Conductors, Semiconductors and Insulators
Conductors require free negative charge carriers : electrons (shown in red)
These conductor electrons are the outermost or “ valence” electrons in the furthest
orbit of the atom and are free to move about.
In conductors, they are not tightly bound in by the positive nucleus that is why are free
to move around. The black dot here represents the nucleus plus all the electron below
the outer orbit: so it is positive.
Conductors
In terms of energy bands, the valance band overlaps with the conduction band. This
results in the electrons being free to flow under and electric field applied by the
battery. In the diagram below the electrons flow from right to left.
+
-
Insulators
In an insulator all the outermost electrons within the structure are “bound
in” and unable to move freely.
Above is shown the structure of a plastic, where all of the outermost
electrons are covalently bonded.
That is each outermost electron is attracted to both its own nucleus and
simultaneously its nearest neighbouring nucleus. It is thus unable to move
freely as a mobile charge carrier.
Insulators
In terms of a band diagram and insulator looks like this:-
The valance band is the shell or
orbit of a materials outermost
electrons.
There are no electrons in the conduction band and a very large band gap between
the valance band and the conduction band. The valence band is completely filled.
Only if a very high voltage is placed across an insulator
(e.g. a plastic) will electrons jump up into the conduction
band from the valance band. This would then rupture the
plastic with a small lightening spark.
Semiconductors
Intrinsic Silicon
In an intrinsic semiconductor all the outermost electrons within the structure appear to be
“bound in” and unable to move freely -just like an insulator. And at absolute zero no conduction
takes place.
However only a small amount of thermal energy is needed to free an electron from its location
and become a free negative charge carrier . At the same time a hole is left behind: i.e. a free
positive charge carrier is created. So at room temperature there are an equal number of both
free positive and negative charge carriers i.e. holes and electrons.
How “holes”-positive charge carriers move
Cathode
-ve
Anode
+ve
As electrons jump into the adjacent hole in the structure, the
hole appears to move across the structure, much the same
way as a positive charge would. We call these positive charge
carriers “holes”.
If a voltage is applied to an intrinsic
semiconductor, then both the electron and the
holes can contribute to a small current flow.
Cathode
-ve
Anode
+ve
The holes flow towards the cathode away from the
anode, and the electrons towards the anode and away
from the cathode.
Intrinsic semiconductor –band diagram
At absolute zero there is no thermal energy and so all the electrons are bound into the valance band.
Above zero however , since the band gap is very small, thermal excitation can cause electrons to jump
from the valance band into the conduction band leaving a corresponding hole behind in the valance
band.
Temperature at
absolute zero
Temperature above
absolute zero
Intrinsic semiconductor band diagram –current flow.
If a voltage is applied to an intrinsic
semiconductor, then both the electron and the
holes can contribute to a small current flow.
n type doping
A neutral valence 5 atom is added, resulting a free electron
being produced. At absolute zero -with no thermal energy
available, the electron lies just below the conduction band.
However with only a little thermal energy it moves up to
the conduction band and the electrons are then free to
move.
The resistance decreases with n type doping, more negative
free charge carriers become available, but the
semiconductor remains NEUTRAL.
p type doping
p-type band structure at absolute zero
In p type doping, neutral valence 3 atoms are added, resulting in
free “holes” being produced (at room temperature).
p-type band structure above absolute zero
At absolute zero -with no thermal energy available, the electron
(that will move to produce a hole) lies at the top of the valance
band. No free “holes” are present.
But above 0k ,with only a little thermal energy the electron
moves up to the acceptor level and the holes are then free to
move at the top of the valance band.
The resistance decreases with p type doping, more positive free
charge carriers become available, but the semiconductor
remains NEUTRAL.
p-n junction
With a diode or a p-n junction, p type and n type semiconductors are joined.
At the junction a depletion layer forms: electrons from the n type jump into
the holes at the p type.
This leaves the n type side of the junction positive and makes the p type side
negative.
Reverse bias
Vs
If the p-n junction is connected to a dc supply with
the positive side connected to the n-type and the
negative side to the p-type the depletion layer is
widened.
A tiny leakage negligible current is the result.
Vs
“Neglegible”
leakage current
(micro amperes)
I (μA)
p-n junction in forward bias
Vs
I (mA)
A p-n junction is a diode
– current flows in forward
bias but not in reverse bias
Vo
Vs
The voltage required to overcome the depletion layer
is known as the “striking voltage” Vo.
If the p-n junction is connected to a dc supply
with the negative side connected to the n-type
and the positive side to the p-type the
depletion layer is reduced to zero, as the
supply voltage is increased.
A p-n junction (diode) graph
I (mA)
Forward bias:
After overcoming the
depletion layer
the current then
increases with the
voltage
Vs (volts)
Reverse bias:
“Neglegible”
leakage current
(micro amperes)
When a light emitting diode is operating in
forward bias and a current is flowing, the
electrons and holes meet at the junction and
recombine, resulting in a photon being emitted.
The energy of the photon is equal to the energy
gap.
Since the energy of the photon has the equation
ℎ𝑐
𝐸𝑝ℎ = ℎ𝑓 = λ
It means the bigger the frequency given off by
the LED the bigger the band gap.
Or the longer the wavelength given off by the
LED the smaller the band gap.
LED in forward bias-colour and striking voltage
I (mA)
Vs (volts)
Vo Vo
Vo
Vo Vo Vo
As the frequency of the light of the LED increases so the striking voltage Vo increases
It turn out that E=eVo gives an approximation to the band gap energy the electron jumped to produce the photon.
(https://www.ecse.rpi.edu/~schubert/Light-Emitting-Diodes-dot-org/LED-slide-show.pdf page 35
“Forward voltage is approximately equal to Eg / e” ( i.e. Egap≈eVo)
Work done W=QV
The work done moving a charge across a voltage is W=QxV
Hence the energy moving an electron charge e through a voltage
Vo is W=QV=eVo
The charge on an electron is e=1.6x10^-19 coulombs.
So the work done moving an electron across 1V is
W=QV=eV=1.6x10^-19 x1=1.6x10^-19 J
i.e. 1eV=1.6x10^-19 J
Eph=hf=hc/λ
When a photon of light is emitted, an electron moves from a high energy level to a
lower energy level.
At a p-n junction of an LED in
forward bias, electrons from
the conduction band descend
to the valence band and, as
electron hole recombination
takes place, photons are
emitted.
The bigger the band gap the
higher the frequency and the
longer the wavelength.
Irradiance=Power/Area (Intensity)
Units watts per metre squared, Wm-2
Irradiance is the light energy per second per square metre
So I=Nhf
Since Eph=hf (=hc/λ)
and N=number of photons per second per square metre
The irradiance is a measure of the number of photons being produced per second
(per m2) so will increase as the number of electron hole recombinations occur.
As the current increases, electron hole recombination will increase so the irradiance
should increase.
Intensity
?? –investigate!! –by experiment
current
Back up data from internet –by google image
https://www.google.co.uk/search?q=Luminance+versus+current+for+a+LED&safe=strict&biw=1280&bih=878&tbm=isch&tbo=u&source=univ&sa=X&ved=0CBwQsARqFQoTCPX0vN3NmckCFYhXGgodRVALnA
https://engineering.purdue.edu/~ece495/Power_Electronics_Lab/LED_Basics.pdf
http://www.societyofrobots.com/electronics_led_tutorial.shtml
http://www.screens.ru/en/2003/7.html
http://www.screens.ru/en/2003/7.html
How threshold voltage relates to LED colour
http://www.chemistry.wustl.edu/~edudev/LabTutorials/PeriodicProperties/MetalBonding/MetalBonding.html
Bonds, Bands, and Doping:
How Do LEDs Work?