arsenide (GaAs) and indium antimonide (InSb). Materials that have very few free electrons, which cannot Insulators move easily. For example, plastic, glass and wood. The atomic model Learning Content: State that electrons in a free atom occupy discrete energy levels. The simple model of the atom contains a positive nucleus surrounded by negative electrons. Each electron can have an energy determined by the energy levels of the particular atom. The electrons in an isolated or free atom occupy discrete energy levels. However, when atoms are close to each other the electrons can also use the energy levels of the neighbouring atoms. Electrons are constantly in random motion due to the heat energy that they gain from their warm surroundings. This is called thermal agitation. This thermal energy allows some electrons to move from a lower energy level to a higher one. Learning Content: State that when the atoms come together to form solids, the electrons then become contained in energy bands separated by gaps. When the atoms are all regularly arranged in what is called the crystal lattice of a solid, the energy levels become grouped together in a band. This is a continuous range of allowed energies rather than a single level. There will also be groups of energies that are not allowed, in what is known as a band gap. Similar to the energy levels of an individual atom, the electrons will fill the lower bands first. The Fermi level gives a rough idea of which levels electrons will generally fill up to, but there will always be some electrons with individual energies above this. When atoms combine to form a solid the electrons align themselves into energy bands. The two highest energy bands are known as the valence band and the conduction band (see diagrams on the following pages). The conduction band is the higher of the energy bands. The electrons are held tightly in the valence band, but are able to move freely in the conduction band. So if there are electrons in the conduction band the solid will conduct. If there are no electrons in the conduction band, the solid will not conduct. Electrons at Work Energy Band Diagrams Simple diagrams can be used to explain the difference between the three types of solid. Conductor Learning content: State that in metals which are good conductors, the highest occupied band is not completely full and this allows the electrons to move and therefore conduct. This is the conduction band. Conduction band Valence band In a conductor, the valence band is not full which means that the conduction band overlaps it. Thermal agitation of electrons boosts electrons from the valence band into the conduction band. As a result, there are many electrons in the conduction band which are able to move. When an e.m.f. is applied to the solid, these are the electrons that flow to produce the current. Insulator Learning Content: State that in an insulator the highest occupied band (called the valence band) is full. For an insulator the gap between the valence band and the conduction band is large and at room temperature there is not enough energy available to move electrons from the valence band into the conduction band where they would be able to contribute to conduction. Conduction band In an insulator the valence band is full and there is a large energy gap to the next available energy level in the conduction band. The gap is so large that even with thermal agitation, the electrons almost never cross the gap and the solid never conducts. The electrons remain in the valence band. Valence band If the applied voltage is high enough (beyond the breakdown voltage) sufficient electrons can be lifted to the conduction band to allow current to flow (e.g. lightning in air). Often this flow of current causes permanent damage. Within a gas this voltage is often referred to as the striking voltage. In a fluorescent lamp, e.g. a neon lamp, this is the voltage at which the gas will start to conduct and the lamp will light. Semi-Conductor Conduction band Valence band Learning Content: State that in a semi-conductor the gap between the valence band and conduction band is smaller and at room temperature there is sufficient energy available to move some electrons from the valence band into the conduction band allowing some conduction to take place. Semiconductors are like insulators as their valence band is full. However the gap between the two bands is relatively small and at room temperature some electrons have enough energy to jump the gap. The solid will conduct to some extent. Electrons at Work The Fermi Level If an atom is cooled to absolute zero temperature (0 K) the thermal energy available to its electrons is zero. If all its electrons were removed and replaced one by one, each electron would occupy the lowest available energy level at the time. Since electrons cannot occupy the same level, the electrons would fill up the atom from the bottom up. The Fermi Level is the name given to the highest occupied energy level of the electron in the valence band. This would be occupied by the last electron to be replaced. Electron energy No overlap Conduction band Fermi level Conduction band Band overlap Valence band Conductor Conduction band Band gap Valence band Valence band Semiconductor Insulator In a conductor, there is no energy gap between the top Fermi level of the valence band and the lowest energy level of the conduction band. At normal room temperature, there is some thermal energy available to the electrons. Effectively this means that the valence band and the conduction bands overlap. In contrast, for semi-conductor there is a small energy band gap, and for an insulator there is a large energy band gap. The Effect of temperature on a semi-conductor Learning Content: State that an increase in temperature increases the conductivity of a semi-conductor. Conduction band Valence band Thermal agitation of electrons increases with temperature. As the temperature increases, some electrons in the valence band gain enough thermal energy to boost them up to the conduction band. This causes the number of conduction electrons to increase and so the semiconductor conducts better. In an insulator, the energy gap is still much greater than the energy which can be gained from thermal agitation so very few electrons are available to conduct. Note: Electron bands also control the optical properties of materials. They explain why a hot solid can emit a continuous spectrum rather than a discrete spectrum as emitted by a hot gas. In a solid, the atoms are close enough together to form continuous bands. The exact energies of these bands determine which frequencies a material will absorb or transmit and therefore what colour it emits. Electrons at Work Section 2b) p-n junctions Manufacturing Semiconductors The electrical properties of semiconductors make them very important in electronic devices like transistors, diodes and light-dependent resistors (LDRs). In such devices the electrical properties are dramatically changed by the addition of very small amounts of impurities. The process of adding impurities to these semiconductors is known as doping. The development of doped semiconductors in the 1950s led to the invention of the transistor and the start of the ‘solid state’ revolution that transformed the whole face of electronics. Bonding in semiconductors The most commonly used semiconductors are silicon and germanium. Both these materials have a valency of four, that is they have four outer electrons available for bonding. In a pure crystal, each atom is bonded covalently to another four atoms. All of its outer electrons are bonded and therefore there are few free electrons available to conduct. This makes the resistance very large. The few electrons that are available come from imperfections in the crystal lattice and thermal ionisation due to heating. A higher temperature will thus result in more free electrons, increasing the conductivity and decreasing the resistance, as in a thermistor. Holes When an electron leaves its position in the crystal lattice, there is a space left behind that is positively charged. This lack of an electron is called a positive hole. This hole may be filled by an electron from a neighbouring atom, which will in turn leave a hole there. Although it is technically the electron that moves, the effect is the same as if it was the hole that moved through the crystal lattice. The hole can then be thought of as the charge carrier. In an undoped semiconductor, the number of holes is equal to the number of electrons. Current consists of drifting electrons in one direction and drifting holes in the other. Electrons at Work Doping Learning Content: State that the addition of impurity atoms to a pure semiconductor (a process called doping) decreases its resistance. If an impurity such as arsenic (As), which has five outer electrons, is present in the crystal lattice, then four of its electrons will be used in bonding with the silicon. The fifth will be free to move about and conduct. Since the ability of the crystal to conduct is increased, the resistance of the semiconductor is therefore reduced. The addition of an impurity like this is called doping. n-type semiconductors Learning Content: Explain how doping can form an n-type semiconductor in which the majority of the charge carriers are negative, or a p-type semiconductor in which the majority of the charge carriers are positive. This type of semiconductor is called n-type, since most conduction is by the movement of free electrons, which are, of course, negatively charged. p-type semiconductors Learning Content: Explain how doping can form a p-type semiconductor in which the majority of the charge carriers are positive. The semiconductor may also be doped with an element like indium (In), which has only three outer electrons. This produces a hole in the crystal lattice, where an electron is ‘missing’. An electron from the next atom can move into the hole created as described previously. Conduction can thus take place by the movement of positive holes. This is called a p-type semiconductor, as most conduction takes place by the movement of positively charged ‘holes’. Electrons at Work Additional Notes on doping The doping material is not simply added to the semiconductor crystal. It has to be grown into the lattice when the crystal is made so that it becomes part of the atomic lattice. The quantity of impurity is extremely small; it may be as low as one atom in a million. If it were too large it would disturb the regular crystal lattice. Although p-type and n-type semiconductors have different charge carriers, they are still both overall neutral (just as metal can conduct but is normally neutral). In terms of band structure we can represent the electrons as dots in the conduction band, and holes as circles in the valence band. The majority charge carriers are electrons in n-type and holes in p-type, respectively. In an intrinsic semiconductor (one which has not been doped) the creation of an electronhole pair means that every electron which is boosted to the conduction band leaves behind a hole in the valence band. This is shown below in (a). Electron energy Conduction band Conduction band Conduction band Fermi level increased Fermi level Fermi level decreased Valence band Valence band Valence band (a) Intrinsic semiconductor (b) n-type semiconductor (c) p-type semiconductor Figure (b) shows an n-type semiconductor. The addition of Group 5 “donor” atoms into the semiconductor introduces electrons in new energy levels which are just below the conduction band. This raises the Fermi level. It takes much less energy to boost these electrons into the conduction band. As these are electrons from group 5 atoms, when they move into the conduction band, they do not leave a hole behind in the valence band. At higher temperatures, a few electrons are boosted directly from the valence band up to the conduction band, creating some holes in the valence band which are free to move. Holes are the minority charge carriers in an n-type material. In a p-type semiconductor (c) the addition of Group 3 acceptor atoms lowers the Fermi level. However, because of the impurities, there are now stable energy levels for the valence electrons to fill which are much closer than the original conduction band. Valance electrons can move into this level, leaving behind a hole in the valence band. This hole is the majority charge carrier and is free to move. As before, at higher temperatures, some electrons are boosted directly up to the conduction band. These electrons are free to move and are the minority charge carriers in the p-type material. Electrons at Work P-N JUNCTIONS Learning content: When p-type and n-type material are joined, a layer is formed at the junction. The electrical properties of this layer are used in a number of devices. When a semiconductor is grown so that one half is p-type and the other half is n-type, the product is called a p–n junction and it functions as a diode. Thermal agitation means that at temperatures other than absolute zero, the electrons in the n-type material and the holes in the p-type material will constantly diffuse. Diffusion is the spread of any particle through random motion from high concentration regions to low concentration regions. Electrons in the n-type material near the junction will be able to diffuse across it. Electron energy p-n junction Conduction band Fermi level Fermi level flat as electron drift balances diffusion - + - + Filled sites where there is an excess of charge eVi Diffusion Valence band p-type - + Drift + n-type Excess of n-type electrons diffuse across junction to fill holes on p-side which becomes – while n side becomes +. Any free electrons in junction drift back to ntype, holes drift back to p-type. When an electron meets a hole, they recombine, i.e. the electrons ‘fill in’ the holes. This is shown as a filled-site in the diagram above. This creates a charge imbalance: excess negative charge in the ptype region and excess positive in the n-type. This creates a slope in the conduction level which acts as a potential barrier (Vi ≈ 0.7 V for silicon) since it would require work of eVi to be done in order to get electrons to move against the barrier (e is the electron charge).1 The build up of charge on either side of the junction causes any free electrons/holes in the junction to drift back across the junction. Once this drift balances the diffusion in the opposite direction, equilibrium is reached and the Fermi level (where you are likely to find electrons) is flat across the junction. When no external voltage is applied to a p–n junction we refer to it as unbiased. 1 Any attempt to use this inbuilt voltage to do electrical work would prove futile, however, since in a complete circuit the curves in the conduction and valence bands would even out. Electrons at Work Biasing the diode To bias a semiconductor device means to apply a voltage to it. A diode may be biased in two ways. The forward-based diode When the p-side is attached to the positive side of a battery (Va = applied voltage) then the electrons at that side have less potential energy than under no bias. This lowers the Fermi level and the conduction bands on the p-side from where they were originally. We say it is forward biased. Electron energy Conduction band Junction e(Vi – Va) = W Fermi level W = eVa Valence band p-type n-type As the applied voltage (Va) approaches the built in voltage (Vi), more electrons will have sufficient energy to flow up the now smaller barrier and an appreciable current will be detected. Once the applied voltage reaches the in-built voltage there is no potential barrier and the p–n junction presents almost no resistance, like a conductor. The holes are similarly able to flow in the opposite direction across the junction towards the negative side of the battery. Electrons at Work The reverse-biased diode The applied voltage can either act against or with the in-built potential barrier. When the p-side is attached to the negative side of a battery (Va, the applied voltage is now negative) then the electrons at that side have more potential energy than previously. This has the effect of raising the bands on the p-side from where they were originally. We say it is reverse biased. Electron energy Conduction band p-n Junction e(Vi + Va) = W Fermi level W = eVa Valence band p-type n-type Almost no conduction can take place since the battery is trying to make electrons flow ‘up the slope’ of the difference in the conduction bands. The holes face a similar problem in flowing in the opposite direction. The tiny current that does flow is termed reverse leakage current and comes from the few electrons which have enough energy from thermal ionisation to make it up the barrier. Electrons at Work The light-emitting diode Learning Content: LEDs are p-n junctions which emit photons when a current is passed through the junction. We have seen that in a forward-biased p-n junction diode, holes and electrons pass through the junction in opposite directions. Sometimes holes and electrons will meet and recombine. When this happens, energy is emitted in the form of a photon. For each recombination of electron and hole, one photon of radiation is emitted. In most semiconductors this takes the form of heat, resulting in a temperature rise. In some semiconductors such as gallium arsenic phosphide, however, the energy is emitted as light. If the junction is close to the surface of the material, this light may be able to escape. This makes what we call a Light Emitting Diode (LED). The colour of the emitted light (red, yellow, green, blue) depends on the relative quantities of the three constituent materials. The recombination energy can be calculated using E = hf if the frequency of the light emitted is measured. The LED does not work in reverse bias since the charge carriers do not travel across the junction towards each other so cannot recombine. The photodiode A p-n junction in a transparent coating will react to light, producing electronhole pairs. This can be used to generate electricity. The photovoltaic mode Learning Content: Solar cells are p-n junctions designed so that a potential difference is produced when photons enter the layer. This is the photovoltaic effect. A p–n junction in a transparent coating will react to light in what is called the photovoltaic effect. Each individual photon that is incident on the junction has its energy absorbed, assuming the energy is larger than the band gap. In the p -type material this will create excess electrons in the conduction band and in the n -type it will create excess holes in the valence band. Some of these charge carriers will then diffuse to the junction and be swept across the built-in electric field of the junction. The light has supplied energy to the circuit, enabling a flow of current, ie it provides the emf in the circuit. More intense light (more photons) will lead to more electron–hole pairs being produced and therefore a higher current. In fact the current is proportional to the light intensity. Electrons at Work Energy Band diagram for the photodiode The band diagram below shows that Incoming photons boost electrons from the valence band of the p-type material to the conduction-band (top-left of the diagram). holes are created by the removal of electrons from the valence band of the n-type material (bottom right). These particles can then drift back across the junction due to the static charge that is present on either side of the p-n junction (+ on the n-side and – on the p-side!) Electron energy Incident photons Incident photons Conduction band Electron drift Fermi level Valence band Hole drift p-type n-type As a result a current flows in the circuit. The p–n junction can supply power to a load, eg a motor. Many photodiodes connected together form a solar cell. It is interesting to note that there is no bias applied to a solar cell and the photodiode therefore acts like an LED in reverse. END OF ELECTRICITY UNIT NOTES!