# Transistor as two diodes

Morning all,

I know that a transistor is 2 diodes joined at the hip.

But if I wanted to prove that to myself, what variety of diode would make for a good experiment. Let's say I wanted to work in 5v sort of range, both on the high side of the collector for the load and on the base to switch it. Low current, say switching an led with series resistor.

I imagine that the diodes in a real transistor are different from each other; if not the collector and emitter would be interchangeable.

TIA as always,

Jim

Good morning (evening here on the Left Coast)!

If you are thinking of trying to make a transistor from two diodes, that won't work. A key difference between a transistor and two diodes is that the base region (the hip) is extremely thin, so its properties are quite different than "one end of a diode". The difference has to do with short range electric fields that form at the PN and NP junctions when the junctions are created, which sweep current carriers out of the region.

Ok... so although a transistor is conceptually two diodes, it can't be made from two physical ones?

Damn, I was thinking that would make a nifty experiment.

One of the stranger consequence of transistors NOT behaving as two diodes is saturation behavior. The collector to emitter junction can have a voltage drop of less than 0.2 volts when the transistor is saturated, yet the base-emitter junction will show a voltage drop of typically 0.6 - 0.7 volts at the same time (for reasonably small currents). Furthermore, in that situation the "conceptual diode" that forms the C-B junction should be reverse biased! Try the experiment and see.

Another way to think of it is that a transistor is NPN. The only thing you can do with two diodes is NP-PN which is not the same.

You can think of a transistor as two back-to-back diodes for testing purposes only. i.e. You can pull a transistor out of the circuit you can use a multimeter to check if it's "blown".

If you think about how a transistor really works... With two back-to-back diodes it would be impossible to get any current flow between collector & emitter.

Yeah DVD^2, that's what I was thinking....

When I put two real diodes back to back, and which each tested OK on diode test on my meter, it fails both ways across the putative C and E. On a real transistor, it passes the test one way.

The reason a transistor works is that the emitter spews forth charge carriers straight into the depletion layer of the reverse-biased base-collector junction - here they get accelerated easily by the electric field and contribute current to the base-collector circuit that otherwise would not flow.

With separate diodes those charge carriers from the "emitter" just recombine just like any charge carriers in a forward biased junction, never get near that strong electric field of the base-collector depletion layer.

spews forth

Hey, don't get all technical on me now 8)

JimboZA:

spews forth

Hey, don't get all technical on me now 8)

!!

Actually I think a good way to think about a BJT as two diodes plus a magic route for emitter current to jump straight to the collector, but that the base current and the current through this magic route are strongly coupled.

The coupling is due to the relative likelyhood of recombination and diffuse to the collector, which mainly depends on doping and geometry.

The point you are all missing is that for a transistor to work the two junctions making up the diodes have to be made on the same crystal lattice.

I imagine that the diodes in a real transistor are different from each other; if not the collector and emitter would be interchangeable.

In fact to a certain extent they are. You will still get the same transistor action only the resultant gain will be smaller, due to the geometric configuration of the two.

Grumpy_Mike: The point you are all missing is that for a transistor to work the two junctions making up the diodes have to be made on the same crystal lattice.

Isn't that basically what Keith was saying though, Mike-

Another way to think of it is that a transistor is NPN. The only thing you can do with two diodes is NP-PN which is not the same.

Not quite. It is the single crystal structure that makes the whole thing work. A NPIPN structure on the same lattice would work as a photo transistor. By the way an I type semiconductor stands for Intrinsic that is an are when ther are no excess charge carriers. Normally these regions are used in PIN diodes which have a fast recovery time.

Right, that's enough transistors!

Interesting as it certainly was, my simple civil engineer's brain is now in danger of overload.

Thanks everyone, for that discussion.

When my wife was doing her teaching practice one memorable phrase she herd a five year old say was" this book tells me more about penguins than I want to know"

More stuff you didn't need to know... when the team at Bell Labs told their boss they had success in the lab, he was supposedly not pleased... he wanted them to show him a Field Effect Transistor and not the transistor we now know as Bipolar Junction Transistor. The BJT was proven to have the characteristics they wanted (signal amplification) so it was a success and announced to the world a month later.

Grumpy_Mike: When my wife was doing her teaching practice one memorable phrase she herd a five year old say was" this book tells me more about penguins than I want to know" :)

:D

From the mouths of babes........ or as Art Linkletter would say "kids say the darnest things"

My kids when growing up with color tv would yell out to me "hey dad the TV is all gray again" whenever the stations would play old black and white TV replays.