This unit addresses aspects of the following syllabus outcomes:
H1.2 differentiates between the properties and structure of materials and justifies the selection of materials in engineering applications
H4.1 investigates the extent of technological change in engineering
Source: Board of Studies NSW (2011) Engineering Studies Stage 6 syllabus.
Metals are able to conduct electricity while other materials, such as plastics and ceramics generally cannot.
A simple explanation of this phenomenon is that different elements have atoms with different numbers of electrons. These electrons can only move in specific places around the atom. It is like rows of seats in a stadium. A few electrons get to sit in the first row around the ground, and when that's filled the next electrons sit in the next row and so on. Just like in a sports stadium; it's harder to get out when you've got people sitting on each side of you so electrons in a filled row stay put. In an insulator, every row is completely filled. Consequently, the electrons rarely move out of their usual orbit. No moving electron means no electricity can pass through. But, if you're sitting in the back row and the seats aren't full, you could simply get up, switch seats, maybe even leave the stadium. In a metal, the last row isn't filled with electrons. The outer electrons have little attraction to the nucleus of the atom and readily wander off to other atoms. This translates to many moving electrons, which means metals can easily conduct electricity.
To properly explain the difference between conductors and insulators we need some knowledge of band theory. Electrons occupy energy levels in an atom, from the lowest energies upwards. The highest filled level is known as the valence band. Electrons in the valence band do not participate in the conduction process. The first unfilled level above the valence band is known as the conduction band. In metals, there is no forbidden gap since the conduction band and the valence band overlap, allowing free electrons to participate in the conduction process. Insulators, on the other hand, have an energy gap that is far greater than the available energy of the electron.
Metals, then, conduct electricity easily because the energy levels between the conduction and valence band are closely spaced or there are more energy levels available than there are electrons to fill them so very little energy is required to find new energies for electrons to occupy.
Figure 1: The relationship between valence and conduction bands.
For further information on band theory seek advice from the Britney Spears web site.
What is a semiconductor?
We are usually able to categorise materials as either conductors or insulators. However, there are a number of materials that have conductivity levels between those of metals and insulators. Shining a light on them, or injecting charges, can modify their conductivity briefly. These materials are known as semiconductors. These substances first became interesting to physicists in the late 1920s.
Silicon is an element commonly found in nature, being the major element in sand. On the periodic table it is found next to aluminium, below carbon and above germanium. Carbon, silicon and germanium all have four electrons in their outer shells, allowing them to form covalently bonded crystal lattices with four other atoms. The crystalline form of carbon is diamond. The other two form semiconductors. Silicon forms a silvery, metallic looking substance.
Figure 2: Silicon sits next to aluminium and below carbon in the periodic table.
At room temperature, the thermal energy of the atoms in a semiconductor may allow a small number of electrons to participate in the conduction process. As the temperature rises, the thermal energy of the valence electrons increases, allowing more of them to breach the energy gap into the conduction band. As an electron moves from the valence band to the conductance band it leaves behind a vacancy which may be filled by another electron. This vacancy is known as a hole and has a positive charge. As electrons flow through the semiconductor, holes ”flow” in the opposite direction.
Silicon can be changed into a conductor by doping. A small amount of an impurity is deliberately mixed into the silicon crystal. Silicon has four valence electrons and each atom shares an electron with a neighbouring atom. Aluminium, however, has three valence electrons. When a small proportion of aluminium atoms, (less than one in a million), is incorporated into the crystal the aluminium atom has an insufficient number of bonds to share bonds with the surrounding silicon atoms. The material will now have electron vacancies or holes. This type of semiconductor is known as p-type as it creates positive charge carriers.
Elements such as As, P and Sb, possess an extra electron in the valence band. When added as a dopant to silicon, the dopant atom contributes an additional electron to the crystal and produce n-type semiconductors.
How do they behave?
Whether intrinsic (naturally occurring) or extrinsic (produced by doping), semiconductors have small band gaps between their valency and conductance band as shown in the following diagram.
Figure 3: There will be a small band gap in semiconductors.
These semiconductor materials are used to make devices such as diodes and transistors. Thousands of these devices can be doped into a single silicon chip, thus creating an integrated circuit. The two most basic semiconductor devices are diodes and transistors. Semiconductors have had a monumental impact on our society.
Using a multimeter (your school will have one) set to resistance; check the conductivity of a diode. To do this touch one of the probes to each end of the diode and check the meter. Reverse the probes and check again. Note your results.
Figure 4: A typical diode.
By themselves, a p-type semiconductor is no more useful than an n-type semiconductor. The truly interesting effect begins when the two are combined in various ways. A diode consists simply of an n-type and p-type semiconductor junction. When current attempts to flow in one direction it produces forward bias and allows conduction. If the polarity is swapped, reverse bias prevents current flow.
igure 5: The diode
A device, then, that blocks current in one direction while letting current flow in another direction is called a diode. Battery operated items often contain a diode to protects the device in case batteries are accidentally inserted backward.
Click here for more detail on how a diode works.
Figure 6: Two typical transistors used in the modern circuits.
When a transistor is forward-biased, there is a small amount of voltage necessary to start the hole-electron combination process at the junction. In silicon, this voltage is about 0.7 V. A transistor, which is constructed from three layers rather than the two used in diodes, can provide this voltage and hence act as a switch or it may be used as an amplifier.
Figure 7: The transistor as a switch.
A transistor looks like two diodes wired back-to-back. You can create either an “npn” or a “pnp” sandwich. No current can flow through the device since the junctions stop the flow in both directions. However, when you apply a small current to the centre section of the sandwich (base), a much larger current can flow through the sandwich (emitter to collector). In this way a transistor acts as a switch. A small forward bias applied to the emitter-base junction and a larger reverse bias applied to the collector-base junction turns the current flow between emitter and collector on and off.
Search for a copy of the Dick Smith textbook called Funway into Electronics Volume 1. This text is common in schools for teaching both Technics Electronics and Science. Project 2 outlined on pages 18 and 19 is especially suited to this topic. It shows in a practical way how transistors, diodes, resistors and LEDs are commonly used. The components are inexpensive and easily available.
The light emitting diode (LED)
When electrons move back from the conductance band to the valence band, the energy released is in the form of photons. In a LED these photons react with the crystal placed on the cup (see diagram below) and emit light.
Figure 8: The parts of an LED.
The colour of the emitted light has nothing to do with the colour of the screen (plastic case) which only improves the wavelength of the light emitted by the crystal. It is the structure of the crystal that determines the colour of the light generated electronically.
Figure 9: A typical LED
The integrated circuit
A silicon chip is a piece of silicon that can hold thousands of transistors. With transistors acting as switches, you can create Boolean gates, and with Boolean gates you can create microprocessor chips.
Figure 10: An integrated circuit.