Electronics > Beginners
PN junctions
kosine:
And to make things even more confusing, you have to think in terms of actual electron flow rather than conventional current flow. So everything is backwards, with electrons coming out of the emitter (hence the name). Some get captured by the thin base, the rest go straight through to the collector - which is usually at a higher potential than the base and attracts the electrons more strongly. (It's very similar to how a triode vacuum tube works, but it still takes a bit of getting used to.)
When a transistor is driven hard to saturation, the collector voltage goes lower than the base (minimum VCE), so more electrons get captured by the base and fewer by the collector. This is why you need lots of base current to drive a BJT to saturation. Even though we tend to think in terms of applying more current, we're actually extracting more electrons because they're not making it to the collector as readily. (Talking NPN here, and obviously simplifying things a little.)
The emitter is usually more heavily doped to ensure plenty of excess electrons and help overcome this effect. This is a key feature of making a transistor, and another reason why two diodes won't work. (Along with the very thin base region and the need for the whole thing to be a single crystal.)
Technically you also have to consider the hole movement as well, which is why they're called "bipolar" transistors. The exponential Schockley equation is a result of holes & electrons recombining. This happens at random and basically exhibits a kind of exponential half-life decay. But they're also constantly being re-created by thermal energy (or an applied voltage), so a point of equilibrium is reached where there's a constant number of holes & electrons. The number being created then matches the number being lost through recombining.
The availability of holes & electrons (charge carriers) obviously affects how well the PN junction will conduct, and this is why the effective resistance changes with both temperature and voltage. And it happens in an exponential manner, because you're counteracting the inherent exponential half-life of random recombination.
Might be going overboard at this point, but the PN-junction is certainly a fascinating beast!
T3sl4co1l:
--- Quote from: fourfathom on July 26, 2019, 03:08:47 am ---I had a boss once who was convinced that was how transistors worked. I had a design with an NPN transistor operating as a saturated switch in the ground leg of an amplifier, the whole circuit running off a 1.2V NiCd cell. My boss insisted that it wouldn't work, since the transistor "was two diodes back to back" and it was impossible to get the collector-emitter voltage under 0.6 volts. I tried to explain, and he didn't get it. I wired up a transistor with base and collector resistors and a power supply, showing him how we could drive the collector voltage to well under 0.1V. He claimed that I was "pulling the ground up". Somehow I got him to accept the design, but he often changed other designs behind my back for truly stupid reasons. I eventually quit. In his defense, he was pretty good with vacuum tubes (this was in the 1970s).
--- End quote ---
That's another thing I used to wonder about, Vce(sat). Turns out the ~0.6V junction drop is a built-in potential, like a battery in series. The collector current goes through two junctions which cancel out, so the collector voltage does indeed get referenced to emitter voltage. The cancellation isn't perfect, and the difference arises from the doping gradient, on the order of 10s of mV. The emitter is usually more heavily doped than the collector.
This is why most transistors have a minimum saturation (for all possible Ic and Ib) in the 10s of mV.
Conversely, you might rightfully ask: if we invert the transistor, can we get that back (negative)? Well, you can't get something for nothing, of course, so Vec(sat) as it were, cannot be negative. But it does happen that it's very close to zero in this configuration, and this fact was used in early precision DACs (12-16 bit), switched with inverted BJTs (and driven by carefully balanced current sources and sinks).
Tim
T3sl4co1l:
--- Quote from: kosine on July 26, 2019, 05:11:13 pm ---And to make things even more confusing, you have to think in terms of actual electron flow rather than conventional current flow. So everything is backwards, with electrons coming out of the emitter (hence the name). Some get captured by the thin base, the rest go straight through to the collector - which is usually at a higher potential than the base and attracts the electrons more strongly. (It's very similar to how a triode vacuum tube works, but it still takes a bit of getting used to.)
--- End quote ---
Indeed, one can draw the quantum energy diagram for a vacuum tube; it's merely an extension of the semiconductor band diagram, where valence electrons are promoted to the conduction band. Well, if we keep going up, eventually the conduction band ends, and the unbound band begins -- that is, electrons having enough energy to leave the material entirely, namely, above the work potential.
The current flow equations then have to be changed, for the lack of hole current and recombination (of course), ballistic rather than diffusion movement, and unbalanced charge (space charge). This gives the Child-Langmuir law, and combined with electrode geometry, we can generate the family of curves of real devices.
Alternately, we can start with what we know about tubes, assume a charge-neutralizing gas (namely, the sea of ions that is a crystal), and basically invent FETs long before we have chemical processing refined enough to actually create them.
A neutral gas (with free ions and electrons; a plasma) can actually be used to some benefit in real tubes. de Forest believed gas was actually necessary, and it does increase the gain under certain conditions. It turns out, hard vacuum tubes are only a little harder to make (much better pumping, and tighter electrode tolerances), but far more general (they don't have weird glow-discharge modes).
Tim
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