Higher Physics - Unit 3 - Optoelectroics and Semi-conductors

I created my own revision notes for all units of the Higher Physics course.

These are revision notes for part of the 3rd unit (Radiation and Matter), section 3 (Optoelectronics and Semi-conductors). I've uploaded all the subsections of this section in different files as they are otherwise too large.

This is a collection of 'definitions' for some of the key information in Unit 3.3. This covers:

• Photoelectric effect
• Work function
• Threshold frequency
• Photons
• Electrons - ground state
• Electrons - excited state
• Electrons - ionisation level
• Line emission spectra
• Line absorption spectra
• Spontaneous emission
• Stimulated emission
• LASER
• Doping
• n-type semiconductor
• p-type semiconductor
• Forward biased p-n junction diode
• Reverse biased p-n junction diode
• Photodiode
• Photodiode in photovoltaic mode
• Photodiode in photoconductive mode
• LED
• MOSFET - how it works
• MOSFET voltages
• MOSFET

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• Created by: Visser
• Created on: 08-05-10 17:11

Photoelectric effect

When certain metals are exposed to the correct frequency of electromagnetic radiation, electrons are ejected from the surface. The interaction between light and electrons is called the photoelectric effect.

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Work function

The work function is the minimum energy required to remove an electron from a metal (or solid).

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Threshold Frequency

The minimum frequency required to eject an electron from a surface.

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Photon

A quantum of visible light or other form of electromagnetic radiation demonstrating both particle and wave properties.

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Electrons - ground state

The lowest energy level of an electron.

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Electrons - excited state

An electron which moves from its usual energy level to a higher energy level

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Electrons - ionisation level

The energy level furthest from the ground level. Electrons at the ionisation level are so far from the nuclear that they are beyond its influence. They are ionised (freed).

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Line emission spectra

As the energy levels have different values, each of the possible electron transitions within an atom will produce a photon with a different energy. This means that each electron transition will produce a photon of different frequency and hence a different colour.

The brightness of a spectral line is determined by the number of electrons making the particular energy change.

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Line absorption spectra

· Sodium flames absorbs photons to fill energy gaps.

· Emits photons in all directions.

· Black lines appear where the colours of these particular energies would have been.

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Spontaneous emission

Electrons can often be raised from their ground state to an excited state; when they return to their ground state they emit photons of a particular frequency in the process.

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Stimulated emission

If an electron is in an excited state and the energy of the stimulating photon matches the energy gap (E2 - E1), then the electron will jump down and emit an identical photon.

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LASER

Light Amplification by the Stimulated Emission of Radiation

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Doping

The deliberate addition of impurities into a semiconductor decreases its resistance.

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n-type semiconductor

The pure semiconductor with four electrons in its outer shell is doped with an impurity with five electrons in its outer shell. Four of the electrons in the outer shell of the impurity are ‘used up’ in bonding with the surrounding atoms, but the fifth electron is a free charge carrier. N-type material: the majority of the free charge carriers are negative

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p-type semiconductor

The pure semiconductor with four electrons in its outer shell is doped with an impurity with three electrons in its outer shell. The three electrons of the impurity are ‘used up’ in bonding with the surrounding atoms and there is a ‘hole’ where the ‘missing’ electron should be. P-type material: the majority of the free charge carriers are positive

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Forward biased p-n junction

Negative terminal --> n-type

Positive terminal --> p-type

electrons --> to positive terminal = charge carriers enough

holes --> to negative terminal energy to overcome voltage barrier

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Reverse biased p-n junction

Negative terminal --> p-type
Positive terminal -->n-type

electrons→ to positive terminal

holes→ to negative terminal = depletion layer widens

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Photodiode

When light strikes the p-n junction, each photon of light creates electron/hole pairs.

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Photodiode - photovoltaic mode

When light shines on the photodiode, a voltage is produced. In photovoltaic mode it can supply power to a load.

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Photodiode - photoconductive mode

No light: it acts like a normal reverse biased diode, blocking all current.

Light: electron/hole pairs are created and a current is able to flow in the reverse directiona LEAKAGE CURRENT!

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LED

In forward biased p-n junction diodes, electrons and holes going in opposite directions ‘recombine’. Each electron/hole pair which recombines produces energy. Usually this is heat energy, but in an LED the energy is emitted as photons of light.

One electron filling one hole = one photon of energy (E=hf)

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MOSFET - how it works

· +V supplied to the gate = electrons within the substrate are attracted to the region below the gate (because it is formed by electrons it is an n-channel)

· V applied between the source and drain (+ drain, - source) – a current flows through the channel from the source to the drain

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MOSFET voltages

VGS > 2V the MOSFET is switched on. A current now flows from the source to the drain.

There are two ways to increase the drain current:
Increase VGS (beyond the threshold)
Increase VDS

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MOSFET

n-channel enhancement mode

Metal Oxide Semiconductor Field Effect Transistor

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