# BJT how to explain in terms of conventional current flow?

Conventional current flow always confuses me.

For electron flow, i can explain how the BJT functions. When there is a small current flow from Base to Emitter, more positive holes are created which reduces the depletion region. Electrons from Emitter then diffuse across the depletion region and move through the Base-Collector depletion region which is thin.

How do i explain this in terms of conventional current flow?

Thanks

• How do you explain PNP transistors by comparison? – pjc50 Aug 9 '16 at 13:31
• Conventional flow is the exact opposite way around. Flip all arrows of the electron flow and ta da, you got conventional flow. It's best to learn electronic circuitry through conventional flow rather than learning electron flow of the components then flipping the arrows to get the conventional flow. – Bradman175 Aug 9 '16 at 13:31
• Do not consider currents only. Don`t forget the role of the diffusion voltage Vdiff and the external voltage that works against Vdiff (thereby reducing the width of the effective base region). – LvW Aug 9 '16 at 14:27
• Typically, BJT operation is not explained in terms of conventional current flow or electron current flow, but in terms of minority carrier injection and diffusion, which means electrons in the P regions and holes in the N regions. – The Photon Aug 9 '16 at 15:52

## 2 Answers

Conventional current flow is nothing more than a mathematical convention for assigning voltage polarities across components in a circuit. Period. It is completely unrelated to the concepts of analyzing majority and minority charge carriers ("hole flow" and "electon flow") in materials.

In your figure, blacken out the interior of the BJT so you do not know what's inside it or its theory of operation at the atomic lattice level. Now reverse the directions of all the current arrows, keep the voltage values and polarities the same, and bingo: you have the conventional current flow analysis of the circuit.

• It is completely related, through sign conventions on carriers’ charge. – Incnis Mrsi Aug 26 '16 at 17:12
• @Incnis Mrsi |It is completely related... Well, yes, and its because Conventional Current IS electron flow. (Many beginners' books get this wrong, and incorrectly say that electron flow and Conventional Current are two different things. Nope. And they're confusing their students.) Conventional Current is when we add up all the various charge-carrier flows into a single value, just like any ammeter does. Electrons are part of this sum. In other words, Conventional Current is not a flow of positives. – wbeaty Dec 9 '16 at 6:17

Conventional Current cannot be used to explain the inner workings transistors. Why? Because one main purpose of Conventional Current is to conceal the actual charge-flows. Transistors are based on just those charge-flows: p and n and majority and minority and diffusion and field-driven. Conventional Current erases them, and replaces them all with a sum: the amperes. To calculate conventional current, we look at one point in a conductor and add up all the various flows into a single value, including both the electrons and holes (and the proton flows if it's acid, and the various ions if it's plasma or wet dirt or human tissues.) Conventional current has some more common names: electric current. Also "amperes."

In other words, electron flow is the Conventional Current. The electron motion is a part of the sum that gives us the total amperes inside the silicon.

Or put it this way: Conventional Current is what your clamp-on ammeter measures. The ammeter doesn't care if both holes and electrons are together creating the amps. It just sums them all together.

Some beginners' textbooks get this stuff wrong. If you're unlucky, and you're doing this for a class which uses one of these books, then you probably should figure out which "answers" the teacher wants. If you give the actual correct information, you'll be marked wrong.

Heh, I first encountered such stuff in fourth grade. The teacher said that nickel wasn't magnetic. The textbook said that only iron and steel were magnetic. The teacher takes out a (American) nickel and proves that a magnet won't attract it. So, if I don't want to be marked wrong, I have to lie, and say that nickel metal isn't magnetic. (This was 1960s, no internet, no good refs in the tiny school library.) Don't even get me started.