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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 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.