Key Stage 5 – What is a semiconductor?

Notes for teachers

At a glance

The digital age is built on an electrical component called a semiconductor. These are really important in electrical devices, as their electrical properties can be changed. Silicon is the most popular material for a semiconductor, hence the naming of “Silicon Valley” where most of the world’s prominent technology

companies operate in, in California.

Researchers studying semiconductors are looking to create new semiconductors that could revolutionize our technology.

Learning Outcomes

1. Recognise I-V graphs and the limitations of Ohm’s Law

2. Describe the difference between an Ohmic conductor and a semiconductor

3. Identify and analyse uses and limitations of semiconductors

Each student will need

· Key Stage 5 – What is a semiconductor? activity sheet.

· Graph paper, calculators

· Prior knowledge: Students are expected to already have an understanding of Ohm’s law and how to measure current and voltage.

http://www.oxfordsparks.ox.ac.uk/content/soluble-semiconductors-revolution-printing-21st-century

· OPTIONAL: Thermistor (or diode), voltage supply and multi-meter.

Possible Lesson Activities

1. Starter activity: Recognise I-V graphs and the limitations of Ohm’s Law

· Show students the I-V curve of a wire, without the axis labelled. Tell them it’s a wire and ask if they know what the axis should be. Then show the one for a filament lamp, and see if they get it from that. This leads into a curve-component sorting exercise. This is just a KS4 recap, so warn students that you expect them to be able to justify their answers.

▪ At the end of the exercise, cold call students to justify their answers. (Prompt for keywords such as temperature, resistance, electrons …). Depending on your class, you may want to pre-warn students who you will be cold-calling.

▪ This could also be done as group work as a think, pair, share exercise (students compare answers), depending on size of class.

· It is important to get students into the frame of mind of analysing the physics to create their own opinion of the technology. (You may want to explain this to the students.) Using the ‘thinking pose’ for encouragement will be very useful when receiving students answers in the cold call, and encouraging students to expand on each other’s answers and including key words as above.

· To lead onto the main activity, ask students what they know of Ohm’s law. Also mention how filament bulbs are not as popular they used to be, due to the development of LEDs, light emitting diode, which are a type of semiconductor.

· MISCONCEPTIONS: “Heat will increase speed of electrons, making them move faster through the circuit and lowering resistance.” The electrons will move faster, as they have higher kinetic energy, however there will be more collisions with the structure of the conductor, hence it slows electron flow down.

2. Main activity: Describe the difference between an Ohmic conductor and a semiconductor

· Remind the class of Ohm’s Law, identifying what each term means. (It was made by Georg Ohm in 1826). If the graph from the starter is linear, then a component obeys Ohm’s law and it is called Ohmic.

· Introduce and watch the Oxford Sparks animation.

· Using band theory, explain the difference between insulators and conductors and apply to semiconductors. Follow-up with the key questions on the activity sheet.

▪The band theory of conduction describes conductors as having two bands in which electrons can exist: valence band and conduction band. Electrons must be in one or the other.

http://www.oxfordsparks.ox.ac.uk/content/soluble-semiconductors-revolution-printing-21st-century

▪ In insulators, there is a large energy gap between the bands (called a band gap). As a result, the conduction band has no electrons in it and the valence band is full.

▪ In conductors, there is no band gap and the bands may even overlap slightly. This means electrons can freely move between the bands.

▪ In semiconductors, there is a band gap like in an insulator, however, it is much smaller. As the temperature of a semiconductor increases, electrons gain enough energy to cross the band gap, going from the valence band to the conduction band. Impurities in the semi-conductors which is added to alter the size of the band gap – this is called doping.

· MISCONCEPTIONS: Some students may confuse a superconductor and a semiconductor. Clearly identify the difference that a superconductor has zero resistance. You can ask the students what happens to the kinetic energy of the lattice ions when a conductor is cooled.

3. Plenary: Identify and analyse uses and limitations of semiconductors

· Silicon is the most popular material for a semi-conductor, hence the naming of “Silicon Valley” where most of the world prominent technology companies operate in, in California.

· The research group at Oxford looks into creating a different kind of semi-conductor called a soluble semiconductors. As stated in animation, these are cheaper, more efficient and more technologically usable.

· Mention how in the early years after the discovery of the LASER, many critics described it as a “solution in search of problem”, meaning they could not imagine a use of the LASER and that it was pointless. Is soluble semiconductors the same? However, Graphene, which won its discoverers the Nobel Prize in physics in 2010, is a type of semi-conductor that has a zero band gap.

Using the animation, get the students to work in pairs to judge the research and create a speech to persuade others or their judgement. Students should expand upon the key uses and limitations of semiconductors and soluble semiconductors.

Students should focus on producing a well-reasoned answer to “Will the researchers achieve their goal?”(As teacher, you should say your opinions on this.) Students should also include key points on how a semiconductor is different to a conductor.

OPTIONAL: You could turn this activity into a Fermi-styled question, named after the Physicist Enrico Fermi who would often ask questions that seem unreasonable, but could be estimated with thoughtful reasoning. For example:

▪ “What companies might use a soluble semiconductor?”

▪ “How much are those companies worth?”

▪ “How many devices do you have that include semiconductors?”

▪ “What is a reasonable mark-up price the company may make?”

http://www.oxfordsparks.ox.ac.uk/content/soluble-semiconductors-revolution-printing-21st-century

· Once the students have judged the research group, point out that the group leader, Donal Bradley, has been recognised by awards including the EU Descartes Prize, the Royal Society Bakerian Medal and the IOP and IET Faraday Medals. Donal was elected a Fellow of the Royal Society in 2004 and was awarded a CBE in 2010 for services to science. Ask the students if this changes their judgement.

▪ Finish the lesson by asking a pair to share their speech.

Web links

· Oxford Sparks animation on Semiconductors: http://www.oxfordsparks.ox.ac.uk/content/soluble-semiconductors-revolution-printing-21st-century

· The semiconductor research group at Oxford: https://www2.physics.ox.ac.uk/research/physics-and-application-of-soluble-semiconductors

· Donal Bradley’s profile on the Royal Society website: https://royalsociety.org/people/donal-bradley-11123/

· Winners of the 2010 Nobel Prize in physics: https://www.nobelprize.org/prizes/physics/2010/press-release/

http://www.oxfordsparks.ox.ac.uk/content/soluble-semiconductors-revolution-printing-21st-century