How Transistors Really Work
Q: OK, then how do diodes REALLY work? And FETs? And for
that matter, Resistors and Inductors and Batteries?
A: I have about three lifetimes of backlog articles to write! I know...
I need to die, and then twenty years later someone will go
through my vast piles of paper and start compiling it into books.
Q: Why is your article so weird? It must be wrong, since it's very
different than textbooks.
A: If you don't know my goals and techniques I used in writing
this article, you might come away with a totally wrong impression:
- The article is absolutely NOT for science and engineering students.
The article will be aimed at the general public (i.e. 5th grade
conceptual level.) As with most of my writing, my goal is "teaching
physics to your grandmother."
- My intent is not teaching, it's "UNteaching." There are
plenty of articles about transistor basics. Rather than writing yet
another one, instead I want to build a time
machine to communicate with the distant past. I want to send messages
to myself, back when I was a student. With the
benefits of future
hindsight I can discover all the things which blocked my progress back
then. Then I can send a message to my past self about how to avoid all
the problems, and turn myself into Ultra-Genius-Student. No time machine?
Rats. I guess I'll just have to write articles to pass all my
learning-tricks to the students of today. I suspect that they'll be
suffering from many of the same learning-barriers that I did, so my
information will free up their progress too.
- All the common electrical misconceptions listed here and here will be carefully avoided. Since
these misconceptions are extremely widespread in textbooks, their removal
will make for some odd writing. For example, rather than teaching that
current flows in wires and that batteries are sources of electric current,
I'll explain that a metal circuit is full of movable electricity, and that
batteries are electricity-pumps. This reinforces the idea that Electric
Current is not a substance, and that batteries are not "sources" of any
such substance. When we realize that the charges which flow during an
electric current are supplied by the conductors, then we can almost *see*
how electric circuits work.
- The article will employ no mathematics whatsoever (well, maybe
I'll use some generalized proportional reasoning!) When writing for the
public, equations are totally forbidden. Including any math will make
about as much sense as including any Latin or Japanese.
- Jargon and terminology-shortcuts will be unpacked as much as possible.
Complicated terms will be replaced by their own definitions. For example,
I'll go through the article and delete any mention of "electric current,"
replacing it with the phrase "charge-flow" or perhaps "flows of
Q: Transistors are CURRENT CONTROLLED DEVICES! Anyone who says
DIFFERENTLY is a DISGUSTING LIAR!!!
OK OK, you're right, transistors really are current controlled devices!
(And I back away slowly, feeling around for a heavy object in case I have
to defend myself if your violent response turns physical.)
Viewing the transistor as a current-controlled device is incredibly
useful. Doing so lets us understand how circuits function. And it's the
central technique for amplifier design. If engineers couldn't use the
hfe current-gain equation, they'd be screwed. But it's really just a
fiction, like pretending that atoms are tiny solar systems. If we want
to understand how the guts of the transistor
works, we have to abandon the "current controlled" idea. Make no
mistake... as long as we stay
outside the transistor package, we can ignore this whole issue. From
outside, a transistor is a current controlled device. But that won't help
us understand how the insides work.
From HOROWITZ AND HILL P65
problem only crops up when we delve into the interacting semiconductor
junctions. I myself went through this painful learning
process when I finally "saw" what transistors really are (and it truely is
painful to discover that your most solid beliefs,
at their core, are wrong.) The central problem is this:
There is no mechanism which allows the Base current to
directly affect the Collector current.
When trying to understand transistors, I'd always searched for the special
thing which lets Ib control Ic.
I never found it.
I finally learned that it doesn't exist. Transistors aren't
And that was the moment when I experienced the big "AHA," and I finally
understood how the inside of transistors work. The scales fell from my
eyes. Next I wanted to help everyone else get past the same barrier which
stopped my own progress. What were the "scales" on my eyes? Simple: it
was my belief that Ib controls Ic, and that transistors are current
amplifiers. Once I dropped this belief, I was no longer blind.
Or in other words, I finally could "hear" what the advanced textbooks
were saying all along: a transistor's internal functioning is described by
the well-known Ebers-Moll equation. Ebers-Moll only exists because the
electrostatic field created by Vbe determines the depletion layer width
which then determines Ic. Even deeper is Gummel-Poon, which points out
that the charge stored on the sides of the depletion zone is controlling
the thickness of the depletion zone, and we adjust the charge to adjust
the Vbe voltage. Of course we can always ignore all this and use Ib to
force Vbe to take on a certain value. That lets us give Ib *INDIRECT*
control of Ic. Then we can ignore Vbe and the guts of the transistor, and
pretend that Ib controls Ic. But this "pretending" sometimes seems to
increase into deep psychological Denial, where people on forums angrily
rant against anyone who dares suggest that Vbe and the Ebers-Moll equation
is crucial to understanding the internal functioning of the BJT.
Speaking of eyes... there's a famous fight scene in the movie "THEY
LIVE," where the main character is trying to force another man to put on
the special sunglasses which lets humans see the hidden alien messages on
billboards. JUST PUT ON THE GLASSES!! The scene is famous for being
the longest grueling fight in any movie. And it's all about a
fight to avoid clear vision and preserve cherished misconceptions. Sounds
Q: I KNOW WHAT YOU'RE TRYING TO DO. YOU'RE REALLY JUST A NITPICKER, AND
YOU'RE TRYING TO CONFUSE EVERYONE.
Please see the following article I wrote in response to a similar comment
on my other 'misconception' pages:
AM I JUST A PEDANTIC SCIENCE-NITPICKER?
I receive huge amounts of email about this and other Electricity articles.
Most of them claim that my articles have removed their long-standing
confusion. But I've found that a certain tiny minority of people simply
HATES my explanations. Since their email forms only a few
percent of the messages I've received, I don't take them very seriously.
I suspect that most of them are the folks who've spent years learning
physics/electronics in the usual way, and are angry that anyone would dare
to find fault with it. But if I want to go back and help newcomers get
past the stumbling blocks that slowed me down, then I damn well am going
to search out and destroy all those errors which harmed my early
Who do I think I am, to be questioning the contents of widely used
textbooks?!!! (I'm nobody. Remember, it's the little kid who points out
the flaws in the Emperor, while all the more sensible people are busy