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How Do Bipolar Transistors Work?
©2003 William J. Beaty
EXCESSIVELY SHORT VERSION
A transistor is essentially a diode. In diodes, the rate
of charge flow is determined by the height of the potential barrier at the
junction. It's voltage controlled: place the right polarity of voltage
across the diode terminals to turn it on. But a transistor a very weird
diode: if you turn it on with 0.7V placed across two terminals here, then
the main diode current goes through a totally different terminal
over there! Vbe determines Ic (as well as Ib.) When Bell Labs had an
informal contest to name their new invention, one engineer pointed out
that it acts like a resistor, but a resistor where the input voltage is
transferred across the device to control the resulting current. A
"Transfer Resistor" or "Trans-sistor."
ALSO:
-
How capacitors REALLY work
-
How electricity really works articles index
LONGER, MORE DETAILED SHORT VERSION...
- Seen from outside, Bipolar Transistors look like current amplifiers.
Engineers and technicians can treat transistors as if they are current
amplifiers. That's fine as long as we're viewing the transistor as a
sealed 3-wire component: a small black box. But inside, transistors are
actually voltage amplifiers. (Most non-physics books get this wrong.) In
fact, transistors are like diodes: they're voltage-controlled insulators.
[Those who disagree can start by investigating the physics behind the
Ebers-Moll and Gummel-Poon models, or by referring to Horowitz & Hill's
book THE ART OF ELECTRONICS, or even see what author
W.
Win Hill himself says when people insist that BJTs are "current
controlled." Here's some more Win
Hill on CR4 forum, and even more.
And his textbook AOE,
partial preview on google books.
- Overall, bipolar transistors act like a thin layer of insulator. The
thickness of the insulating layer can be electrically altered in order to
control electric current. It's like closing a switch: if we make the
insulator between the switch contacts thin enough, the transistor turns
"on." Make it slightly thicker, and the transistor turns partially on.
- A bipolar transistor is like three hunks of silicon in a row. Usually
it's NPN; two hunks of n-type separated by a hunk of p-type.
Doped silicon is a good conductor. At the places
where the n-type silicon is touching the p-type, a very thin layer of
insulator spontaneously appears. Because the three hunks of silicon are
normally separated by these insulating layers, an unpowered transistor
starts out in the "turned off" state.
- In the three hunks of silicon, the center region is called the "Base."
One of the side regions is
called the "Emitter." By applying a small voltage between Base and
Emitter, we can make the thin layer of insulator become even thinner. If
it's thin enough, it stops insulating, and some charges flow across it.
(Imagine
bringing two wires closer and closer until the electrons start jumping
across the microscopic gap.)
- The transistor's Base/Emitter voltage controls the insulator thickness.
This insulator then
controls the whole transistor. A large Base/Emitter voltage will shrink
the insulating region and turn the transistor on. A smaller voltage can
turn the transistor half-way on, and this gives us linear amplifying
action rather than just purely on-off switching.
- But a transistor is not a simple diode! Exactly. There's a the third
hunk of
silicon (the hunk called the "Collector.") And another insulating layer
appears between Base and Collector. Since
it doesn't block the charges coming from the Base, is it even an
insulator? It acts just like an insulating vacuum; it doesn't block
charges, but it's insulating because it contains no charges of its own.
However, the Base-Collector insulator layer
does keep the other sections from seeing any effects of the
large power supply voltage placed between Collector and Base. Whether the
Collector
voltage is low or high, we get the same amperage through that insulating
layer. Therefore the Base acts as if it is being shielded from the
Collector. Yet charges flow right through the insulating region, going
from Emitter through Base and into Collector. Therefore the Collector
acts as if it's NOT insulated from the Base. Which is it? Both!
- Why do transistors act as amplifiers? That's simple to explain: all
valves are amplifiers. Water valves, air valves, and vacuum tubes are
amplifiers. Carbon microphones are amplifiers (paired with a
loudspeaker,
the carbon microphone was the first audio amp, and was placed in long
distance
telephone lines. Thomas Edison made big bucks!) It takes very little
energy
to open or close a valve, yet this tiny energy controls huge pressures and
huge flows. A transistor is like a valve, a valve where a tiny change in
voltage can open or close the large valve by different amounts. A small
amount of
"signal" energy causes the transistor to control the large current which
is pumped by a battery. The shape of the output waveform is the same as
the shape of the input waveform. We usually say that the signal has been
"amplified," when really it has been created from the battery's energy.
And if battery voltage stays the same, then any changes in charge flow are
changes in the output energy flow. Wattage. Small watts in, big watts
out. That's
what amplifiers are all about. But any valve can do this.
- Most transistors are Silicon, and Silicon junctions "turn on" at around
0.4V to 0.8V. Place half a volt on the Base of a transistor and the
transistor barely starts turning on.
- But why do so many books say "Transistors Amplify Current?" Because
it's an incredibly useful rule of thumb. It's part of a mental model
which treats
transistors as black boxes. The base current can be used to determine the
BE voltage, and the BE
voltage then sets the collector current's value. As long as we
don't look inside
the black box, we can
pretend that the charges injected into the base are directly affecting
the collector current. But
some misguided authors go farther, they try to use
current-amplification to explain the stuff inside the box. Those authors
are ignoring the role played by the Emitter depletion layer, and ignoring
the role played by Vbe. They are
confused by the leakage current going through the Base wire. Yes, the
Base/Emitter voltage controls the thickness of the insulating region and
thus controls the main transistor current. But also a tiny bit of charge
leaks through the Base connection. It SEEMS like this leakage can
directly collide against the collector's flowing charges.
- It just so happens that the tiny leakage current in the Base connection
is proportional to the transistor's main Emitter/Collector current.
(This makes perfect sense, since both the Base leakage current and the
main Emitter/Collector current are controlled by the insulator thickness,
which is set by the Base/Emitter voltage.) ...so it SEEMS as though the
Collector current is directly controlled by the Base current. You can
even simplify things by pretending that this is true. But in reality,
Base current communicates with Collector current via changes in
Base-Emitter voltage. It's the Base/Emitter voltage which runs things.
You'll never understand "transistor action" and the simple physics inside
the box if you think that Base current directly controls the Collector
current. It doesn't. The control is there, of course, but it's one stage
separated.
- A bipolar transistor has a voltage-controlled input, while it's output
is a variable control system which creates a constant current. It's
vaguely like a resistor, but where the voltage in one place creates a
current in a second place ...yet the voltage in that second place does not
affect the output current. Change the collector voltage, and the
collector current remains relatively constant. It's not like a
transformer where volts and current are swapped, and where wattage stays
the same. In transistors, the effects of voltage are TRANSFERRED to
another separate path in the circuitry. "Transfer Resistor." Transistor.
MAIN, MUCH LONGER ARTICLE
SEE ALSO:
-
How capacitors REALLY work
-
How electricity really works articles index
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