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. 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 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 Example Find the recombination energy for the following LEDs – a) red – λ = 650 nm b) blue – f = 4.5 x 1014 Hz 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 Photovoltaic Mode 1 Light source Photodiode has no bias voltage applied, but is illuminated by a light source. V Electron-hole pairs are produced, giving a potential difference Voltage, V The output voltage increases as the irradiance of the source increases Irradiance, I 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 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 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. Resistance, R Irradiance, I MOSFET 1 Stands for – Metal Oxide Semiconductor Field Effect Transistor 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.