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 whereh – 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 10 14 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. 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 V Light source Irradiance, I Voltage, V

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. 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. Irradiance, I Resistance, R

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.