Hermatage Semiconductor Devices - Kelso High School

woundcallousSemiconductor

Nov 1, 2013 (3 years and 11 months ago)

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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 10
14

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




M
etal


O
xide


S
emiconductor


F
ield


E
ffect


T
ransistor

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.