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.
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 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; 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 "conventional current.")
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 by ignoring the base current. Instead we look elsewhere for understanding. See the diagram below.
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.
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!
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 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.
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.
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.
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.
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.
review:
10. THE TRANSISTOR CAN ACT LIKE A SWITCH (OR LIKE A PARTIALLY-ON SWITCH.)
11. CONNECT A POWER SUPPLY OR BATTERY FROM COLLECTOR TO EMITTER TO CREATE A BIG FLOW OF CHARGE (BUT WHY?)
12. THERE'S ANOTHER DEPLETION ZONE BETWEEN COLLECTOR AND BASE.
13. THIS NEW DEPLETION ZONE ACTS LIKE AN INSULATING AIR GAP.
14. ANY ELECTRONS WHICH RANDOMLY WANDER ALL THE WAY ACROSS THE BASE ARE GRABBED BY THE COLLECTOR; THEY'RE FORCEDACROSS THE UPPER DEPLETION ZONE.
15. THE BASE DEPLETION ZONE CONTROLS THE COLLECTOR BATTERY CURRENT. BUT CHANGES IN THE COLLECTOR VOLTAGE HAVE LITTLE EFFECT.
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.
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."
17. PURE LUCK?: THE BASE LEAKAGE IS PROPORTIONAL TO COLLECTOR CURRENT.
18. TRANSISTORS ARE *NOT* CURRENT AMPLIFIERS. BUT IT CERTAINLY SIMPLIFIES THINGS IF WE PRETEND THAT THEY ARE.
