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Higher Physics

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

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  1. Higher Physics Semiconductor Diodes

  2. 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.

  3. 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

  4. 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

  5. Photodiode • 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

  6. Light source Voltage, V 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

  7. 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

  8. 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.

  9. 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.

  10. MOSFET 1 • Stands for – Metal Oxide Semiconductor Field Effect Transistor

  11. 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.

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