introductory semiconductor properties micro-interfacing (2002) james mackey
TRANSCRIPT
Introductory Semiconductor Properties
Micro-Interfacing (2002)
James Mackey
Silicon Atom
4 outer electrons are more loosely bound
Silicon Atom Simplified
Only the electrons participating are shown
+14 +4
Germanium Atom
4 outer electrons are more loosely bound
Germanium Atom Simplified
Only the electrons participating are shown
+32 +4
+4 +4 +4+4 +4 +4
+4 +4 +4+4 +4 +4
+4 +4 +4+4 +4 +4
+4 +4 +4+4 +4 +4
+4 +4 +4+4 +4 +4
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.
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+4 +4 +5+4 +4 +4
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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.
This is now an n-type silicon or germanium
Excess electron
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+4 +4 +3+4 +4 +4
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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
This is now a p-type silicon or germanium
VACANCY or hole
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+4 +4 +5+4 +4 +4
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+-
If a potential is applied to the crystal
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ElectronsHoles
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-
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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
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N P
-
- -
- -
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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.
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N P
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As the electrons diffuse across the junction, the N regions becomes + while the P region becomes - , producing a junction potential which limits diffusion.
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N P
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depletion layer
+ -
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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).
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N P
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+ - - -
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No bias - only a few thermally produced carriers
Reverse bias - only a few minority carriers contribute to the conduction process (holes in p-type and electrons in n-type)
N
N P
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.
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+ +
+ - - -
- - -
- - -
N P
Forward bias - the majority carriers in each region contribute to the current, which is much larger than previous cases.
e- e-
This operation form the basis of the semiconductor diode
p-type n-type
Anode Cathode
material
designation
symbol
conventional or + current
.5-1.0 -.5
1.0
10
5
15
20
25
mA
A
-10
-50
-30
perfect diode
Real diode
Note that the reverse current scale is microamps
The forward current scale is milliamps
Current-Voltage characteristics for a pn junction diode
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.
npn transistor pnp transistorn
n
pn
p
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.
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.
In the P-type (filling) layer conduction can take place by the movement of the free holes in the valence band.
n p 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.
Emitter Base Collector
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!
Now consider what happens when we apply a relatively small Emitter-Base voltage whose polarity is designed toforward-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 Emitter-Base 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
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!
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.
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 itspositive 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.
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.
To prevent this happening we use the applied Emitter-Base voltage to remove the captured electrons from the Base and maintain the number of holes it contains.
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 negatively-charged 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.