| Glossary for Revision |
| https://quizlet.com/90855867/122-conductors-semiconductors-and-insulators-flash-cards/ |
| Conductors |
Conductivity is the ability of a materials to conduct charge carriers (electrons or positive holes) (all metals, semi metals like carbon-graphite, antimony and arsenic) |
| Insulators |
Materials that have very few charge carriers (free electrons or positive holes). (plastic, glass and wood) |
| Semiconductors |
These materials lie between the extremes of good conductors and good insulators. They are crystalline materials that are insulators when pure but will conduct when an impurity is added and/or in response to light, heat, voltage, etc (silicon (Si), germanium (Ge), gallium arsenide (GaAs) |
| Band structure |
Electrons in an isolated atom occupy discrete energy levels. When atoms are close to each other these electrons can use the energy levels of their neighbours. When the atoms are all regularly arranged in a 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, 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 |
| In a conductor: |
the highest occupied band, known as the conduction band, is not completely full. This allows the electrons to move in and out from neighbouring atoms and therefore conduct easily |
| In an insulator: |
the highest occupied band is full. This is called the valnce band, by analogy with the valence electrons of an individual atom. The first unfilled band above the valence band above the valence band is the conduction band. For an insulator the gap between the valence and conduction bands 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. Normally, there is almost no electrical conduction in an insulator. 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. Often this flow of current causes permanent damage. Within a gas this voltage is often referred to as the striking voltage, particularly within the context of a fluorescent lamp since this is the voltage at which the gas will start to conduct and the lamp will light. |
| In a semiconductor: |
the gap between the valence band and the 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. An increase in temperature increases the conductivity of the semiconductor as more electrons have enough energy to make the jump to the conduction band. This is the basis of an NTC thermistor. NTC stands for “negative temperature coefficient” (increased temperature means reduced resistance). This makes current increase so conductivity increases. |
| Optical properties of materials |
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 the solid the atoms are close enough together to form continuous bands. The exact energies available in these bands also control at which frequencies a material will absorb or transmit and therefore what colour will appear |
| Bonding in semiconductors |
The most commonly used semiconductors are silicon and germanium. Both these materials have a valency of 4 (they have 4 outer electrons available for bonding. In a pure crystal, each atom is bonded covalently to another 4 atoms: all of its outer electrons are bonded and therefore there are few free electrons available to conduct. This makes resistance very large. Such pure crystals are known as intrinsic semiconductors. 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. Even though electrons are moving, the effect is the same as if it was the hole that moved through the crystal lattice. The hole can be thought of as a positive charge carrier. In complex semiconductors it is easier to calculate what is happening in terms of 1 moving positive hole, rather than many electrons |
| In an intrinsic semiconductor the number of holes is equal to |
the number of electrons. The generally small currents consist of drifting electrons in 1 direction and drifting holes in the other. |
| Doping |
Semiconductor’s electrical properties are dramatically changed by the addition of very small amounts of impurities. Once doped they are known as extrinsic semiconductors. Solid state semiconductors are much smaller and use much less power than valve transistors. |
| Doping |
Doping a semiconductor involves growing impurities such as boron or arsenic into an intrinsic semiconductor such as silicon |
| An in intrinsic semiconductor is |
an undoped semiconductor |
| Fermi level |
Energy of last occupied level by an electron, below this energy are completely occupied and above it are completely unoccupied |
| N-type semiconductors |
If an impurity such as arsenic with 5 outer electrons is present in the crystal lattice then 4 of its electrons will be used in bonding with the silicon. The 5th 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. Because of the extra electrons present, the Fermi level is closer to the conduction band than in an intrinsic semiconductor. This type of conductor is called n – type, since most conduction is by the movement of free electrons (-ve) |
| P-type semiconductors |
The semiconductor may also be doped with an element like Indium, which has 3 outer electrons. This produces a hole in the crystal lattice, where an electron is “missing”. Because of this lack of electrons, the Fermi level is closer to the valence band than in an intrinsic semiconductor. 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. Most conduction takes place by the movement of positively charged holes |
| Notes on doping |
The doping material cannot be added to the semiconductor crystal. It has to be grown into the lattice when the crystal is grown so that it becomes part of the atomic lattice. Overall charge on semiconductors are still neutral The quantity of the impurity is extremely small (could be 1 atom in 1 million). If it were too large it would disturb the regular crystal lattice. |
| minority charge carriers |
In n – type and p – type there will always be small numbers of the other type of charge carrier, known as minority charge carriers, due to thermal ionisation. |
| p-n junctions |
When a semiconductor is grown so that 1 half is p-type and 1 half is n-type, the product is called a p-n junction and it functions as a diode. A diode is a discrete component that allows current to flow in one direction only. |
| At temperatures other than absolute Zero kelvin, the electrons in the n-type and the holes in the p-type material will constantly |
diffuse(particles will spread from high concentration regions to low concentration regions). Those near the junction will be able to diffuse across it. |
| Reverse-biased |
Cell connected negative end to p-type and positive end to n-type |
| Forward-biased |
Cell connected positive end to p-type and negative end to n-type. |
| Reverse biased – charge carriers |
When the p-side is attached to the negative side of a battery (V↓a) 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. Almost no conduction can take place since the battery is trying to make electrons flow “up the slope” of the difference in 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 the thermal ionisation to make it up the barrier. |
| Forward biased – charge carriers |
When the p-side is attached to the positive side of the battery (V↓a) then the electrons at that side have less potential energy than under no bias. This has the effect of lowering the bands on the p-side from where they were originally. We sat it is forward biased. As the applied voltage approaches the built in voltage, 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 has 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. |
| In the junction region of a forward-biased LED |
electrons move from the conduction band to the valence band to emit photons. |
| 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 to hole, 1 photon of radiation is emitted. Most of the time heat energy is released but in some semiconductors like gallium arsenic phosphide, 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 an LED. |
| The colour of light emitted from an LED depends on |
On the elements and relative quantities of the three constituent materials. The higher the recombination energy the higher the frequency of light. |
| The LED does not work in reverse bias since the charge carriers |
do not/can not travel across the junction towards each other so cannot recombine |
| Photodiode |
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 material 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 current to flow (it is the emf in the circuit). More intense light (more photons) will lead to more electron-hole pairs being produced and therefore a higher current. Current is proportional to light intensity. |
| Photodiode |
The incoming light provides energy for an electron within the valance band of the p-type to be removed from a positive hole and moved up to the conduction band in the n-type material. As this electron is moved up into the conduction band it has an increase in energy. Since EMF is the energy per coulomb of charge an EMF is generated. |
| Photovoltaic mode |
The p-n junction can supply power to a load (motor). Many photo-diodes connected together form a solar cell. This is described as photovoltaic mode. There is no bias applied to a solar cell and it acts like an LED in reverse. The increased movement of charge across a p-n junction can reduce resistance of component containing the junction. |
| Photoconductive mode |
When connected to a power supply a photodiode will act as a LDR. This is described as photoconductive mode. The LDR is connected in reverse bias, which leads to a large depletion region. When light hits the junction, electrons and holes are split apart. This leads to free charge carriers in the depletion region. The free charge carriers reduce overall resistance of the diode, allowing current to flow. Conductivity of diode is being changed. |
| Addition of impurity atoms to a pure semiconductor(doping) decreases its |
Resistance |
| Applications of p-n junctions |
Photovoltaic cell /LED /Photoconductive mode(LDR) |
| What is photovoltaic effect? |
A process in which a photovoltaic cell converts photons of light into electricity. |
| Depletion layer |
Near the junction, electrons diffuse across to combine with holes, creating a “depletion region”. |
| Majority charge carriers in n-type |
The electrons in the conduction band are free move towards the positive terminal of an applied p.d. |
| Majority charge carriers in p-type |
The “positive holes” in the valance band move towards the negative terminal of an applied p.d. |
| Majority charge carriers across the p-n junction (forward biased) |
With the applied p.d. in the direction shown electrons in the n-type material move to the left and holes in the p-type material move to the right. The depletion layer in the centre becomes thinner and thinner and if the p.d. of the supply is greater than the barrier potential(0.7 V for silicon-based semiconductors) the barrier is broken down and a current flows through the device. |
| How light is produced at the p-n junction of an LED |
When the diode is forward biased the free electrons in the conduction band of the n-type material are given energy by the supply to overcome the energy barrier generated by the depletion layer at the junction. Once these electrons overcome the energy barrier they drop down from the conduction band to the valance band of the p-type material and combine with a positive hole in the valance band of the p-type material. As the electron drops between the bands it loses and energy and emits this as light. |
| Use band theory to explain how electrical conduction takes place in a pure semiconductor such as silicon. |
Your explanation should include the terms: electrons, valence band and conduction band. most/majority of electrons in valance band or “fewer electrons in conduction band” band gap is small electrons are excited to conduction band charge can flow when electrons are in conduction band |
| What charge carriers actually move across the p-n junction? |
Electrons |
and it is sometimes referred to as average deviation or absolute uncertainty.
Remind you of anyone?
