# Current in BJT at EBJ and its mechanism about Diffusion and Drift Current

I have seen this post and I'm still confused about what mechanism would be applied on majority carrier, and it seems to me that the post I cite only explain about how the minority carrier move from base to emitter and collector would provide minority carriers to base.

This the a little bit different from what I have learnt about, and I've learnt that base would take minority from outside the device (so that this contributes to base current $$\i_B\$$ rather that collector.

Can somebody explain what happens to the majority carriers at the EBJ (emitter-base junction), and where the base takes the minority current?

We should follow Horowitz/Hill's AOA textbook, and view BJT's as transconductance devices, where the base current is seen as an unwanted leakage only. (In other words, base current does not control the collector current. Only the Vbe controls collector current, by altering the potential-barrier in the EB junction, as described by the Ebers-Moll equation.)

So, when base current is zero, the base contains a high density of majority carriers as usual (the holes, if an NPN device.) Then, as Vbe rises past the EB junction threshold, minority carriers flood into the base region from the emitter region. The base region then behaves as a conductive shield-box, a Faraday cage, so only the minority carriers which randomly drift into the CB depletion zone will contact the large e-field in the CB junction, and be accelerated towards the collector. To promote this current, and avoid recombination (and large Ib,) the base is made thin, and given only light doping.

The "leakage" into the BJT base terminal can only occur when a few majority and minority carriers in the base region are recombining: colliding and canceling each other. This constitutes a current in the base terminal only, not part of the large CE current. Ordinarily it would cause the Vbe value to reduce (similar to a capacitor being discharged by a leakage-current.) But if the Vbe supply happens to be a voltage-source, instead the recombination only produces the current in the base terminal, but without dragging down Vbe. (And, since Vbe determines both this small majority current as well as the injected large minority current, they will remain roughly proportional to each other: the beta/hfe value.)

Vbe determines the potential-barrier in the EB junction, which determines the rate of minority carrier injection from E to B regions (electron injection, in the case of NPN devices.)

One source of confusion is the wrong assertion that "transistors are current-amplifiers," as if base-current can directly influence collector current. But there is no physical mechanism for this.

Instead, if the current in the base terminal is allowed to influence Vbe, (via the usual V/I curve for the diode junction,) then next, Vbe will determine Ie and thence Ic. Ib can only affect Ic if first it causes the value of Vbe to vary. The "transconductance" concept describes transistor action, while hfe/Beta only describe an indirect coupling between currents in base and emitter terminals, which is actually mediated via changes in Vbe. The Vbe controls the transistor. However, by injecting an Ib into the base terminal, Ib can control Vbe. (I note that in FETs, the gate current is able to "control" the drain current in similar fashion ...by altering the value of Vgs, while not somehow influencing drain current directly. In FETs we consider the gate current to be unwanted leakage, a consequence of applying a voltage between G and S, and in the BJT, the base current plays a similar role. Just think about it, all FETs have a beta/hfe value! Inject some nanoamps into the gate, and this current controls the drain current! Now imagine an entire technical community who actually believed such a thing, and will fight furiously against any who dare to question it.)

In AOA textbook 3rd edition, BJT is first explained with "Transistor-man," who observes the base current, then adjusts the collector current. "Transistor-man" is actually the EB potential-barrier. He's the changing value of Vbe! Heh. Then next, we ignore transistor-man, and pretend that Ib itself can somehow can control Ic.

Or put differently, "current gain" is the oversimplified, current-based "for dummiles" explanation of BJT operation, aimed at kids in grade-school. It's designed to teach beginning students, the ones who don't quite know what voltage actually is, much less e-fields. They cannot handle exponential functions, and are told that turned-on silicon diodes always have a fixed 0.7V across them. (And then, obviously Vbe cannot affect the emitter current, since Vbe is an unchanging value, by definition!)

When we later arrive at engineering school, we're supposed to abandon hfe as the central explanation, and instead learn about diode VI equation, potential-hills and electrostatics of semiconductor junctions, i.e. Ebers-Moll model, with the e-fields inside BJT depletion layers, which determine forces applied to mobile carriers found there, and therefore determine net currents measured at the transistor terminals.

• It is really surprising that in many books and other "knowledge sources" it is still claimed - without any attempt to give an explanation - that the BJT would be current-controlled. The explanations of the design and function of a simple basic emitter circuit alone provide clear indications of voltage control (Re-feedback, low-impedance base voltage divider, voltage gain dependent only on transconductance and not on the beta factor, etc.).
– LvW
Commented Dec 16, 2023 at 9:26
• @LvW Win Hill, the AOA author, says he was taught "current gain" in college, and it screwed him up! In first design job, a wise old engineer had to take him aside and debunk that stuff, instead showing him the Ebers-Moll-based approach. The same thing happened to me ...but I had to figure it out on my own. (For low-price high-volume optosensor, where an extra SMT transistor was a big deal, and actual ICs were forbidden.) Yep, diff amp and cascode, where hfe is irrelevant, as long as Ib "leakage" isn't large. see cr4.globalspec.com/comment/720374/Re-Voltage-vs-Current Commented Dec 19, 2023 at 11:42
• What surprises me the most: Everywhere in technique and science, people are asking for proof and verification. But there is one exception: The bipolar transistor. For many people, the assertion that the BJT would be current-controlled is sufficient - probably because of the misinterpretation of the relationship Ib=Ic/B (confusion of cause and effect). Who has an explanation for this phenomenon?
– LvW
Commented Dec 19, 2023 at 12:23
• @LvW I was noticing similar things in the 1980s. One physics teacher defined it explicitly, where anything learned in childhood becomes unquestioned dogma,. Victims would almost rather die than question such knowledge, much less consider changing it. This may be origin of most physics misconceptions, where Bill Nye and Magic Schoolbus taught wrong stuff, which those students can never "un-learn." Electrons zoom through wires at lightspeed. Wings fly by Bernoulli, and veinous blood is bright blue like paint. Batteries store electrons, your cornea is merely a window, etc., etc. Commented Dec 21, 2023 at 1:49
• Aha! personal.ems.psu.edu/~fraser/BadScience.html "It seems that anything people have learned prior to puberty takes on the status of an immutable truth (this is something well understood by parents, governments, and religions.) ...they know in their hearts that their 8gr teacher, or their mummy was actually right and that you are just a contrarian who is attempting to destroy the established order. The damage is done, the mind is frozen and the prepubescent dogma lasts a lifetime." So, transistors amplify current, gravity in space is zero, winter is when Earth is further from the sun! Commented Dec 21, 2023 at 2:05