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 HOW DO TRANSISTORS WORK? PART II ©1995 William J. Beaty, BSEE
Physics? That's where you find new insights on things people have been thinking about for a long time. If you don't have two or three separate approaches to explaining something, then you don't really understand it.

OK, everything we know is wrong, and transistors aren't really "current amplifiers." <grin> Instead the base voltage is the important thing, not the base current.

```         |
______|______
|             |
| COLLECTOR N |
|_____________|
|             |  ---->
| BASE      P |__________
|=============|          |  +
|             |      ____|____
| EMITTER  N  |        _____
|_____________|      _________
|               _____
|_________________|  -
<----- ```
With a small voltage applied, the depletion layer gets thin, charges start crossing it, and a small flow of charge appears in the battery circuit. The "switch" is only partly closed!
 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.
 ``` <------ ____________________ | | | | | ______|______ | | | | Colletor | COLLECTOR N | + | Batt. |_____________| ____|____ | |______________ _____ | BASE P | | _________ |=============| | + _____ 9V | | Base ____|____ _________ | EMITTER N | Batt. _____ _____ |_____________| 0.5V _________ _________ | _____ _____ | | - - | |_____________________| |____________________| -------> ```
 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.
 ``` IT'S ALL SHORTED OUT, IT GETS HOT AND SMOKES _____________________ __________________ | | | | Collector | | | | Battery | + ____|____|___ | ____|____ | | | _____ | BASE P | | _________ |=============| | + _____ 9V | | ____|____ _________ | EMITTER N | _____ Base _____ |_____________| _________ Battery _________ | _____ .5V _____ | IT'S A PN DIODE | - | - | | |_______________________|_____________________| ```
 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.
 ``` <------ _______________________ | | | | | ______|______ | | | Collector | | COLLECTOR N | Battery | + |_____________| thick depletion layer | _____________ ____|____ | |______________ _____ | BASE P | | _________ |=============| | + _____ 9V | | ____|____ _________ | EMITTER N | _____ Base _____ |_____________| _________ Battery _________ | _____ .5V _____ | | - | - |_____________________| |_______________________| -------> ```
 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. See, a traffic jam. 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 down into the P, and repels electrons back up into the N Collector segment.) 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 down 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 big 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's 9V battery provides the "suction" that drives the main vertical current. But if we change the collector's battery voltage, the vertical flow of charge does not change. Doesn't change? It's because the collector battery only attracts what electrons it's given by the Base segment. 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. If you make people think they're thinking, they'll love you. But if you really make them think, they'll hate you     - Don Marquis Whew. All the stuff above is a very large chunk to swallow. Don't be suprised if it takes your brain awhile to connect all the puzzle-pieces together. It took me ages to see all of this (and it only happened years after I took the two semesters of engineering school exclusively focused on the Ebers-Moll mathematical model describing this entire subject.) The voltage-control viewpoint shown by the Ebers-Moll explanation does appear widely in textbooks, but it certainly isn't widely learned. If it had been learned, then people wouldn't get angry when they hear that transistors are voltage-controlled; that the collector current is proportional to the voltage across the base-emitter junction. We'd better recap: 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 IN ORDER 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.
 ``` ________ Light / \ Bulb | ________/\/\/\________ | | | | | \________/ | v | | | | | ______|______ | | | Collector | | COLLECTOR N | Thick depletion Battery | + |_____________| layer with electrons | _____________ <-- passing through ____|____ | |______________ _____ | BASE P | | _________ |=============| | + _____ 9V | | ____|____ _________ | EMITTER N | _____ Base _____ |_____________| _________ Battery _________ | _____ .7V _____ | | - | - |_____________________| |______________________| ------> ```
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