Transcript conduction band - Caldervale High School

Notes p.22 Conductors, Insulators & Semiconductors
Atomic Energy Levels for Free Atoms
Electrons in “free atoms” occupy discrete energy
levels.
nucleus
Electrons in
various orbits
which
correspond to
discrete
energy levels
The highest energy electron
orbits are those furthest
from the nucleus of the
atom.
Electrons in “free atoms”
have these specific
energies.
Energy of electrons
An energy level diagram
represents these energies as
horizontal lines.
Energy “Bands” for Atoms in Solids
Instead, the energy levels of the
individual atoms get slightly
changed to produce continuous
“bands” of permitted energies
separated by band gaps where
electrons do not stay.
Like this …
Band Gap
When atoms come together to
form solids, the electron energy
levels cannot be shared between
electrons of neighbouring atoms.
Band Diagram Labelling
The lower of these 2 bands is
called the valence band.
The upper is called the conduction
band.
conduction
band
Band Gap
While there could be several
permitted energy bands in a solid,
it is the 2 outermost bands and
the gap between them that
determine whether the solid is a
conductor, an insulator or a
semiconductor.
valence
band
Conduction
A conducting material must
have either electrons in its
conduction band or spaces in
its valence band (under
certain conditions).
Conduction band
empty or
part filled
Band Gap
Conduction in a material can
only happen if there is a
partially filled band and
therefore spaces for electrons
to move into.
full
valence band
Conductors, Insulators & Semiconductors
Conductors have a partially filled conduction
band, so electrons can move easily and resistance
is low.
There is really no band gap to speak of! The
conduction and Valence bands overlap.
(eg. copper, silver,…)
Conduction band part filled
Their resistance is high and
current can’t pass easily. (eg.
plastic, rubber,…)
Conduction band
empty
Big Band Gap
Insulators have a large band gap
and full valence band. There are
no electrons in the conduction
band as the band gap from the
valence band is too big for them
to cross.
full
valence band
Semiconductors have a small band gap. Electrons
can be “mobilised” under certain conditions, enough
to cross the gap into the conduction band. For
example by heating or “doping”. So the resistance
of semiconductors can be decreased by heating.
This is the basis of a thermistor.
(eg. silicon, germanium,..)
Conduction band
Normally
empty
Small Band Gap
Valence band
full
In Summary
(shading represents electron occupation)
Insulator
Conductor
Semiconductor
Conduction
band
Conduction
Conduction
band
band
Valence
band
Valence
band
Valence
band
Bonding in Semiconductors
Basic Points …
•In atoms, outer shell electrons are the ones that
bond to other atoms.
•Atoms are neutral (no. of electrons = no. of
protons).
•If a material has “mobile electrons” it does
NOT mean the material is negative. It can still
be overall neutral!
Doping Semiconductors
One way to make electrons mobile in a material is
to “dope” the material with impurity atoms that
have more or less electrons in their outer shells.
Forming an n-type Semiconductor
Dope a semiconductor “host” material with impurity atoms
that have more electrons in their outer shell than the host
material.
So mobile charges are –ve electrons.
Forming an n-type Semiconductor
Dope a semiconductor “host” material with impurity atoms
that have more electrons in their outer shell than the host
material.
So mobile charges are –ve electrons.
silicon
arsenic
Forming an n-type Semiconductor
Dope a semiconductor “host” material with impurity atoms
that have more electrons in their outer shell than the
host material.
Extra
So mobile charges are –ve electrons.
silicon
arsenic
electrons
in
Conduction
band.
Valence
band full.
Forming a p-type Semiconductor
Dope a semiconductor “host” material with impurity atoms
that have fewer electrons in their outer shell than the
host material.
So mobile charges are +ve “holes”.
silicon
indium
Mobile
holes in
Valence
band.
Conduction
band has
fewer
electrons.
PROBLEMS: page 27 & 28, Q. 1 - 4