Transcript Forward bias

Introductory Semiconductor Properties
Micro-Interfacing
(2002)
James Mackey
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Silicon Atom
4 outer electrons are more
loosely bound
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Silicon Atom Simplified
Only the electrons participating
are shown
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Germanium Atom
4 outer electrons are more
loosely bound
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Germanium Atom Simplified
Only the electrons participating
are shown
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A 2-dimensional representation of a portion of a crystal of silicon or
germanium. The bonding is covalent (one electron for each of two
atoms), and each atom has 8 valence electrons around it.
If a small amount of an impurity atom with 5 valence electrons (P, As, Sb) is
added to the silicon or germanium crystal......the conductance of the crystal
changes significantly.
Excess
electron
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This is now an n-type silicon or germanium
If a impurity with 3 valence electrons (B, In, Al) is added to the crystal, then
the result is an electron deficit or a hole
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This is now a p-type silicon or germanium
VACANCY or
hole
If a potential is applied to the crystal
-
-
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Electrons
In addition to the electron-hole motion
produced by the external potential, at room
temperature there is always a certain number
of electron-hole pairs generated as an
electron escapes from its binding site leaving
behind a hole.
NORMALLY these thermally generated pairs
are much less significant than the potential
generated pairs and can be ignored.
As long as a potential is present these
electron-hole pairs are continuously produced.
To make a semiconductor junction, p-regions
and n-regions are produced in a single crystal
P
N
+
+
+
+
-
-
-
-
+
+
-
+
-
-
The n-type material is “rich” in electrons and
deficient in holes, while the p-type is “rich” in
holes and deficient in electrons.
As the junction if formed, impurity atoms near
the junction will supply electrons to diffuse
across the junction.
P
N
+
+
+
+
-
-
-
+
+
-
+
-
-
As the electrons diffuse across the junction, the N regions
becomes + while the P region becomes - , producing a
junction potential which limits diffusion.
depletion layer
N
+
P
+
+
+
-
-
-
+
-
+
+
-
+
+
-
-
The junction potential is about 0.7 volts for silicon
and 0.4 volts for germanium. The junction acts
like a small voltage and a small capacitor (the
depletion zone acts like a thin dielectric).
P
N
+
+
+
+
-
-
-
+
+
-
+
-
-
No bias - only a few thermally produced carriers
N
+
+
+
-
-
+
-
-
-
P
-
-
Reverse bias - only a few minority carriers contribute to the conduction
process (holes in p-type and electrons in n-type)
N
P
+
+
+
-
-
+
-
-
e-
-
e-
When the material is reverse-biased, the depletion
layer grows in size, reducing current flow even
more.
Forward-biasing causes the depletion layer to shrink
in size, enhancing the flow of charges through the
device.
Forward bias - the majority carriers in each region contribute to the
current, which is much larger than previous cases.
N
P
+
+
+
-
-
+
-
-
-
-
e-
This operation form
the basis of the
semiconductor diode
e-
p-type
n-type
Anode
Cathode
material
designation
symbol
conventional
or + current
Current-Voltage characteristics for a pn junction
diode
mA
perfect
diode
The forward current
scale is milliamps
Real
diode
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20
15
10
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Note that the reverse
current scale is microamps
A transistor is a device made from 3 regions, i.e.
N P N or P N P
where the middle region is very thin (an is called
the base), constitutes a transistor. The injection
of small currents in the middle region has a large
effect on the current between the large regions,
N to N, or P to P.
p
n
p
n
npn transistor
pnp transistor
n
p
A Bipolar Transistor essentially consists of a pair of PN Junction
Diodes that are joined back-to-back. This forms a sort
of a sandwich where one kind of semiconductor is placed in
between two others. There are therefore two kinds of Bipolar
sandwich, the NPN and PNP varieties. The three layers of the
sandwich are conventionally called the Collector, Base, and
Emitter.
Some of the basic properties exhibited by a
Bipolar Transistor are immediately recognizable
as being diode-like. However, when the 'filling' of
the sandwich is fairly thin some interesting
effects become possible that allow us to use the
Transistor as an amplifier or a switch. To see how
the Bipolar Transistor works we can concentrate
on the NPN variety.
Figure 1 shows the energy levels in an NPN transistor when we
aren't externally applying any voltages.
We can see that the
arrangement looks
like a back-to-back
pair of PN Diode
junctions with a thin
P-type filling
between two N-type
slices of
'bread'.
In each of the N-type layers conduction can take place by the
free movement of electrons in the conduction band.
In the P-type (filling) layer conduction can take place by
the movement of the free holes in the valence band.
n
p
However, in the absence of any externally
applied electric field, we find that depletion zones
form at both PN-Junctions, so no charge wants to
move from one layer to another.
n
Consider now what happens when we apply a moderate
voltage between the Collector and Base parts of the
transistor.
The polarity of the applied
voltage is chosen to increase
the force pulling the N-type
electrons and P-type holes
apart. (i.e. we make the
Collector positive with respect
to the Base.)
This widens the depletion zone between the
Collector and base and so no current will flow.
In effect we have reverse-biased the Base-Collector diode
junction. The precise value of the Base-Collector voltage we
choose doesn't really matter to what happens provided we
don't make it too big and blow up the transistor!
Emitter
Base
Collector
Now consider what happens when we apply a relatively
small Emitter-Base voltage whose polarity is designed to
forward-bias the Emitter-Base junction.
This 'pushes' electrons from the
Emitter into the Base region and
sets up a current flow across the
Emitter-Base boundary.
Once the electrons have managed to get into the Base
region they can respond to the attractive force from the
positively-biased Collector region.
As a result the electrons which get into the
Base move swiftly towards the Collector and
cross into the Collector region.
Emitter
Base
Collector
Hence we see a Emitter-Collector current
whose magnitude is set by the chosen EmitterBase voltage we have applied.
To maintain the flow through the transistor we
have to keep on putting 'fresh' electrons into the
emitter and removing the new arrivals from the
Collector. Hence we see an external current flowing
in the circuit.
Emitter
Base
Collector
The precise value of the chosen Emitter-Base
voltage isn't important to our argument here, but
it does determine the amount of current we'll see.
For the sake of example we've chosen a half a volt.
Since the Emitter-Base junction is a PN diode we can
expect to see a current when we apply forward
voltages of this sort of size.
In practice with a Bipolar transistor made using Silicon
we can expect to have to use an Emitter-Base voltage
in the range from around a half volt up to almost one
volt. Higher voltages tend to produce so much current
that they can destroy the transistor!
It is worth noting that the magnitude of the
current we see isn't really affected by the chosen
Base-Collector voltage. This is because the current
is mainly set by how easy it is for electrons to get
from the Emitter into the Base region.
Most (but not all!) the electrons that get into the Base move
straight on into the Collector provided the Collector voltage is
positive enough to draw them out of the Base region. However,
a few of the electrons get 'lost' on the way across the Base.
This process is illustrated in the figure shown.
Some of the free
electrons crossing
the Base encounter
a hole and 'drop
into it'. As a result,
the Base region
loses one of its
positive charges
(holes) each time
this happens.
If we didn't do anything about this we'd find that the
Base potential would become more negative (i.e.
'less positive' because of the removal of the holes)
until it was negative enough to repel any more
electrons from crossing the Emitter-Base junction.
The current flow would then stop.
To prevent this happening we use the applied EmitterBase voltage to remove the captured electrons from the
Base and maintain the number of holes it contains.
This has the overall effect that we see some of the
electrons which enter the transistor via the Emitter
emerging again from the Base rather than the Collector.
For most practical Bipolar Transistors only about 1% of
the free electrons which try to cross Base region get
caught in this way.
Hence we see a Base Current, IB, which is typically
around one hundred times smaller than the Emitter
Current, IE.
An alternate way of looking at
transistors is to use the source,
gate, and drain picture.
Transistors consist of
three terminals; the
source, the gate, and the
drain.
In the n-type transistor,
both the source and the
drain are negativelycharged and sit on a
positively-charged well of
p-silicon.
When positive voltage is
applied to the gate,
electrons in the p-silicon
are attracted to the area
under the gate forming an
electron channel between
the source and the drain.
When positive voltage is
applied to the drain, the
electrons are pulled from the
source to the drain. In this
state the transistor is on.
If the voltage at the
gate is removed,
electrons aren't
attracted to the area
between the source
and drain. The
pathway is broken and
the transistor is turned
off.