HOW DO TRANSISTORS WORK?
|©1995 William Beaty, BSEE|
"The difference between a conviction and a prejudice is that you can explain a conviction without getting angry." - anonWhen I first became interested in electronics as a kid, I sat down and figured out how bipolar transistors work.
Well, sort of.
I read many articles which explained the "Common Base" amplifier.
Common-base is the setup which was used by the inventors of the
transistor. In those explanations, the Base is a grounded piece of
Germanium and the input signal is applied to the Emitter. Since
common-base amplifiers are rarely used in transistor circuitry, I ended up
having to dream up my own explanation. I based it upon the little bits I
already knew about the Common Emitter configuration. Common Emitter is
the one where the Emitter is grounded, the Base is the input, and where
the output is taken across a resistor connected to the Collector. My
home-made explanation sort of worked, but I wasn't satisfied. I was full
of niggling doubts. And why the hell were the textbooks using Common Base
to introduce transistors to the newbies? It just didn't make any
When I went into engineering school, I found it extremely odd that there
were still no good explanations of bipolar transistors. Sure,
there were detailed mathematical treatments. Just multiply the Base
current by "hfe" to obtain the Collector current. Or, treat the
transistor as a two-port network with a system of equations inside. Ebers-Moll
and all that. But these were similar to black-box circuits, and none of
them said HOW a transistor works, how can a small current have any
effect on a larger one???? And nobody else seemed curious. Everyone
else in the class seemed to think that to memorize the equations was the
same as learning concepts and gaining understanding of the device. (R. Feynman calls this the Euclidean or "Greek
viewpoint;" the love of mathematics, as opposed to the physicists' "Babylonian viewpoint" where concepts are far more important than
equations.) I'm a total Babylonian. For me, math is useless at the start.
The equations are like those black-box Spice programs which might work
but they don't tell you any details of what's happening inside a device in
the real world. I can learn the math, but that just means I can run a
"mental spice program" without needing any computer, and I still don't
know how transistors work. First tell me what "Transistor Action" is all
about. Show me animated pictures, use analogies. Only after I've attained
a visual and gut-level understanding of something, only then is the math
useful to me for refining it and adding all the details. However, for me
the math alone is not a genuine explanation. Math is just a tool or a
recipe, a crutch for those who want nothing except the final numerical
result, and it certainly does not confer expert knowledge.
Now many years have passed and I think I see the problem...
Traditional transistor explanations basically *suck.*The ones I see in high school textbooks and hobby magazines are terrible. They're full of errors and contradictions. They misuse the word "current" as if it were a substance that flows. They don't explain insulators properly. And they try to prove that the base current can have a direct effect on the collector current. Textbooks for engineers spend their time deriving equations which will end up in software simulations, but still they don't sit down and describe what's happening in a direct clear fashion. And then there's all those authors who use Common-base amplifiers to introduce transistors to newbies. Are they just fools who follow a tradition only because it's traditional? Why don't they ever make efforts to improve the explanations? Were they written in stone by god? Well, if nobody but me thinks the explanation is open to improvements, then I'd better put my money where my mouth is. (And if I'm right, then it should be very easy to write a vastly improved explanation.)
Below are my ideas on how transistors really work. They're *not*
based on the traditional explanations found widely in technician's texts
and hobbyist magazines. Instead they're based on engineering textbooks:
semiconductor physics and the details behind the Ebers-Moll model. I'm
translating the usual math models into a verbal/intuitive version. As
you'll soon see, several new concepts are required. It might be easier
for you to just memorize the equations rather than to imagine what really
goes on inside. But if you DO manage to decode my explanations and crude
ASCII artwork, I think you'll be in the elite minority who really
understands transistors. I've found that even most working engineers
have no good mental picture of bipolar transistor operation. So, if you
attain a clear understanding of transistors, you'll surpass many of the
Voltage-driven charge-flows |
First of all, you must abandon the idea that current travels in
transistors or flows inside of wires. Yes, you heard me right. Current does not flow. Electric
current never flows, since an electric current is not a stuff.
Electric current is a flow of something else. (Ask yourself this:
what's the stuff that flows in a river, is it called "current?" Or is it
The stuff that moves within wires is not named Electric Current. Intead
it is called Electric Charge. It's the charge that flows, never the current.
The motion of charges can vanish, and the motion can appear. But the
motion itself doesn't flow along, it's the charges which flow. And in
rivers (or in plumbing,) it's the water that flows, not the "current."
Analogy: we cannot understand plumbing until we stop assuming that the
pipes are empty ...while also believing in a magical stuff called
We must learn that pipes are already full; that "water" flows inside them.
The same is true with circuits. Wires are not filled with "flow of
Current," instead they are pre-filled with charge. Charge which can move.
Electric charge is real stuff; it's carried by physical particles, and it
can move around with a real velocity and a real direction. Charge behaves
much like a "stuff," like a gas or a liquid. But electric current is
different from charge: charge is like a stuff, but current is not a stuff.
(If current is like wind, then charge is like Nitrogen!)
If we experiment with concepts; if we decide to ignore "current,"
and instead we go and carefully examine the behavior of the moving charges
great detail, we can burn off the clouds of fog that block our
understanding of electronics.
Second: the charges found within conductors do not push themselves
along, but instead they're pushed by "potential difference;" they're
pushed by the voltage-fields within the conductive material. Charges are
not squirted out of the power supply as if the power supply was some sort
of water tank. If you imagine that the charges leave through the negative
terminal of the power supply; and if you think that the charges then
spread throughout the hollow pipes of the circuit, then you've made a
fundamental mistake. If you think that the charges are provided by the
power supply, then you've made a fundamental mistake. Wires do not act
like "empty electron-pipes." A power supply does not provide any
electrons. Power supplies certainly create currents, or they
cause currents, but remember, we're removing that word "current."
To create a flow of charges, a power supply does not inject any
charges into the wires. The power supply is only a pump. A pump can
supply a pumping pressure. Pumps never supply the water being pumped.
Third: have you discovered the big 'secret' of visualizing electric circuits?
ALL CONDUCTORS ARE ALREADY FULL OF CHARGEWires and silicon ...both behave like pre-filled water pipes or water tanks. The "water" is the vast population of movable charged particles of the conductor. Electric circuits are based on the "full pipes analogy." This simple idea is usually obscured by the phrases "flow of current," or "power supplies send out current." We end up thinking that wires are like hollow pipes. We end up visualizing a mysterious substance called Current which flows through them. Nope. (Once we get rid of that word "current," we can discover fairly stunning insights into simple circuits, eh?)
OK, since the "pipes" are already full of "liquid," then in order to
understand circuitry, we should NOT trace out the path starting at the
terminals of the power supply. Instead, we can start with any component on
the schematic. If a voltage is applied across that component, then the
charges within that component will start to flow. Let's modify the old
"flashlight explanation" which we all were taught in grade school. Here's
the corrected version:
AN ACCURATE FLASHLIGHT EXPLANATION:
The truth will set you free ...but first it will piss you off! -anon
1. THE STUFF THAT FLOWS THROUGH CONDUCTORS IS CALLED CHARGE. ("CURRENT" DOESN'T FLOW.)One last thing: The difference between a conductor and an insulator is simple: conductors are like pre-filled water pipes, while insulators are like pipes choked with ice. Both contain the "electric stuff;" conductors and insulators both are full of electrically charged particles. But the "stuff" inside an insulator can't move. When we apply a pressure-difference along a water pipe, the water flows. But with an empty pipe, there's nothing there, so the flow does not occur. And with an ice-choked pipe, the stuff is trapped and doesn't budge. (In other words, voltage causes charge-flow in conductors, but it can't cause charge-flow in insulators because the charges are either missing, or immobilized.) Many intro textbooks get their definitions wrong. They define a conductor as something through which charges can flow, and insulators supposedly block charges. Nope. Air and vacuum don't block charges, yet air and vacuum are good insulators! In fact, a conductor is something that contains movable charges, while an insulator is something that lacks them. (If a book gets this foundational idea wrong, then most of its later explanations are like buildings built on a pile of garbage, and they tend to collapse.)
One more last thing before diving into transistors. Silicon is very different than metal. Metals are full of movable charges... but so is doped silicon. How are they different? Sure, there's that matter of the "band gap," and the difference between electrons versus holes, but that's not the important thing. The important difference is quite simple: metals have vast quantities of movable charge, but silicon has far less. For example in copper, every single copper atom donates one movable electron to the "sea of charge." Copper's "electric fluid" is very dense; it's just as dense as the copper metal. But in doped silicon, only one in every billion atoms donates a movable charge. Silicon is like a big empty space with an occasional wandering charge. In silicon, you can sweep all the charges out of the material by using a few volts of potential, while in a metal it would take billions of volts to accomplish the same thing. Or in other words:
6. THE CHARGE INSIDE OF SEMICONDUCTORS IS LIKE A COMPRESSIBLE GAS, WHILE THE CHARGE INSIDE OF METALS IS LIKE A DENSE AND INCOMPRESSIBLE LIQUID.Sweeping away the charges in a material is the same as converting that material from a conductor to an insulator. If silicon is like a rubber hose, then it's a hose which contains compressible gas. We can easily squeeze it shut and stop the flow. But if copper is also like a rubber hose, then instead, it's like a hose full of iron slugs. You can squeeze and squeeze, but you can't smash them out of the way. But with air hoses and with silicon conductors, even a small sideways pressure can pinch the pathway shut and stop the flow.
OK, let's look at the way that transistors are usually explained.
To turn on an NPN transistor, a voltage is applied across the base and
emitter terminals. This causes electrons in the Base wire to move away
from the transistor itself and flow out towards the power supply. This in
turn yanks electrons out of the P-type base region, leaving 'holes'
behind, and the 'holes' act like positive charges which are pushed in the
opposite direction from the direction of electron current. What SEEMS to
happen is that the base wire injects positive charges into the base
region. It spews holes. It injects charge.
(Note that I'm describing charge flow here, not positive-charge
That's part of the conventional explanation. Why is all of this important
to transistor operation? ***It's not!*** The base current is not
important to transistor operation. It's just a byproduct of the REAL
operation, which involves an insulating layer called the Depletion Region.
By focusing attention on the current in the Base lead, most authors go up
a dead end in their explanations. To avoid this fate, we must start out
by ignoring the base current. Instead we look elsewhere for
understanding. See the diagram below. |
| ______|______ | | \ | COLLECTOR | | Full of | | > wandering | n-doped | | electrons |_____________| / | | \ | BASE | | full of | |-- > wandering "holes" | p-doped | | |_____________| / | | \ | EMITTER | | full of | | > wandering | n-doped | | electrons |_____________| / | |
|The Depletion Region is an insulating layer existing between the base region and the emitter region. Why is it there? It exists because the Base region is p-doped silicon; the insulating layer appears because p-type silicon is full of naturally-occurring movable "holes," and because the p-type silicon is touching n-type silicon.|
| ______|______ | | | COLLECTOR N | |_____________| | | | BASE P |-- |_____________| insulating _____________ <-- "depletion layer" | | | EMITTER N | |_____________| | |
The Depletion layer appears when electrons fall into holes. The p-type
silicon has electrons too, but they act like the closely-packed beads of
an abacus, and the "holes" are like gaps in the rows of beads. Move one
bead, and a hole has moved the other way. Touch the p-type silicon
against the n-type, and lone wandering electrons from the n-type silicon
will fall into the holes. Also, holes in the p-type's Base region can flow
out among the movable electrons from the N-type Emitter region and many
swallow electrons and are cancelled. Holes eat electrons, and this leaves
a thin region between N and P sections which lacks movable charges. |
Remember: a conductor is not a substance which allows charges to
pass. (Don't forget #3 above!) Actually a conductor is any substance
which contains charges which are movable. Anything that lacks
movable charges is an insulator. Inside the depletion layer, all the
opposite charges have fallen together and vanished. The gaps in the
abacus beads are gone, so no beads can move anymore. It's packed solid
with immobile charges, so the silicon has turned into an insulator. When
there's no voltage applied across the base/emitter terminals, this
insulating layer grows fairly thick, and the transistor acts like a switch
which has been turned off.
I like to visualize that a transistor's silicon as normally like a shiny
silver conductor (sort of like metal) ...except for that insulating layer
between the P and N regions which acts more like a layer of insulating
glass. Silicon is like a metal which can become glass!
| ______|______ | | \ | COLLECTOR N | | Shiny silver |_____________| > conductive | | | | BASE P |-- / |_____________| Glasslike insulating _____________ <-- "depletion layer" | | \ | EMITTER N | > Shiny silver |_____________| / conductive | |
Whenever voltage is applied between base and emitter, this insulating
layer changes thickness. If (+)voltage is applied to the p-type
(to the base wire,) while a (-) voltage polarity is applied to the n-type,
(to the emitter wire,) then electrons in the n-type are pushed towards the
holes in the p-type. The insulating layer becomes so thin that the clouds
of electrons and holes start meeting and combining. A current therefore
exists in the base/emitter circuit. But this current is not important to
transistor action. What's important to notice is that the *VOLTAGE*
across the base/emitter has caused the insulating Depletion Layer to
become so thin that the charges can now flow across it. It's as if the
transistor contains a layer of glass whose thickness can be varied when we
alter a Base-Emitter voltage. The layer becomes thinner when BE voltage
is increased. This happens because the voltage pushes the holes and the
electrons towards each other, reducing the size of the empty insulating
region between the clouds of holes and electrons, and allowing the
stragglers to jump across the insulator. The depletion layer is a
voltage-controlled switch which "closes" when the right polarity of
voltage is applied. It is also a proportional switch, since a
small voltage can close it only partially. For silicon material, charges
first start jumping across whenever the voltage is around 0.3V. Raise the
voltage to 0.7V and the current gets very high. (That's for silicon.
Other materials have different turn-on voltages.) The larger the voltage,
the thinner the insulating layer, so the higher the current in the entire
transistor. By applying the right voltage, we can thicken or thin the
depletion layer as desired, creating an open, closed, or partially open
See what's happening here? The transistor is not controlled by current. Instead it is controlled by the base/emitter voltage.
7. THE P-TYPE AND N-TYPE ARE CONDUCTORS BECAUSE THEY CONTAIN MOVABLE CHARGES.
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.
It is possible to make a transistor using Galena (lead sulfide, PbS). Silvery hunks of Galena are often available from rock shops and science museum stores. You can even make your own by melting sulfur and lead powder over a flame. Look up keywords such as "cat's whisker diode" and "crystal radio" to find out more.
The trick to making a transistor is to use a hyper-clean, 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 preferably 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 act 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. Old Germanium audio power transistors probably do the same, while giving much larger semiconductor area on which to play.
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.
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