Semiconductors

Electrical Quantities

Define a semiconductor.

A material with a lower resistivity than an insulator but a higher resistivity than a conductor, usually with only a small number of delocalised electrons available for conduction.

Give an example of a semiconductor.

Silicon.

How does conduction in a semiconductor differ from conduction in a metal?

Semiconductors have far fewer delocalised charge carriers than metals, so they generally conduct less current for the same applied voltage.

What happens to the discrete electron energy levels when many atoms form a solid?

The energy levels broaden into energy bands.

What is an energy band?

A range of allowed energies that electrons in a solid can have.

What is the valence band?

A range of electron energies in which electrons remain associated with particular atoms and do not contribute to electrical conduction.

What is the conduction band?

A range of electron energies in which electrons are delocalised and can move through the material to conduct current.

What is the band gap?

The forbidden range of energies between the valence band and the conduction band.

What must happen for an electron in a semiconductor to conduct?

It must gain enough energy to move from the valence band into the conduction band.

What happens to an electron once it enters the conduction band?

It becomes delocalised and can move through the semiconductor as a charge carrier.

Why does a semiconductor conduct less well than a metal at room temperature?

It has fewer electrons in the conduction band and therefore fewer charge carriers.

Describe the energy bands in an insulator.

It has a full valence band, an empty conduction band and a large energy gap between them.

Describe the energy bands in a semiconductor.

It has an almost full valence band and an almost empty conduction band separated by a relatively small energy gap.

Describe the energy bands in a metal.

The conduction band is partially filled or overlaps with the valence band, so many electrons are available for conduction.

Why are metals good electrical conductors?

They have many delocalised electrons available in the conduction band.

What happens to a semiconductor when its temperature increases?

More electrons gain enough thermal energy to move from the valence band into the conduction band.

How does increasing temperature affect the number of charge carriers in a semiconductor?

The number of charge carriers increases.

How does increasing temperature affect the resistance of a semiconductor?

Its resistance decreases.

How does increasing temperature affect the resistivity of a semiconductor?

Its resistivity decreases.

Why does the resistance of a semiconductor decrease as temperature increases?

More electrons gain enough energy to enter the conduction band, greatly increasing the number of available charge carriers.

Why doesn't increased collision frequency make semiconductor resistance increase with temperature?

Although collisions increase, the increase in the number of charge carriers is a much larger effect.

How does the temperature behaviour of a semiconductor differ from a metal?

A semiconductor's resistance generally decreases with temperature, whereas a metal's resistance increases with temperature.

What is a hole in a semiconductor?

A vacant electron state left behind when an electron leaves the valence band.

What charge is assigned to a hole?

Positive charge.

Are holes actual positively charged particles?

No. A hole represents the absence of an electron.

How can a hole appear to move through a semiconductor?

A neighbouring electron moves into the hole, leaving a new hole behind, so the vacancy appears to move.

How does the direction of hole movement compare with electron movement?

Holes appear to move in the opposite direction to electrons.

In which direction do holes move when a potential difference is applied?

Towards the negative terminal.

How do holes contribute to current in a semiconductor?

They behave as positive charge carriers and contribute to the overall current.

What are the two types of charge carrier involved in semiconductor conduction?

Delocalised electrons and positive holes.

Does hole conduction involve movement of positive lattice ions?

No. The positive ions remain fixed; it is the absence of an electron that appears to move.

What is a semiconductor diode?

A semiconductor component designed to allow current to flow mainly in one direction.

How is a semiconductor diode formed?

By joining different types of semiconductor, creating an energy barrier at the junction.

What does the energy barrier in a diode do?

It restricts the movement of charge carriers across the junction.

What happens when a diode is forward biased?

The energy barrier can be overcome and the diode conducts once a sufficient forward voltage is applied.

What is the forward junction potential?

The minimum forward potential difference needed for significant conduction through a diode.

What is the typical forward junction potential of a silicon diode?

About 0.6 V.

What happens to current after the forward junction potential is exceeded?

The current increases very rapidly with increasing voltage.

What happens when a diode is reverse biased at low voltages?

Only a very small leakage current flows.

What is leakage current in a diode?

The very small current that flows through a reverse-biased diode.

What is reverse breakdown?

The rapid increase in reverse current when the reverse voltage becomes sufficiently large.

What is the reverse breakdown voltage?

The magnitude of reverse voltage at which a diode begins to conduct a very large reverse current.

Why can a diode conduct during reverse breakdown?

The large reverse voltage allows charge carriers to overcome the junction's energy barrier.

What is an LDR?

A light-dependent resistor whose resistance depends on the intensity of light falling on it.

How does increasing light intensity affect the resistance of an LDR?

Its resistance decreases.

Why does an LDR's resistance decrease in brighter light?

Photons transfer energy to electrons, promoting more electrons from the valence band into the conduction band and increasing the number of charge carriers.

What happens to the resistance of an LDR in darkness?

Its resistance is high because there are relatively few conduction electrons.

What type of material is used to make an LDR?

Semiconductor material.

What is a thermistor?

A component whose resistance changes significantly with temperature.

What type of thermistor is commonly used?

A negative temperature coefficient (NTC) thermistor.

What does negative temperature coefficient mean?

The resistance decreases as temperature increases.

Why does the resistance of an NTC thermistor decrease when temperature rises?

Thermal energy promotes more electrons into the conduction band, increasing the number of charge carriers.

What similarity is there between an LDR and an NTC thermistor?

Both use energy to promote electrons into the conduction band, increasing charge carriers and reducing resistance.

What is the difference between how an LDR and thermistor gain energy?

An LDR gains energy from incident light, whereas a thermistor gains thermal energy from its surroundings.

Define an electrical insulator using energy bands.

A material with a large energy gap between its valence band and conduction band, leaving virtually no electrons available for conduction under normal conditions.

Why do insulators normally have very high resistance?

Their large band gap means electrons cannot easily gain enough energy to enter the conduction band.

Why are there virtually no conduction electrons in an insulator under normal conditions?

The energy gap between the valence and conduction bands is too large for electrons to cross with ordinary thermal energy.

Can an insulator ever conduct electricity?

Yes. A sufficiently large electric field can cause electrical breakdown and make it conduct.

What is electrical breakdown in an insulator?

The process in which a very large electric field provides enough energy for charge carriers to conduct through the material.

Give an example of electrical breakdown in an insulator.

A spark passing through air.

Why can air conduct during a spark?

A sufficiently large electric field ionises the air, producing mobile charge carriers.

What generally happens to the resistance of a metal as its temperature decreases?

Its resistance decreases.

Why does cooling a metal decrease its resistance?

The lattice ions vibrate less, causing fewer collisions with conduction electrons.

What happens to some materials below a particular critical temperature?

Their electrical resistance suddenly falls to zero.

Define superconductivity.

The property of a material having zero electrical resistivity below its critical temperature.

Define critical temperature.

The temperature below which a material's resistivity suddenly falls to zero and it becomes superconducting.

What is the resistance of an ideal superconductor below its critical temperature?

Zero.

Why can superconductors carry very large currents efficiently?

They have zero resistance, so electrical energy is not dissipated due to resistance.

Why are superconductors useful for powerful electromagnets?

Very large currents can flow without the large energy losses and heating associated with resistance.

Give an application of superconductors.

Powerful electromagnets in particle accelerators.

Why must many superconductors be kept extremely cold?

Their critical temperatures are very low, so they must remain below these temperatures to maintain superconductivity.

Approximately what temperature do the superconducting electromagnets at the Large Hadron Collider operate at?

About 1.9 K.

What is the Meissner effect?

The expulsion of magnetic fields from a superconductor.

How can the Meissner effect cause magnetic levitation?

A superconductor excludes the magnetic field, producing forces that can cause a magnet to levitate above it.

What is absolute zero?

The lowest possible temperature, equal to 0 K.

What is 0 K in degrees Celsius?

−273.15 °C.

How do you approximately convert Celsius to Kelvin?

Add 273: T(K) ≈ T(°C) + 273.

How do you approximately convert Kelvin to Celsius?

Subtract 273: T(°C) ≈ T(K) − 273.

What is 0 °C in Kelvin?

273.15 K, approximately 273 K.

Why is absolute zero the lowest possible temperature?

It represents the minimum possible internal energy of a system.