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Higher Physics. Semiconductor Diodes. Light Emitting Diode 1. An LED is a forward biased diode When a current flows, electron-hole pairs combine at the p-n junction.

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Higher physics

Higher Physics

Semiconductor Diodes

Light emitting diode 1
Light Emitting Diode 1

  • An LED is a forward biased diode

  • When a current flows, electron-hole pairs combine at the p-n junction.

  • The doping of the p-type and n-type materials is carefully tuned so that there is excess energy when electron-hole pairs combine.

Light emitting diode 2
Light Emitting Diode 2

  • The recombination energy of the electron-hole pairs is released by de-excitation of the electrons

  • This leads to photon emission

  • Photon Energy, E = hf

    where h – Planck’s Constant

    f – light frequency

Light emitting diode 3
Light Emitting Diode 3

  • Example

    Find the recombination energy for the following LEDs –

    a) red – λ = 650 nm

    b) blue – f = 4.5 x 1014 Hz


  • A photodiode has a

    p-n junction where electron-hole pairs are generated by absorbed photons from incident light

  • Photodiodes can be operated in two different modes

Photovoltaic mode 1

Light source

Voltage, V


Irradiance, I

Photovoltaic Mode 1

  • Photodiode has no bias voltage applied, but is illuminated by a light source.

  • Electron-hole pairs are produced, giving a potential difference

  • The output voltage increases as the irradiance of the source increases

Photovoltaic mode 2
Photovoltaic Mode 2

  • Output voltage can be used to power devices e.g. photo cell for calculator

  • Can be connected in series to give larger voltage outputs

  • In this mode the photodiode operates in exactly the opposite way to an LED

Photoconductive mode 1
Photoconductive Mode 1

  • In this mode the photodiode is connected in reverse bias.

  • If it is kept dark, it acts a reverse-biased p-n junction and will not conduct.

  • If it is illuminated, the junction will release electrons and create electron-hole pairs.

  • This provides a number of free charge carriers in the depletion layer, decreasing the resistance and enabling a current to flow.

Photoconductive mode 2

Resistance, R

Irradiance, I

Photoconductive Mode 2

  • A greater irradiance gives more free charge carriers and therefore less resistance.

  • The photodiode acts as a light dependent resistor (LDR)

  • Because the electron-hole pairs recombine quickly LDRs have a very fast response time, allowing them to be used in situations where light levels change rapidly.

Mosfet 1

  • Stands for –







Mosfet 2

  • Operate like npn transistors (have threshold voltage to ‘switch on’)

  • Work in a different way, due to charge distributions inside the transistor material.