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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.