Created and maintained by Bill Beaty. Mail me at:
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OK, everything we know is wrong, and transistors aren't really "current amplifiers." <grin> Instead the base voltage is the important thing, not the base current.
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______|______
| |
| COLLECTOR N |
|_____________|
| | ----> With a small voltage applied,
| BASE P |____________ the depletion layer gets thin,
|=============| | + thin, charges start crossing
| | ____|____ it, and a small flow of charge
| EMITTER N | _____ appears in the battery circuit.
|_____________| _________ The "switch" is only partly
| _____ closed!
|___________________| -
<-----
The changing thickness of the insulating depletion layer switches
the transistor on and off. And since BASE VOLTAGE is what changes the
thickness,
we can ignore the current in the base wire. But wait a
minute, WHICH flow of charge is being
switched on and off? Ah, we have another entire circuit to add to our
diagram. We connect another battery across the entire transistor,
between emitter and collector. Let's use a common 9-volt battery.
<------
_______________________
| |
| |
| ______|______
| | |
Collector | | COLLECTOR N |
Battery | + |_____________|
____|____ | |______________
_____ | BASE P | |
_________ |=============| | +
_____ 9V | | ____|____
_________ | EMITTER N | _____ Base
_____ |_____________| _________ Battery
_________ | _____ .5V
_____ | | -
| - |_____________________|
|_______________________|
------->
So the Base Battery turns on the transistor's "switch", and this lets the
9-volt Collector-Battery drive a large flow of charge vertically through
the entire thing.
What use then is the "collector's" silicon? Won't the voltage from the
collector battery override control from the base? And why have THREE
silicon segments at all? Won't the second Depletion Layer turn everything
off? And why not just connect the top wire to the Base
section directly? The answers are in the last of these questions. If we got rid of the collector, we'd accidentally connect the two batteries together, since silicon is a good conductor. We'd end up with a diode instead (see below.) The batteries would fight each other, and the diode would just act like a short circuit between the two batteries.
IT'S ALL SHORTED OUT, IT
GETS HOT AND SMOKES
_____________________ __________________
| | | |
Collector | | | |
Battery | + ____|____|___ |
____|____ | | |
_____ | BASE P | |
_________ |=============| | +
_____ 9V | | ____|____
_________ | EMITTER N | _____ Base
_____ |_____________| _________ Battery
_________ | _____ .5V
_____ | IT'S A PN DIODE | -
| - | |
|_______________________|_____________________|
Obviously the collector is required. Obviously the collector segment does
something really strange!
Notice that the collector battery is applying a (+) polarity to the collector, but the collector is n-type silicon. Isn't this backwards? Won't there be a whole second Depletion Layer forming between collector and base? YES! And since we're using a 9-volt battery to pull the movable holes in the p-type away from the electrons in the n-type, this depletion layer will be a thick one. It should act like a turned-off switch, eh? It does... and yet it doesn't. I personally think this is the strangest part of transistor action, and it took me a good while before my brain stopped rejecting the weirdness so I could "see" it all happening at once. <------
_______________________
| |
| |
| ______|______
| | |
Collector | | COLLECTOR N |
Battery | + |_____________| thick depletion layer
| _____________
____|____ | |______________
_____ | BASE P | |
_________ |=============| | +
_____ 9V | | ____|____
_________ | EMITTER N | _____ Base
_____ |_____________| _________ Battery
_________ | _____ .5V
_____ | | -
| - |_____________________|
|_______________________|
------->
OK, this new depletion layer keeps the Collector Battery from affecting
the
rest of the transistor. If we increase the voltage of that 9V battery,
the insulating layer between Base and Collector segments just gets
thicker, and the Base/Emitter segments below the Collector
never feel the voltage-force from that battery. Yes, the
"upper surface" of the Base segment in the upper depletion zone
does feel the force from the 9V battery, but the rest of the
circuit does not. (It's like waving a highly-charged balloon near
a flashlight's circuit. Nothing happens to the charge flow in
the flashlight.)
HOWEVER!
Because the Base battery has already thinned out the insulating emitter
depletion
layer,
this means that swarms of movable electrons can pour from the Emitter and
upwards into
the Base segment. Only a few will actually flow upwards into the Base,
since it would cause a traffic jam if the
Base wire wasn't able to immediately suck those electrons out again. (Or
more accurately, if the electrons in the Base don't leave again, and
aren't cancelled by holes,
then any extra electrons would cause the Base segment to become
negatively charged, which would repel any more electrons coming upwards
from the Emitter.
So now we have a sparse cloud of a few
electrons entering the p-type silicon of the Base section from below, and
some of those
electrons wander upwards into the "upper surface" of the Base segment.
What happens? They're suddenly exposed to the attraction of the 9V
battery positive voltage.
The upper depletion region doesn't act so much like a
hunk of insulating glass, instead it acts like an insulating air gap.
It's only insulating if there are no movable charges present. It doesn't
block the flow of charges, but if no charges exist there, the voltage
cannot create a charge flow.
PS, Don't forget, there
were always plenty of
holes already in the Base segment, but any holes which dare to wander
upwards out
of the Base
segment will be pushed back down by the positive polarity of the 9V
battery.
(That's what makes the depletion zone act like an insulator in the first
place: it repels holes back into the P, and repels electrons back into
the N.)
Imagine that the Collector segment is conductive metal. The Base segment
is also like a metal, and the depletion region between them is like an
empty space. Next, "static electricity" happens!
We've electrically charged the Collector segment to positive 9 volts.
Stick some rice-crispies in the empty gap, and if they're negatively
charged they'll be sucked upwards. Well, the few wandering electrons in
the Base segment act JUST LIKE negatively charged objects, and if they
should wander up to the surface of the base layer, up they'll go. They'll
be sucked across the gap into the Collector and then forced to go around
the rest of the collector circuit. This can only happen if they get to
the "upper surface" of the Base segment. When they were within
the Base
segment, the Base acted like a conductive metal shield, and the wandering
electrons didn't "see" the strong attractive field coming from the
Collector segment.
Some electrons are yanked upwards and go missing from the Base. But this
relieves the
"traffic jam!" The Base region loses some electrons upwards. As
soon as the positively charged Collector has yanked some electrons out of
the Base segment, more electrons can finally pour in from below... which
gives us more wandering electrons
to be yanked upwards, and so on. A fairly huge vertical charge flow
appears.
The "traffic jam effect," as well as the valve-action of the thin
depletion zone between base and emitter, these team up to control the main
vertical current through the whole transistor. Any electron which wanders
across the very thin Emitter depletion zone can also wander across the
thin Base segment and end up becoming part of the large flow of charge in
the
Collector Battery circuit. The Base Battery voltage controls the width of
the thin
depletion zone, and this controls the amount of electrons pouring up into
the collector. The Collector battery provides the "suction" that drives
the main vertical current. But if we change the voltage of the collector
battery, the vertical flow of charge does not change. The collector
battery only attracts what electrons it's given. It can't alter the
collector current. This is an interesting situation known as a "constant
current power supply."
Note that it's important to make the Base segment be fairly thin so we
maximize the "traffic jam" effect (and minimize the number of charges that
unnecessarily leak out of the Base wire.) We're relying on the natural
ability of electrons to wander across the Base section all by themselves.
No voltage is pushing them in that direction. The Base Battery is pulling
them slowly sideways towards the Base wire. The Collector battery can't
start yanking on them at all, not until they reach the "upper surface"
of the Base segment.
If you make people think they're thinking, they'll love you. But if you really make them think, they'll hate you - Don Marquis
Whew. All the stuff above is a very large chunk to swallow. Don't be
suprised if it takes your brain awhile to connect all the puzzle-pieces
together. It took me ages to see all of this (and it only happened years
after I took two semesters of engineering school focused on the
Ebers-Moll
mathematical model describing this entire subject.) The voltage-control
viewpoint shown by the Ebers-Moll explanation does appear widely in
textbooks, but it
certainly isn't widely learned. If it had been learned, then people
wouldn't get angry when they hear that
transistors are voltage-controlled; that the collector
current is proportional to the voltage across the base-emitter junction.
We'd better recap:
10. THE TRANSISTOR CAN ACT LIKE A SWITCH (OR LIKE A PARTIALLY-ON SWITCH.)If we crank up the Base Battery voltage, the emitter's depletion layer thins, the "switch" is fully on, and a very large flow of charge might appear in the collector circuit. Uh oh. The transistor (as a switch) is trying to short out the collector battery. So lets have it switch something. Give it a light bulb in series.
________ Light
/ \ Bulb
| ________/\/\/\________
| | |
| | \________/ |
v | |
| |
| ______|______
| | |
Collector | | COLLECTOR N | Thick depletion
Battery | + |_____________| layer with electrons
| _____________ <-- passing through
____|____ | |______________
_____ | BASE P | |
_________ |=============| | +
_____ 9V | | ____|____
_________ | EMITTER N | _____ Base
_____ |_____________| _________ Battery
_________ | _____ .7V
_____ | | -
| - |_____________________|
|______________________|
------>
And finally we take one last look at the flow of charge in the base wire.
Even though it's really the *voltage* between base and emitter which
controls the transistor, we don't ignore the base-wire's current entirely.
It has an important use. Just by coincidence the tiny base/emitter
current is proportional to the large collector/emitter current. So if we
know the value of flowing charge in the base wire, we can multiply its
value by this "Current Gain" factor, and then figure out just what the
charge-flow in the Collector wire should be. The transistor ACTS as if it
is amplifying current. But it's really using a small change in *voltage*
to create a large change in current. (It's more than just coincidence
that the charge flowing in the Base and Collector are proportional. In
fact, both flows are controlled by the Base/Emitter voltage, which
controls the thickness of the Emitter's depletion layer.) The Collector
current is large because the Emitter's thin depletion layer lets huge
amounts of electrons escape up into the Collector region. The current in
the Base wire is small because only a few electrons are needed in order to
change the BE voltage and the thickness of the Emitter's depletion zone.
A voltage in one place controls a flow of charge in another. This fact
even determines the name of the entire device. Changing a voltage causes
a change in current, so the device behaves somewhat like a RESISTOR. But
the voltage that controls the current is on an entirely different wire.
It's as if the effects of the voltage are TRANSFERRED from the Base side
of the circuit to the Collector side. Transfer resistor. Transistor.
16. BASE VOLTAGE CONTROLS COLLECTOR CURRENT.So, was this explanation too big and nasty? It certainly would be easier if all the textbook authors themselves had a better idea of how transistors work. It would be easier if they stopped telling people that transistors "amplify current." And it certainly would be easier if I get off my butt and create some animations to illustrate the above text!
PS Lilienfeld's patent numbers are:
These patents caused Bardeen, Brattain, and Shockley some grief, and caused the US Patent Office to disallow the Bell Labs FET patents in later years.
PPS The trick to making a transistor is to use a freshly-cleaved crystal face, to sharpen your cat's-whisker contacts by dissolving the tips using electrolysis, and then to put the tips within 0.05mm of each other (or preferrably within 0.01mm). Obviously the latter is the hardest part. Better use a microscope! The authors of the following article found that the base/emitter junction was critical: it HAD to behave as a good rectifier. The base/collector junction wasn't as important. They got some power gain, but their beta was in the single digits. Others have mentioned that if you break open a 1N34 glass diode to expose the Germanium chip, you can make a crude transistor with a similar procedure. Crystal Triode Action in Lead Sulphide, P. C. Banbury, H.A. Gebbie, C. A. Hogarth, pp78-86. SEMI-CONDUCTING MATERIALS, Conference proceedings, H.K. Henisch (ed), 1951 Butterworth's scientific publications LTD 1951.PPPS WHAT ARE TRANSISTORS USED FOR? Ah, that's a whole 'nother article. But here's one example. Computers are made out of processors and memory. Processors are made out of "state machines" and "data selectors," while memory is made out of data selectors and the flipflops that store the individual bits. State machines in turn are are made out of data selectors, and data selectors are made out of nand-gates or nor-gates. Memory flipflops are made out of nand-gates or nor-gates. EVERYTHING is made out of Nand or Nor gates. And... nand-gates and nor-gates are made out of transistors.
So... computers are entirely made out of transistors. If computers are
like animals, then animals are made of tissues, which are made of cells,
which are made of organelles, which are made of proteins, which are made
of molecules, which are made of atoms. Yet an animal is entirely made of
atoms, and everything else is just interesting patterns in those atoms.
Digital electronics has similar levels of complexity and organization, and
in digital electronics, the transistor is the "atom." The transistor
looks too simple though. It looks uninteresting. Ah, but when you have
clusters of transistors hooked together in various ways, THEN you'll learn
all the fascinating things you can do with them.
PPPPPPPS
PPPPPPPPPPPS
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