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 called "water?")
Since a current is a flow of charge, the common expression
"flow of current"
should be avoided, since literally it means "flow of flow of charge."
- MODERN COLLEGE PHYSICS, Richards, Sears, Wehr, Zemanski
So what flows inside of wires?
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. And in rivers or in plumbing, it's the water that
flows, not the "current." We cannot understand plumbing until we stop
believing in a magical stuff called "current." We must learn
that "water" flows inside of pipes. The
same is true with circuits. Wires are not full of current, they are full
of charges that can move. Electric charge is real stuff; it can move
around with a real velocity and a real direction. But electric current is
not stuff. If we decide to ignore
"current," and
then examine the behavior of moving charges in 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 positive or 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. Wires do not act like "empty electron-pipes." A power
supply
does not supply 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 CHARGE
Wires
and silicon ...both behave like pre-filled water pipes or water tanks.
Electric circuits are based on full pipes. This simple idea is usually
obscured by the phrase "power supplies create
current," or "current flows in wires." We end up thinking that wires are
like hollow pipes. We end up thinking that a mysterious substance called
Current is flowing
through them. Nope. (Once we get rid of that word
"current," we can discover fairly stunning insights into simple circuits,
eh?)
If
circuits are like plumbing, then none of the "pipes" of a circuit are
ever empty. This idea is extremely important, and without it we cannot
understand semiconductors ...or even conductors.
Metals contain a vast quantity
of movable
electrons which forms a sort of "electric fluid" within the metal.
A simple block of copper is like a water tank!
Physicists call this fluid by the name "electron sea of metals."
Semiconductors are always full of
this movable "charge-stuff." The movable charge is there even when a
transistor is sitting on the
shelf and
disconnected from everything. When a voltage is applied across a piece of
silicon, those charges already within the material are driven
into motion. Also note that the charge within wires is ...uncharged.
Every movable electron has a positive proton nearby, so even though the
metal contains a vast sea of charge, there is no net charge on average.
Wires contain "uncharged" charge. Better call that "cancelled charge."
Yet even though the electrons are cancelled by the protons, the electrons
can still flow among the protons. Cancelled charge can still move around,
so it's possible to have flows of charge in uncharged metal.
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:
Wires are full of vast
amounts of movable electric charge (all conductors are!) If you connect
some wires into a circle, you form an "electric circuit" which contains a
movable conveyor-belt made of charges within the metal. Next we cut this
circle in a couple of places and we insert a battery and a light bulb into
the cuts. The battery acts as a charge pump, while the light bulb offers
friction. The battery pushes the wires' row of charges forward, then all
the charges flow, then the bulb lights up. Let's follow them.
The charges start out inside the light bulb filament. (No, not inside the
battery. We start at the bulb.) The charges are forced to
flow along
through the filament. Then they flow out into the first wire and move
along to the
battery's first terminal. (At the same time more charges enter the
filament through its
other end.) The battery pumps the charges through itself and back
out again. The
charges leave the second battery terminal, then they flow through the
second wire to the bulb. They wind up back inside the light bulb
filament. At the same
time, the charges in other parts of the circuit are doing the same thing.
It's like a solid belt made out of charges. The battery
acts as a drive wheel which moves the belt. The wires behave as if they
hide
a conveyor belt inside.
The light bulb acts like "friction;" getting hot when its own natural
charges are forced to flow along. The battery speeds up the entire belt,
while the friction of the light bulb slows it down again. And so the belt
runs constantly, and the light bulb gets hot.
The truth will set you free ...but first it will piss
you off!
-anon
Brief review:
1. THE STUFF THAT FLOWS THROUGH CONDUCTORS IS CALLED CHARGE.
("CURRENT" DOESN'T FLOW.)
2. THE CHARGE INSIDE CONDUCTORS IS SWEPT ALONG BY VOLTAGE FIELDS.
3. ALL WIRES ARE "PRE-FILLED" WITH A VAST AMOUNT OF MOVABLE CHARGE
4. BATTERIES AND POWER SUPPLIES ARE CHARGE-PUMPS.
5. LIGHT BULBS AND RESISTORS BOTH ACT "FRICTIONALLY."
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 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 will
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 does not. For
example in copper, every single copper atom donates one movable electron
to the "sea of charge." The "electric fluid" is very dense;
just as dense as the copper metal. But in doped silicon, only one
atom in a billion 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 pulls 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
"conventional current.")
|
______|______
| |
| COLLECTOR N |
|_____________| ELECTRONS ARE PULLED FROM THE
| | -----> BASE REGION AND INTO THE WIRE,
| BASE P |______________ WHICH CREATES POSITIVE "HOLES"
|_____________| | + WHICH SPEW OUT INTO THE BASE
| | ____|____ REGION.
| EMITTER N | _____
|_____________| _________
| _____
|_____________________| -
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 concentrating 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
ignoring the base current. Instead we look elsewhere for understanding.
See the diagram below.
|
______|______
| | \
| COLLECTOR | |
| | > full of wandering electrons
| n-doped | |
|_____________| /
| | \
| BASE | |
| |-- > full of wandering "holes"
| p-doped | |
|_____________| /
| | \
| EMITTER | |
| | > full of wandering electrons
| n-doped | |
|_____________| /
|
|
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; it exists 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 |
|_____________|
|
|
Electrons in p-type silicon 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
wandering electrons from the n-type silicon will fall into the holes.
Also, holes in the p-type's Base region
flow out among the movable electrons from the N-type Emitter region and
many
are cancelled. Holes swallow 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. Lacking mobile charges, 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 is normally like a
shiny
silver conductor (sort of like metal) ...except for this 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
|_____________| /
|
|
When 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
voltage. The
layer becomes thinner when base/emitter 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 start jumping across when 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 switch.
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.
8. A LAYER OF INSULATING MATERIAL APPEARS WHEREVER P-TYPE AND N-TYPE
TOUCH.
9. THE INSULATING LAYER CAN BE MADE THIN BY APPLYING A VOLTAGE.
|
______|______
| |
| COLLECTOR N |
|_____________|
| | ---->
| BASE P |______________
|=============| | + With a small voltage applied,
| | ____|____ the depletion layer gets thin,
| EMITTER N | _____ charges start crossing it,
|_____________| _________ and a small current appears.
| _____ The "switch" is only partly
|_____________________| - closed!
<-----
OK, on to PART TWO
Also see: short version of article
and the
versión Español.
PS
The transistor was invented around 1923, by physicist Dr. J. Edgar
Lilienfeld, the father of the modern electrolytic capacitor. WHAT?!!!
But everyone knows that it was invented at Bell
Labs in 1947. Nope. The original transistor was a 1920s thin-film device
deposited on glass. The base region was a clever idea: crack a piece of
glass, put it back together with metal foil clamped in the crack, then
slice off the extra foil to make a flat surface that goes: glass, metal,
glass. Deposit a thin layer of semiconductor and heat the device, and the
thin metal line
will "dope" that part of the semiconductor layer. Simple! Dr.
Lilienfeld also built MOSFETs using the natural oxide layer found on
aluminum plates. He also built a working transistor radio and showed it
around to various
companies. It was ignored, possibly because he didn't have a solid theory
to explain how his
invention worked, but more probably because it was weird and new. Some
hobbyist should try making a
home-built transistor. [New 2006 info: R. G. Arns says that Bret
Crawford
built sucessful Lilienfeld transistors in 1991
as his MS Physics Thesis. Joel Ross did it again in 1995 with more stable
versions. And more amazing: William Shockley and G. L. Pearson
did so in 1948, publishing in Physical Review for July 15 1948, but
they concealed the fact that it was Lilienfeld's device they were
demonstrating!]
Lilienfeld's patent numbers are:
- #
1,745,175 Method and Apparatus for Controlling Electric Currents
- #
1,877,140 Amplifier for Electric Current
- #
1,900,018 Device for Controlling Electric Current
[Click on IMAGES button to view them.]
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.
Also:
PPS
It is possible to make a transistor using Galena (lead sulfide, PbS).
Galena is 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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