Transcript Band Theory of solids

Band Theory of Solids
In isolated atoms the
electrons are arranged
in energy levels
In solids the outer electron energy levels become
smeared out to form bands
The highest occupied band is called the VALENCE band.
This is full.
For conduction of electrical energy there must be
electrons in the CONDUCTION band. Electrons are free
to move in this band.
Insulators : There is a big energy gap between the
valence and conduction band. Examples are plastics, paper
…..
Conductors : There is an overlap between the valence and
conduction band hence electrons are free to move about.
Examples are copper, lead ….
Semiconductors : There is a small energy gap between
the two bands. Thermal excitation is sufficient to move
electrons from the valence to conduction band. Examples
are silicon ,germanium….
When a conductor is
heated its resistance
increases ; The atoms
vibrate more and the
electrons find it more
difficult to move
through the conductor.
But in a semiconductor
the resistance
decreases with an
increase in
temperature. Electrons
can be excited up to
the conduction band.
Conductivity increases
R
R
T
T
Doping
This is the replacement of a silicon atom in
the crystal structure with a different atom.
Typically 1 atom per 100 million silicon atoms
is replaced. This changes the electrical
properties.
Silicon is in group 4 of the periodic table. It
has 4 valence electrons.
P type silicon.
Replace a silicon atom with a group
3 atom such as aluminium.( 3
valence electrons)
This lowers the number of
electrons in the valence band which
creates a ‘hole’ in the structure.
Electrons can move through the
structure.
This is called p type silicon. P is for
positive as there is a deficiency of
electrons.
N type
Replace a silicon atom with a group
5 atom such as phosphorous ( 5
valence electrons )
The extra electron introduced
here goes into the conduction band
as the valence band is full.
This is n type silicon.
N for negative as ‘additional ‘
electrons have been introduced
into the structure.
pn junction or diode
To be forward biased
( current flows ) the p type
must be connected to the
positive terminal of the
power supply.
pn junction
Electrons flow from the negative
terminal of the battery to the
n-type semiconductor, where they
occupy the conduction band.
The electrons then move into the
conduction band of the p-type
(there is a potential barrier here
that must be overcome)
semiconductor and fall into the
valence band. The excess energy is
released as heat energy.
Electrons then move through wire.
Electrons do not flow from the
p type to the n type and so a pn
junction can be used as a
rectifier. This allows current
to flow through it in one
direction and can be used to
turn a.c to d.c.
There is actually a small
reverse flow current of the
order of micro amps but
compared with the current
flowing from n to p this is
negligible.
LEDs
Electrons flow from the negative
terminal of the battery to the ntype semiconductor, where they
occupy the conduction band.
The electrons then move into the
conduction band of the p-type
semiconductor and fall into the
valence band this releases
energy in the form of light.
The electrons then move through
the wire back to the positive
terminal of the battery, and they
re-circulate.
The energy gap can be altered in the pn junction by
adding different impurities. The bigger the energy gap
the bigger the frequency of light emitted.
We will look at this in unit 3
Photovoltaic cells
These are like leds in reverse.
Photons of light enter the cell
An electron is ejected from the
valence band in the p type producing
a ‘hole’. Charges can flow.
This electron moves up to the
conduction band
The electron moves to the conduction
band of the n type silicon.
If the cell is connected to a load the
electrons will flow around the
external circuit.
The base of the triangle is the p type
The more photons of light that enter the
cell the more electrons are released and
the greater the potential difference.
The p type becomes the positive end and
the n type the negative end of the
‘power supply’