Using Transistors as Diodes - Page 1
Forum section: electronics / projects

author: pelud
author_rank: Regular Contributor
timestamp: November 01, 2017, 11:58:51 pm
content: Hi all,I notice that every once and a while people use transistors for diodes by just using one of the junctions (i.e. for low leakage, etc.) In some cases it seems just more convenient to use another transistor for simple low voltage, low current applications when you already have one in circuit.Assuming it's OK, I've never really been able to figure out what sort of characteristics to expect.I would guess that Vcbo and Vebo would be the equivalent to peak inverse voltage for each of the junctions, but what about current? Is it just the same as the maximum collector current?Also, what to do with the unused terminal? Leave unconnected, or short to the base? Does it make a difference?Thanks,John

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 02, 2017, 12:24:20 am
content: Short to base, and yes, it makes a difference! You get the Ebers-Moll transistor equation then, instead of the diode equation. Which, to the same order of approximation, are identical, so I'm not exactly telling a convincing story here, but suffice it to say, real diodes have more quirks to them (non-ideality parameter, series resistance, recovery) that make this worthwhile, especially when it's as cheap as drawing another connection!Speaking of recovery, because there's very little stored charge (as long as Vbe > Vce(sat), it's in the linear range), the resulting diode is much faster than otherwise.Here's an example ofotherwise:2N3904, B-E, C open. B-E is faster (higher doping), but lower voltage rating and more leakage (..higher doping).B-C, E open. Embarrassingly slow!Note that B-C is the only one really worthwhile (high voltage standoff, low leakage), and it's quite slow by itself (t_rr ~ 87ns).I forget what the case is for the diode-strapped transistor; it might be I didn't take a picture because it's effectively zero. Which really would mean it's in the 10ns or below range, because my pulse generator isn't very impressive. (Reminder that the puny 1N914 takes a mere 4ns to recover!)B-C (E tied to B) won't be as ideal, because inverted hFE is usually pretty bad, and thus you don't get as strong a reduction of the B-E junction's non-ideality (it's just another diode junction, after all). For example, 2N3904 inverted hFE ranges from ~1 at Ie = 10uA, to ~5 at Ic = 10mA, so you can expect a comparable factor on the improvement of characteristics.Not all transistors have poor inverted hFE. Low-Vce(sat) types usually have amazingly good hFE. ZTX651 is about 300 forward and 150 inverted. It would make an excellent diode!Leakage current: not usually very good for B-E junctions, a consequence of the low voltage range (Vebo). Some transistors are intentionally made with high Vebo, which usually come with good inverted hFE as it happens (usually for audio muting and signal switching applications).Even the lowly 2N4401 has comparable B-C leakage to a 1N914 (even better if you're shining a bright light on the thing), and smaller junctions have even less leakage. I know someone who's fond of BFT25's for jellybean-like use, and low leakage. (With an 8V C-B rating, it's comparable to a general-purpose transistor's B-E junction already. B-E rating is even lower. It must be heavily doped to have such low voltage ratings; and despite that, it has fractional pF capacitance. Conclusion: it must be a very small junction indeed!)Tim

author: pigrew
author_rank: Frequent Contributor
timestamp: November 02, 2017, 03:51:34 am
content: I don't have any fancy plots to show, but it's also worth mentioning the diode-connected MOSFET. In it, the gate is connected to the drain. It's very commonly used in analog integrated circuits.We use them since a lot of processes do not have good diodes (they commonly only have huge ESD diodes). They're also used when you need a diode with characteristics that match your transistor.For the BJT, you may also want to read about Gummel plots. They're measurements of the current characteristics with the base and collector shorted, yielding diode-like characteristics.

author: Cervisia
author_rank: Regular Contributor
timestamp: November 02, 2017, 02:28:42 pm
content: See page 70 of Ferranti'sE-Line Transistor Applications.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 02, 2017, 04:52:04 pm
content: I didn't even know ZTX was Ferranti! Cool.Tim

author: Peabody
author_rank: Super Contributor
timestamp: November 02, 2017, 05:39:01 pm
content: Maybe someone here could explain this: I was working on a project that needed reverse battery protection. Normally I would use a P-channel MOSFET, installed "backward", for this. But I saw somewhere that a bipolar PNP transistor could also be used for that. The PNP would provide a low-drop switch for the power rail, but would not let current flow backward, thereby providing both a soft switch and reverse connection protection in the same device. The idea was that if the voltage on the collector is greater than the voltage on the emitter, current cannot flow - regardless of what the base is doing.That actually seemed to work with the reverse voltage a 5V or 6V, with the base open, but alas, not with a 9V battery. As the reverse voltage increases, current begins to flow at almost exactly 8V. And the bottom line is that in reverse bias it basically acts like an 8V zener. I tried several PNPs, all with essentially the same result, so unless there's a datasheet parameter that would affect it, this appears to be an inherent characteristic of a PNP.That of course is substantially different from the reverse voltage behavior of even the lowly 1N914. But if the project only deals with voltages below 8V, a PNP *might* be usable as a diode.I don't know enough about how bipolars are built to understand where this 8V reverse limit comes from. And again, that's with the base open so no current is flowing through it. Can anyone explain why this happens?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 02, 2017, 07:54:25 pm
content: Yup, that's E-B breakdown. You need a specialty type with high Vebo to do reverse protection.I also wouldn't trust E-B breakdown against transients, which would be relevant for hot-plugging situations (say someone jams the wrong polarity charger cord into your device).Tim

author: Cerebus
author_rank: Super Contributor
timestamp: November 03, 2017, 01:55:50 pm
content: Quote from: Cervisia on November 02, 2017, 02:28:42 pmJust in case anybody else is as blind as me, and didn't spot that until now, I'll point that up. I.e. that there's a whole document linked there. These old transistor data and circuit collections can be goldmines of data that the manufacturers can't be arsed to publish nowadays.