Transcript semiconductors

Semi Conductors
• Between conductors and insulators are
materials, which allow an electric
current to flow only under certain
circumstances. These are called
semiconductors.
Semiconductors
• To understand
semiconductors,
we need to
understand the
structure of a
semiconductor
atom.
• Four electrons in
the valence band.
Semiconductors
There are several types of semiconductor
material available, but the most commonly
used materials are:-
Silicon
&
Germanium
Semiconductors
Both these materials have the structure shown
below.
4 electrons
in valence
band
core.
representing
nucleus and
inner bands
Semiconductors
If we imagine that there is another band
just outside the Valance band, called the
conductor band, that electrons could
move into and this conductor band gives
the atom similar properties to a conductor.
Then the conduction abilities of a
Semiconductor material can be altered.
Semiconductors
• In SC materials the gap
(forbidden gap)
between the Valance
band and conduction
band is very small. So a
little extra energy
(heat), closes the gap,
enabling the electrons
to pass into the
conduction band.
germanium
conduction
band
Forbidden
gap
germanium
valence
band
Intrinsic Semiconductors
It is this property that we can use for NTC
(Negative Temperature Coefficient) sensors.
Hence Germanium is adopted for use in NTC
temperature sensors.
What does NTC mean?
Intrinsic Semiconductors
As the temperature of the Germanium material
is increased more electrons are given enough
energy to jump into the conducting band and
hence lower its resistance.
NTC
Intrinsic Semiconductors
The semiconductor materials that naturally act
in this way are normally referred to as Intrinsic
semiconductors.
For other purposes other than temperature
sensing, changing the properties of the natural
semiconductor material are required.
Extrinsic Semiconductor
An Intrinsic semiconductor material can be
altered by adding minute quantities of another
substance, the electrical properties of the
material can be altered.
Impurities such as phosphorus, arsenic,
antimony and bismuth are added to the natural
material to give an excess of electrons.
Impurities such as indium, aluminum, gallium
and boron are added to give an excess of
holes.
Extrinsic Semiconductor
These doped semiconductor materials are
man made and are referred to as
Extrinsic semiconductors.
Semiconductor materials with an excess of
electrons are called N – Type.
Semiconductor materials with an excess of holes
are called P – Type.
N-Type Semiconductor
Materials
Doping elements used to make N – Type
materials all have 5 electrons in there outer
shell and are referred to as pentavalent
elements.
When added to silicon or germanium, four of
these electrons form covalent bonds with the
semiconductor atoms. However, the fifth
electron, although temporarily bound to its
additive atom, can easily be detached.
N-Type Semiconductor
Materials
A semiconductor containing these
additives thus has many free electrons to
form an electric current.
The conductivity of the semiconductor is
now a thousand times greater.
N-Type Semiconductor
Materials
Extra electron
P-Type Semiconductor
Materials
We have seen that introducing atoms,
which have extra electrons, can increase
the conductivity of a semiconductor.
Introducing atoms, which produce extra
holes, can also increase it. Doping
elements used to make P – Type
materials all have 3 electrons in there
outer shell and are referred to as trivalent
elements.
P-Type Semiconductor
Materials
Extra Hole
The PN Junction Diode
Up to now we have examined p-type and n-type
material separately. However, the properties of
these extrinsic semiconductor materials
become useful only when the two types are
combined in some way.
The area where the two types meet is called a
pn junction, and the resulting properties allow
it to be used as a diode.
The PN Junction Diode
A diode is the electrical
equivalent of a one-way
valve,
which
normally
allows current to flow
through it in one direction
only. They can be made to
handle currents varying
from a few micro amps to
many hundreds of amps.
The PN Junction Diode
P
Electron
flow
N
With the N layer connected
to the negative terminal and
P layer connected to the
positive terminal, electrons
flow through the diode.
FORWARD BIAS
The PN Junction Diode
P
N
NO FLOW
If the battery terminals
are reversed then there
will be no flow, as the
electrons and holes are
attracted away from the
junction to there
opposites in the battery.
REVERSE BIAS
Transistors
An even more useful device is made when three
layers of semiconductor material are formed
together in a single continuous crystal of silicon
or germanium as an 'NPN' sandwich, or a 'PNP'
sandwich.
Transistors
Transistors act like a
minute relay. When
the base is
connected to the
opposite source
(e.g. N to positive/
earth) it connects
the emitter to the
collector and current
flows through.
Transistors
E
C
FLOW
B
NO FLOW
Transistor Operation
Small Current to the Base Switches the Collector
and Emitter on.
Transistors
Uses for Transistors
•Switching device
As the base requires very small current to
actually activate the transistor, it can be
used instead of a relay. Benefits include
no mechanical parts at all (solid state)
and with such small currents being used
to switch them on, they can be controlled
by control units.
Transistors
Uses for Transistors
•Amplifier
If two or more transistors are used in
series with each other then, this tiny
current applied to the base can be used
to allow very large currents to flow.