From Ideas to Implementation. Core Module 9.4. Introduction. A new dawn in physics had begun at the start of the twentieth century… …it appeared that the stage had been reached when all the disparate elements of scientific knowledge seemed to be coming together…
Core Module 9.4
A new dawn in physics had begun at the start of the twentieth century…
…it appeared that the stage had been reached when all the disparate elements of scientific knowledge seemed to be coming together…
…many scientists felt that the total knowledge about the universe lay just beyond their horizon…
…future generations of scientists, it was believed, would have nothing to discover…
…this idea was about to be challenged.
However, early in the twentieth century, physics, or classical physics as it is now known was to be shaken to the core by two remarkable new insights about the universe.
The first of these insights was was made by Albert Einstein when he proposed his now famous Theory of Special Relativity in which he showed that space and time were interrelated.
These insights lead to the development of quantum physics, which was primarily developed to explain atomic phenomena and eventually proved to be more fundamental than classical physics.
By extending the laws of quantum physics to large masses, distances and times, it was shown that it was possible to derive the laws of classical physics.
We start with the discovery and identification of cathode rays and how this ultimately led to modern television.
We then look at how black body radiation and the photoelectric effect led to a new view of the nature of electromagnetic radiation with the quantum theory of Max Planck.
We conclude by looking at how the study of sub-atomic particles led to the invention of the transistor and to the discovery of superconductivity.
1. Increased understandings of cathode rays led to the development of television
By the 1850’s, much was known about which solids and liquids were electric conductors or insulators, and it was thought that all gases were electric insulators.
Plucker showed that as the pressure is reduced in the discharge tube (as it is now known), a series of changes progressively take place. We will examine these in the following first hand investigation:
Crooke’s dark space
Faraday’s dark space
As the pressure of the gas inside the tube was reduced, both the anode and cathode were surrounded by a luminous glow. As the pressure was reduced further, the positive glow extended about halfway along the tube (between the positive glow and the negative glow is an area called the Faraday dark space). At even lower pressure, the positive glow broke up into columns called striations. Lower still, an area called the Crooke’s dark space filled the entire tube and a green glow appeared in the glass at the end of the discharge tube opposite the cathode.
The discharge coming from the cathode had by now been named cathode rays.
The Crookes tubes had structures built into or around them to allow the cathode rays to be manipulated. For example, electrodes were built into the cathode ray tube to create an electric field to attempt to change the path of the cathode rays. Magnetic fields were applied to the cathode rays through the glass from outside the tube, and solid objects were placed inside the tube to block the path of the rays.
A fluorescent screen shows that cathode rays can cause fluorescence. This demonstrates that cathode rays have energy. A fluorescent screen can also be used to trace the path of cathode rays being manipulated by other means.
Pairs of electric plates cause the cathode rays to bend towards the positive plate. This shows that cathode rays have negative charges.
Note that Crookes believed that cathode rays could be deflected by an electric field but never succeeded in demonstrating this experimentally.
A lightweight glass paddle wheel, able to rotate freely, is placed in the path of the cathode rays. The cathode rays cause the wheel to spin and move away from the cathode. This demonstrates that the cathode rays have momentum, and therefore mass, and that they are emitted from the cathode.
Use the diagrams to show that the cathode rays are negatively charged
The controversy as to whether cathode rays were streams of charged particles or electromagnetic waves is a great example of the nature of scientific development. This particular debate raged for years.
The reason the debate took so long to clear up was that initially the cathode rays couldnotbe deflected by an electric field.
J.J. Thompson was able to make a cathode ray tube with sufficiently low pressure and using a very high voltage supply he was able to deflect the cathode rays with an electric field. This showed conclusively that cathode rays were beyond doubt streams of charged particles. We now call these charged particles electrons!
Before we look at J.J. Thompson’s experiment, we need to look at the effects of charged particles moving in electric and magnetic fields.
An electric field is said to exist in any region in which an electrically charged object experiences a force. An electric field has both strength and direction.
The electric field between two oppositely charged parallel plates is uniform in strength and direction. The field direction is at right angles to the plates and away from the positive plate.
The electric field due to charged parallel plates
It should be noted that the electric field is not uniform at the edges of the parallel plates:
Note: the syllabus expects you to be able to solve problems using this equation
E = V/d
The electric field strength, E, between two oppositely charged parallel plates is:
Where: E is electric field strength (V/m) V is the potential difference between the plates (V) d is the distance between the plates (m)
In a glass tube from which air has been removed, electrons are accelerated by a uniform electric field between a pair of charged parallel plates, separated by 5 cm. If an electrical potential difference of 4 kV is applied across the plates, determine the electric field strength half way between the plates.
This is a trick question. The field is UNIFORM, hence the field strength is the same in all areas between the plates:
E = V/d
E = 4000 / 0.05
E = 8 x 104 V/m (N/C)
F = qE
A charged object placed in an electric field will experience a force. The magnitude of the force acting on this charged object is given by:
Where: F is the force acting on the charged object (N) q is the magnitude of the charge of the object (C) E is the electric field strength (N/C or V/m)
Moving charges in magnetic field experience a force
As we saw in the topic ‘Motors and Generators’, a magnetic field exerts a force on electric currents – that is, on moving charged particles.
The direction of the force can be determined using the right hand palm rule. Remember that current is in the opposite direction to the flow of electrons.
The magnitude of the force is a maximum when the charge moves perpendicular to the magnetic field:
Can you explain why?
The force, F, on a charge moving through a magnetic field is:
Note: the syllabus expects you to be able to solve problems using this equation
F is the force on the charged object (N)q is the charge of the object (C) v is the velocity of the charged object (m s-1)B is the magnetic field flux density (T)
q is the angle between the velocity and the magnetic field.
The path of a helium nucleus, travelling at 3.0x103 m s-1, makes an angle of 90oto a magnetic field. The electron experiences a force of 1.2x10-15 N while in the field. Calculate the strength of the field.Solution:
A charge of +5.25 mC moving with a velocity of 300 m/s north east, enters a uniform magnetic field of 0.310 T directed from left to right across the page. Calculate the magnetic force acting on the charge.
With the knowledge of how charges are influenced by electric and magnetic fields, we can return to the cathode ray controversy.
Remember that it was J.J Thomson who finally established the true nature of cathode rays – they were negatively charged particles.
In order to do so he carried out an astonishing experiment. He subjected beams of cathode rays to deflection by electric and magnetic fields set at right angles to each other. By doing so, he was able to measure the charge to mass ratioof cathode rays.
Thomson manipulated the strengths of the two fields until the beam passed through both fields undeflected. This occurred when the two forces on the particles in the beam were equal and opposite. By equating the expressions for these two forces, Thomson calculated the velocity of the particles.
FB = FE
therefore, qvB = qE
hence, v = E/B
Thomson then removed the electric field and calculated the radius of the circular path followed by the particles in the uniform magnetic field alone. By equating the force due to the magnetic field to the centripetal force, he was able to calculate that all cathode ray particles had the same charge to mass ratio of 1.76 x 1011 C kg-1 – this was 1800 times larger than the q/m value for a hydrogen ion (a proton).
FB = FC
therefore, qvB = mv2/r
hence, q/m = v/Br
On the basis of all these results, Thomson suggested that the cathode ray particle was a fundamental constituent of the atom (i.e. a sub-atomic particle). Although he originally referred to the particles as ‘corpuscles’, the name electron slowly became accepted as the official name.
The cathode ray TUBE
The end of the CRT is coated on the inside surface with some fluorescent material such as zinc sulfide (ZnS). When an electron strikes the end of the tube, the material fluoresces, that is gives off light. This enables a spot of light to appear wherever an electron (from the electron gun) strikes the end of the tube.
The Electron Gun:
This produces a narrow beam of electrons. It consists of a filament enclosed in a cylindrical cathode electrode, a ring-shaped electrode called the grid and two cylindrical anode electrodes. The filament is heated by passing electric current through it. Electrons are then emitted due to thermionic emission from the heated filament. These electrons are accelerated towards the anodes by the electric field set up between the cathode and anodes.
The dual anode system helps to focus the electron beam into a single beam.
The ‘grid’ is negatively charged, and its magnitude can be varied easily. What effect would it have on the brightness of the screen? Why?
This allows the electron beam to be deflected from the straight-line trajectory with which it leaves the electron gun.
The deflection system usually consists of two sets of parallel plates, one set in the horizontal plane, and the other in the vertical plane. When potential differences are applied between each set of plates, electric fields are set up between the plates.
The electrons in the beam then experience forces vertically while passing between the horizontal plates and horizontally while passing through the vertical plates. Thus, by applying appropriate voltages to the deflection plates, the position of the spot on the end of the screen can be controlled.
A cathode ray oscilloscope (CRO) uses cathode rays to form an image on a fluorescent screen of the waveform of a periodically varying voltage. This allows the waveform to be analysed for frequency, amplitude and irregularities, etc.
A television picture tube is a cathode ray tube in which the cathode rays (beams of electrons) are focused onto a fluorescent screen to form a moving picture. The picture is formed by combining and synchronising the actions of horizontal and vertical electric fields (or sometimes electromagnetic coils), in response to signals from the television signal.
The horizontal field causes the beam to sweep uniformly across the screen from one side to the other, then rapidly back to the start. The vertical field moves the beam down slightly for each successive sweep, filling the screen with a series of horizontal lines, until it reaches the bottom, then returning rapidly to the top. This process is repeated 50 or 60 times a second.
Close up, you can see the filament in the center of the gold colored electron gun.
Looking at the electron gun
The intensity of the electron beam is modulated by the input signal (via the grid) to cause lighter and darker spots along on the fluorescent screen, forming the detail of the picture. The picture is constantly refreshed, giving the appearance of smooth motion.
2. The reconceptu-alisation of the model of light led to an understanding of the photoelectric effect and black body radiation
In 1865 James Maxwell predicted the existence of electromagnetic waves. He derived four highly complex mathematical equations that suggested that an accelerating charge would produce a changing electric field that would in turn produce a changing magnetic field.
By Faraday’s Law, this uniformly changing magnetic field would in turn produce a changing electric field and so on.
Hertz discovers Radio waves
Heinrich Hertz, a German physicist, achieved the first experimental evidence of the existence of electromagnetic waves in 1887. Hertz used an induction coil to produce oscillating electric sparks between two brass spheres separated by a small distance. He used a small loop of wire with a tiny gap in it as the receiver.
AIM: To demonstrate the production and transmission of radio waves.
MATERIALS: Power pack, induction coil, radio, wires.
RISK ASSESSMENT:The spark across the gap of an induction coil generates all wavelengths of EMR – including X-rays and short wavelength UV. These are potentially dangerous.
METHOD: 1. Adjust the gap on the induction coil to about 5mm and set the power pack to 6V DC
2. Adjust the tuner on the radio so that it receives a signal from a radio station. Set the radio at the back of the laboratory.
3. Record any observations when the induction coil is turned on then off.
Hertz was able to calculate the velocity of his radio waves by reflecting the generated waves off a metal sheet and measuring the wavelength of the standing wave set up by interference. He the substituted this wavelength and the frequency of the waves (i.e. the frequency of the induction coil) into the general wave equation, v = f λ. Hertz then calculated the wave speed at 3 x 108 ms-1 – the same as that estimated by Maxwell and measured by Fizeau!
Thus, Hertz’s experiment confirmed Maxwell’s prediction of the existence of EM waves and provided strong experimental support for the idea that light was a form of transverse EM wave.
While conducting his initial experiments, Hertz often placed the receiver in a darkened box to make it easier to see the tiny sparks in the gap. He noticed that the sparks across the gap in the receiver were distinctly weaker when the receiver was in the box.
After much tinkering, Hertz discovered that the sparks jumping the gap in the receiver were greater if ultraviolet light from the initial spark was shone onto the gap. Although this was a most amazing discovery, Hertz did not further investigate this phenomenon but confined his research to the production and study of EM waves.
Remember that he only had a passing interest in this effect, and never actually investigated it.
Can you describe what he would have seen?
When objects are heated, they begin to give off light - first red, then yellow, and finally white.
The dominant wavelength (colour) of a hot object is directly related to the temperature of the object.
To understand exactly how the wavelength of the radiation varies with temperature, experiments were carried out using a ‘black body’.
A black body is a perfect emitter and absorber of energy. An example is a completely sealed oven with a small hole drilled in one side. At a temperature of say 6000 degrees, the inside walls of the oven will emit all types of radiation (including visible light, UV, IR), but will peak in intensity at a particular frequency (e.g. red – it will therefore appear red to our eyes).
The diagram below shows the black body curves (obtained through experiment) of the radiation emitted at different temperatures:
Note: all wavelengths are emitted, but intensity peaks at a certain wavelength that is dependent upon the temperature.
Classical ‘wave theory’ states that light is a wave. It predicted that as the black body became hotter, the wavelength of the peak emitted radiation would become shorter, and the intensity (amount) of this peak wavelength would increase (i.e. as the oven becomes hotter it will produce more blue light than the other wavelengths and therefore appear blue).
This in itself is not a problem (after all it explains what we actually see), but when the predicted curve is plotted we get the following:
Experimental results gave this curve
What happens at the wavelength of UV to the amount of energy released (i.e. the intensity) by this object according to classical (wave) theory?
Experimental results clearly show the radiation intensity curve corresponding to a particular temperature having a definite peak and then declining – take another look at the black body radiation curves:
Note: all wavelengths are emitted, but intensity peaks at a certain wavelength that is dependent upon the temperature.
Planck suggested that all EMR was emitted or absorbed by a black body in discrete quanta (packets of energy) rather than continuously, as suggested by classical physics. This daringhypothesis
led to the successful explanation of the shape of the black body radiation curves. The big problem was that this explanation was based purely on mathematical equations, and the only way it could work was if Planck introduced a ‘fudge’ constant, h, (6.626 x 10-34) into his calculations.
With time and the contributions of many physicists – most notably Albert Einstein (who was now on the threshold of obtaining the experimental evidence needed to support Planck’s quantum theory), Planck’s hypothesis that all EMR is quantised eventually was proven to be correct and ultimately led to the development of a whole new branch of Physics called Quantum Physics. But in order to see how this occurred, we need to return to the photoelectric effect…
As mentioned previously, Hertz stumbled across a curious effect of light when conducting his EM wave experiments. Sparks jumping the gap in his receiver were more vigorous when the receiver was exposed to ultraviolet light. Because both light and electricity were involved in this phenomenon, it was called the photoelectric effect.
In 1900, Philip Lenard (Planck’s student) showed that the photoelectric effect is actually the emission of electrons from the surface of material when the material is illuminated by light of high frequency.
AIM: To determine whether altering the intensity and frequency of the incident light on a metal surface will have an effect on the number of electrons ejected and the kinetic energy of these ejected electrons
BACKGROUND: The photoelectric induced current is a measure of the number of electrons released. The applied voltage (stopping voltage) needed to stop the photoelectric induced current is a measure of the electrons Ek
MATERIALS: Photoelectric apparatus.
- The number of electrons released (the photocurrent) is proportional to the light intensity.
- The emission of photoelectrons was virtually instantaneous (if it occurred at all).
- Emission was frequency dependent. There is a certain threshold frequency below which no photoelectrons were emitted.
- As the intensity of the light increased, the maximum kinetic energy of emitted electrons remained constant. The maximum kinetic energy of emitted electrons was found to depend on the frequency of the light used.
Refer to the following diagrams…
frequency of light (Hz)
threshold frequency f0
The energy required to release an electron from the surface of the cathode is called the work function (j)
The gradient of the line is a constant for all types of metals used as the cathode. This gradient is is equal to 6.626 x 10-34, and is exactly the same value hypothesised by Planck – it is called Planck’s constant, h.
The last three of our experimental results could not be explained by classical wave theory.
Classical theory for instance predicted that electrons in any surface that absorbed low intensity radiation of any frequency should accumulate this energy over time and eventually have sufficient energy to be ejected.
The stage had now well and truly been set for a radical overhaul in our thinking about the nature of light – classical theory had to go!
In order to explain the photoelectric effect, AlbertEinstein proposed that light energy was transmitted in discrete packets of energy rather than as a wave (this is exactly what Planck had hypothesised when he tried to explain black body curves!!).
The amount of energy in each packet is a quantum (a discrete amount) and represents the quantity of energy associated with particular frequency.
Einstein gave the name photon to a quantum of energy.
E = hf
The energy of a photon can be calculated using the following equation:
where: E = energy of the photon (J)
h = Planck’s constant (6.626 x 10-34 J/ s)
f = frequency (Hz)
Note that h is the gradient of the K.E./frequency graph for the photoelectric effect.
This model of light is referred to as the particle model.
KEmax=h (f - fo)
This explanation is summarised in Einstein’s photoelectric equation:
KEmax=(hf - j)
where j is the work function of the metal surface and is the minimum energy required to remove an electron from the surface.
and since, j = hfo
where fo is the threshold frequency below which no photoelectrons are emitted. Then, the equation above becomes:
Planck’s ‘fudge’ constant used in his mathematical explanation of BBR curves was validated by Einstein – after all, he obtained the same value experimentally in his explanation of the photoelectric effect!!
Planck’s Quantum Theory had now been established – all EMR was quantised, and not simple waves as classical theory had suggested.
Today, we view light as having a dual character. Light behaves like a wave under some circumstances and like a particle, or photon, under others. This is known as the wave-particle duality of light (and all EMR).
Thus, all EMR have both wave and particle characteristics – they are basically bundles or packets of wave – NOT a continuous wave as classical theory suggested. This is the basis of Quantum Theory.
For each photon, its energy is related to its frequency, and as we know, frequency is related to both the speed of light and wavelength. We can see the connection in the two equations below:
c = f l
E = h f
Remember that h = Planck’s constant (6.626 x 10-34 J/ s)
What energy would be associated with a photon of red light of frequency 3.85x 1014 Hz?
E = hf
E = (6.626 x 10-34) x (3.85x 1014)
E = 2.55 x 10-19 J
If a photon has an energy of 1.0 J, what will its frequency be?
E = hf
f = E/h
f = 1.0 / (6.626 x 10-34)
f = 1.6 x 10-33 Hz
v = 2.22 x 10-13 m/s
At what velocity would a 1.75 x 10-6 g particle have the same energy as light of wavelength 4.62 x 10-13 m?
c = fl
f = c/l
f = (3.0 x 108) / (4.62 x 10-13)
f = 6.49 x 1020
E = hf
E = (6.626 x 10-34) x (6.49 x 1020)
E = 4.3 x 10-13 J
The electrons that initiate an electric current are produced by the photoelectric effect. They also contain semiconductor material discussed in the next section.
Because photocells can detect the amount of light, they can also be used to measure the concentration of particles in liquids and gases, as these particles scatter light. The more particles, the more light is scattered. Therefore they can also be used as breathalysers, to measure the purity of drinks and to determine the level of pollution.
Einstein and Planck were great friends – both were German and shared many of the same laboratories. They collaborated on many scientific works, yet they had very differing views regarding whether their work should be used for the pursuit of scientific knowledge or the evils of war.
3. Limitations of past technologies and increased research into the structure of the atom resulted in the invention of transistors
A new model of the atom
Einstein and Planck had challenged the way that we look at EMR. Previous models visualised light as waves. Physicists now described light (and all EMR) as photons. In doing so they pioneered a new way of describing both matter and energy. This is the wave-particle duality of matter and energy. This then, meant that the model of the atom had to be adjusted!
lowest energy level (n=1)
The electrons were only allowed to occupy discrete energy levels (the orbits).
The lowest energy state of an atom is called the ground state. This is when the electrons fill up all of the energy levels as close to the nucleus as they can.
- electrons certainly have mass, but should not be considered as tiny ball satellites orbiting the nucleus.
- electrons should be considered as standing waves with whole number wavelengths.
Wolfgang Pauli in his ‘Pauli exclusion principle’ worked out that only 2 electrons could exist in the lowest energy level and 8 electrons were permitted to exist in the second energy level at any one time.
de Broglie could now explain that when electrons jump down a single energy level, they emit a photon and lose one whole wavelength from their shape (therefore this photon has a particular frequency!!). If electrons absorb energy, they move out from the nucleus to the next quantised energy level.
A normal atom at absolute zero temperature has an electron occupying every one of the lower energy levels, starting outward from the nucleus and continuing until all electrons have been placed.
Quantum theory shows that when two atoms are brought close enough together, each of the allowed electron energy levels within the atoms splits into twodistinct but closely spaced energy levels as they ‘overlap’ to form the two-atom system.
Clearly then, as we pack more and more atoms closely together each set of split energy levels contains more and more levels spread over the same energy range allowed at that particular radius from the nuclei.
So, in a crystalline solid, such as a metal or a diamond, where billions upon billions of atoms are packed closely together, the energy levels within each set become so closely spaced in energy that they form a practically continuous energy band.
The spaces between these allowed bands are called forbidden energy gaps. These forbidden energy gaps correspond to the gaps between electron energy levels within the isolated atoms.
If an electron can get into the conduction band, it is called a free electron and can drift from atom to atom in the solid. In other words, some electrons are shared between atoms.
This means that the solid becomes a conductor of electricity.
The wider the forbidden energy gap between the valence and conduction bands, the more likely the solid is to be an insulator.
This is at least partially full
This is full
BIG energy gap
A lower band
In an insulator, there is a substantial gap between the valence band and the conduction band. You need to supply a lot of energy to an insulator in order to get electrons to jump up to the conduction band. Since there are no free electrons in the conduction band, the solid, as expected, acts as an insulator – it has a high electrical resistance.
This is full
A lower band
In a conductor (eg a metal), the valence band and the conduction band actually overlap. This means that there are many free electrons in the conduction band. The valence band is also the conduction band!! It has a low electrical resistance.
Smallish energy gap
This is full
This is full
A lower band
The band diagram for a semiconductor is similar to that of an insulator, except that there is a smaller forbidden energy gap between the valence band and the conduction band than with an insulator. They are therefore insulators, but not quite as insulating as insulators. They only need a bit of energy to excite the electrons up to the conducting band, but once they are there, they are free to move around. They have electrical resistance between those of conductors and insulators.
At low temperatures, all electrons in a semiconductor fill the valence band and it is therefore an insulator. But, as the temperature increases, thermal energy allows some electrons to jump the small gap into the conduction band. This leaves the valence band unfilled.
This means that a‘hole’ has been created in the valence band where an electron has left.
Since this hole represents the absence of an electron, it can be treated as a positive charge.
A nearby electron then moves into the hole, creating a new hole at the position from which it moved, and so on.
As such, current flow is now possible in both the valence band (as a flow of positive holes) and in the conduction band (as a flow of electrons).
Who cares !?!?
You should! Semiconductors have led to the invention of an incredibly important piece of technology – the transistor. With its development, unprecedented progress has taken place – particularly in the field of computers. Enter the silicon chip! We’ll look at this stuff shortly.
But first… a very very silly experiment…
Early devices made of semiconductor materials included diodes and transistors. Diodes are rectifiers – they block the backward and forward movement of electric current in a circuit (i.e. they act as a one way traffic cop – consider their effect on AC current!). Transistors are manufactured to act as switches and amplifiers in integrated circuits.
The most widely used semiconductor materials are made from crystals of elements of Group IV of the periodic table.
In order to see how this can happen, we will look at the example of Germanium (symbol Ge) – a Group IV element.
Ge is very rare on Earth and is quite brittle. It only became economically significant after 1945 when its semiconducting properties were first recognised as being useful in electronics. The first transistor was made of germanium.
In order to fill its valence shell, a Ge atom shares a valence electron with each of four adjacent Ge atoms.
Each of these atoms contributes an electron to the partnership, forming a pair of electrons which act to bond the atoms together – this is a covalent bond.
This occurs over and over with many individual atoms – eventually forming a crystal of Germanium.
Here we can see the regular arrangement of atoms – this is called a crystal lattice.
Despite its rarity and fragile nature, Ge was the semiconductor of choice – for one simple reason – no other Group 4 element was capable of being produced at a suitable purity.
Silicon (Si) is also a group IV element and is the second most abundant element (by weight) in the Earth’s crust. Single crystals are grown slowly from molten silicon. This is a difficult process, and it was not until the late 1950’s that silicon crystals of suitable purity could be produced.
Germanium use was surpassed by silicon in the 1960’s, when it became the new semiconductor of choice for use in solid state devices.
Both Ge and Si do not conduct electricity at room temperature. In fact they are unable to conduct electricity unless the bonds are broken by the application of heat. This is undesirable, as the crystal lattice would break up!
Enter the dope…
Adding an impurity to the crystal lattice of a semi-conductor can actually be a good thing – only if done properly!! Adding an impurity is called doping.
We can dope Si by melting a part of the crystal and adding small quantities of either group III atoms, or group V atoms. It can also be done using transmutation in a nuclear reactor.
A group III element has only 3 electrons in it valence shell e.g. boron, aluminium, gallium and indium.
A group V element has 5 electrons in it valence shell e.g. nitrogen, phosphorous, arsenic and antimony.
By doping a semiconductor we can radically change its electrical properties!
Positive holes in crystal structure where an electron is missing
Indium has 3 valence electrons NOT 4
When a dope group III atom replaces a silicon atom, an electron is missing from the crystal lattice. There is an empty quantum position in the valence band. This is a positive hole!! This is called a p-type semiconductor.
p-type semiconductors have positive holes.
The diagram below shows how p-type semiconductors conduct electricity readily under the application of an external voltage:
n-type semiconductors conduct electricity by the migration of electrons, which are surplus to the crystal structure.
The electrons sit in energy levels above the valence band (i.e they are in the conduction band).
They are called n-type because negative charges move.
To make an n-type, you have to melt the silicon (or Ge) and add small amounts of group V elements.
An n-type semiconductor is shown below:
Arsenic has 5 valence electrons NOT 4
The following diagram shows how n-type semiconductors conduct electricity readily under the application of an external voltage:
n-typeobviously have a large number of negative charge carriers (i.e. free electrons) because they have been doped with Group V elements.
p-typehave a large number of positive holes because they have been doped with Group III elements.
In comparison, undopedsemiconductors have no free electrons or positive holes (unless they are heated up).
Thermionic emission is the spontaneous emission of electrons from solids (and liquids) when they are heated to high temperatures. (Remember the filament inside the electron gun in TV tubes??)
The first application of semiconductors was in radio technology - they replaced the old valve radios.
These valve wirelesses were the first amplifying radios capable of driving a speaker. The valve was a thermionic device called a triode.
You will recall that radio waves set up a current in an antennae (remember Hertz’s experiment) – but this current is very very small. It has to be boosted (amplified) somehow. A triode could do this very effectively. The diagram below shows how a triode works:
The grid becomes negatively charged because it is connected directly to the antennae, and it varies in charge constantly. This causes the electrons that are trying to pass through the grid from the cathode to slow down or even stop. This has the effect of constantly varying the current that arrives at the speaker – sometimes lots of current (a loud sound) and sometimes no current (no sound).
In this way the current produced by the radio wave is amplified!
EXPERIMENT: Thermionic devices.
AIM: To examine a triode in order to identify the cathode, anode and grid, and to determine how this device can act as an amplifier of radio waves.
Thermionic devices dominated the war years, and the allies secret weapon was RADAR.
The race was on to build better valves for higher frequency RADAR’s that could detect smaller targets. Eventually, the scientists realised that there was a frequency limit above which thermionic valve devices could not detect.
This current technology had shortcomings! Research into an alternative technology to replace thermionic devices began.
It soon became apparent that semiconductors could easily, efficiently and cheaply replace valves. All electronic devices made of semiconductors are called ‘solid-state’ devices.
The first solid-state device was called the transistor. Transistors replaced valves, since they did the same job – only better.
Transistors are solid state devices that allow the current in one circuit to vary the current in a second circuit, just like the old valves (triodes) used to do).
They are particularly useful in computers because they act as ‘gates’ in microchips, where they control the flow of charge. The gates are either open or closed (1 or 0). Other transistors acts as amplifiers when information is being retrieved. Miniature transistors pack modern integrated circuits (microprocessors).
e.g.1 The semiconductor diode:
These consists of a piece ofn-type material in contact withp-type material:
The diode therefore prevents backward motion in AC currents.
These amplify the AC current that is induced in radio antennae. They transfer the AC current from the antennae to a mains (high voltage) circuit (they were originally called ‘transfer resistors’ – hence ‘transistor’).
Apnptransistor has 3 slices: the base, collector and emitter. It performs exactly the same function as the old triode valves. It takes a small AC current from the antennae and ‘imprints’ it onto the mains current. This varies the amount of current fed into the speaker of the radio – in this sense, the current coming from the antennae has been ‘amplified’. Examine the following diagram:
Solar cells, or photovoltaic cells, are used to power lights, telephones, radio beacons, calculators and other household devices .
4. Investigations into the electrical properties of particular metals at different temperatures led to the identification of superconductivity and the exploration of possible applications
Diffraction occurs when waves pass through an opening. If the width of the opening is of a similar dimension to the wavelength, maximum diffraction occurs:
William Henry Bragg (1862–1942) and his son, William Lawrence Bragg (1890–1971), used the principle of diffraction to study crystals – in particular to measure the spacing between atoms in crystals. They used X-rays because their wavelength (10-10) is similar to the separation between the atoms. In 1915 they won the Nobel Prize in physics for their work. They were Australian.
Subsequent mathematical analysis of the diffraction pattern permitted the crystal structure to be deduced.
ions in crystal lattice
The diagram shows X-rays being reflected from adjacent atomic planes (i.e. rows in the crystal lattice). The reflected X-rays then interfered constructively and destructively producing the familiar interference pattern on the photographic plate.
They showed that metal crystals are composed of an orderly arrangement of atoms. This is called a crystal lattice.
This eventually resulted in theoretical suggestions that superconductors should exist and finally led the discovery of superconductors themselves!
But before we can look at superconductors, we must first examine the conduction of electricity in metals…
As we already know from the study of band structures, metals are good conductors because their valence band overlaps with their conduction band, resulting inthe presence ofmany free electrons in the conduction band.
Notice that there is no net movement of electrons in any one direction.
Under the influence of an applied potential difference however, the electrons drift in a net direction. This is called an electric current.
The instantaneous speed of the electrons is approximately 1.6x106 m/s, but the average drift velocity is only 1.0x10-5 m/s.
Suggest a reason for this!
Since the electrons are moving enormously fast, they undergo random collisions with other electrons, with impurities in the lattice and imperfections in the crystal lattice.
Metals have resistance
As we saw earlier, conduction electrons undergo collisions with the atoms in the lattice – this causes the electrons to scatter. The collisions cause the atoms in the lattice to vibrate, and the conductor becomes hotter. As the temperature increases, the atoms in the lattice vibrate more, increasing the chance of collision (and scattering) and so resistance increases further…
These impurities are usually other atoms that ‘corrupt’ the crystal lattice. This then increases the chance of collision between the free electrons and the lattice, hence the resistance increases.
Think carefully about this!!
…if the resistance of a conductor decreases as temperature decreases…
…and if you could reduce the temperature of the conductor low enough…
…then theoretically you should reach a stage where it has zero resistance…
…and a conductor like that would be SUPER!
Superconductors are materials which, below a particular temperature called the critical temperature allow electrons to move through the crystal lattice unimpeded.
This means that these materials have lost all resistance to the flow of electricity.
The first superconducting material was discovered by Heike Onnes, in 1911. His results are shown on the next page:
Since Onnes discovered the superconducting property of mercury, many more materials have been shown to have the ability to superconduct.
You can see that the critical temperature at which the transition to the superconducting state occurs is different for different materials.
Ceramic materials and other compounds have been made with even higher critical temperatures than the metal alloys - these are called high-temperature superconductors.
Why do you think there is a special interest in high-temperature superconductors?
In 1957, Bardeen (B), Cooper (C) and Schrieffer (S) developed a theory (known as BCS theory) to explain how superconductors work. Their theory described the formation of pairs of electrons in materials when they are below their critical temperature. These pairs of electrons were called ‘Cooper pairs’.
Realising that it was the interaction of electrons with the vibrations of the crystal lattice that produced superconductivity, they proposed that forcing electrons to pair up into ‘Cooper pairs’ would result in them being able to bypass the obstacles in the crystal lattice which were responsible for electrical resistance.
Here is what they proposed…
3. All of the Cooper pairs represent a highly ordered system and when an external electric field is applied, the pairs move through the lattice with each pair’s motion synchronised to that of every other pair so that none are involved in the random scattering within the lattice (which would otherwise give rise to electrical resistance).
The speed and further miniaturisation of computer chips is limited by heat generation. Super-conductive film can be used. Switching devices (Josephson junctions) are being tested for use in high-speed computers, they are around 10 times faster than the semiconductor switches currently used.
Advantages and applications of Superconductors
Superconducting magnets would not need an iron core – therefore they would be much smaller and lighter. Higher efficiency in generation results in less energy wasted, so less fossil fuels would be burnt - BIG positives for the environment.
Increased efficiency results in less demand for electricity (see above).
No electrical loss therefore thin wires could carry very large currents.
In 1933, W. Meissner discovered that a superconductor does not allow an external magnetic field to penetrate its interior. The exclusion of a magnetic field is called the Meissner effect. A consequence of this is that a superconductor in its superconducting state can cause an external magnet to levitate above it!!
When a superconducting material in its normal state is placed in a magnetic field, the magnetic field strength inside the material is almost the same as outside. This is true of all metals.
But, when the material is cooled so that it is in its super-conducting state, electric currents begin to flow through it, and as we know, these currents will produce their own magnetic field.
The magnetic field caused by the flowing currents not only cancels out the external magnetic field but also extends outside the superconductor itself.
If a magnet is bought close to a superconducting superconductor, the currents set up magnetic fields with poles in such a direction that cause repulsion between the magnet and the superconductor (remember Lenz’s Law??).
This is the cause of magnetic levitation.
EXPERIMENT: Magnetic levitation.
AIM: To observe the Meissner effect – the levitation of a magnet above a superconductor.
MATERIALS: Superconductor, small neodymium magnet, liquid nitrogen, petri dish, styrofoam cup, plastic tweezers, insulated gloves
METHOD: 1. Carefully fill the styrofoam cup with liquid nitrogen. Place the petri dish on top of the cup, and pour in enough liquid nitrogen until it is about 1cm deep. Wait until the boiling subsides.
2. Using the tweezers, carefully place the superconductor in the petri dish. Again wait until the boiling subsides.
3. Using the tweezers, carefully place the small magnet about 2 mm above the centre of the superconductor. Release the magnet.
4. While the magnet is levitating, gently rotate the magnet using the tweezers.
If this is not possible refer to the next page…
The magnetic levitation described above suspends an object so that it is free of contact with any surface – this provides a frictionless surface – particularly appropriate for high-speed trains.
In Japan, levitation is achieved by using helium cooled super-conducting magnets on the train, which interact with the coils in the ‘guideway’ (or track) in such a way that repulsion between the ‘like poles’ causes the train to levitate. Another set of coils along the sides of the guideway propel the suspended train forwards.
In addition to the levitation coils, there are also propulsion coils. These push the suspended train forward.
The advantages of Maglev trains are very high speed, reliability, safety, minimum maintenance and low environmental impact.