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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.
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 Constantf – 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
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
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.
Irradiance, I
Resistance, R
MOSFET 1
Stands for –
MetalOxideSemiconductorFieldEffectTransistor
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.