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In a (hobby) project I'm doing, I noticed that the diodes I needed were more expensive than the NPN transistors, which left me wondering if I could substitute the transistors for the diodes.

I'm aware the base-emitter junction can be used as a diode (usually tying base and collector for better properties), but this limits the reverse voltage to \$V_{EBO}\$ (typically about 6V), which is too low for my application. Therefore I'm interested in using the base-collector junction, like so:

schematic

simulate this circuit – Schematic created using CircuitLab

Researching this, I've found multiple claims that B-C can be used as a diode, but very little information on whether or not this is a good idea, and its properties. So in particular, I'm wondering:

  1. Is it safe to use an NPN BJT in this configuration? Is this as robust as a dedicated diode?
  2. Should I leave the emitter unconnected (A), or tie it to the base (B)? How does this affect the properties?
  3. What would the resulting reverse breakdown voltage be? \$min(V_{CBO}, V_{CEO})\$?
  4. What would the resulting forward voltage be? Can it come close to \$V_{CE(sat)}\$ (at similar currents)?
  5. What forward current could the resulting diode support? Can it come close to \$I_C(max)\$?
  6. How fast would the resulting diode be?

Application

I would appreciate general answers on using B-C as a diode, but for my particular application the diodes (transistors) would be used as a low-current, low-frequency full bridge rectifier. I'm looking to maximize \$V_R\$, and minimize \$V_F\$ at a current of about 10-20 mA. The transistor I had in mind is the MMBTA06WT1G.

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  • \$\begingroup\$ (4) - No, it will be around 0.7v \$\endgroup\$
    – Neil_UK
    Mar 25, 2019 at 17:19
  • \$\begingroup\$ Related: Can I (ab)use a transistor as an ESD protection diode? \$\endgroup\$
    – CL.
    Mar 25, 2019 at 18:29
  • \$\begingroup\$ Could you clarify if the part in question is the one in the picture? Last time I think I've bought 1000 pcs of 1N4148 for around $4 including shipping. Half a cent is hardly expensive, MMBTA06 costs more. \$\endgroup\$
    – Nyos
    Mar 25, 2019 at 18:55
  • \$\begingroup\$ @Nyos The 1N4148 was a stand-in for a generic diode; I've removed it from the schematic. I'm looking for SMD parts, and I'd prefer to avoid MELF. With that, the cheapest diode I could find on Digikey in 100pcs is about 5 cents each, where the transistors are more like 4 cents in 1k qty (I need more transistors anyway). Sure, it's a small difference, maybe not worth the hassle. But I'm also just really curious if it is possible now, even if I decide against it in the end. \$\endgroup\$
    – marcelm
    Mar 25, 2019 at 19:32
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    \$\begingroup\$ Another consideration (if you're making a board, which is what it sounds like) is size. Discrete BJTs can be in bigger packages than diodes (in general). Keep in mind board area cost/layout ease. \$\endgroup\$
    – yhyrcanus
    Mar 25, 2019 at 19:58

1 Answer 1

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(1) Yes, it will work.

(2) If I recall correctly \$V_{CES}\$ (B version) will have \$V_{CES} > V_{CEO}\$.

(3) \$V_{CB0} > V_{CES}\$.

(4) Forward voltage will be around \$0.7V\$.

(5) \$I_{Cmax}\$

(6) Hard to say because keep in mind that the BJT was not designed and optimized to be working as a diode. So, you should use an ordinary diode in your application instead of BJT's. Because you do not get any benefices from using the BJT.

And if you want low forward voltage drop bought yourself a Schottky diode.

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    \$\begingroup\$ (5), no. For example TIP31 Ic(abs max) is 3A but Ib(abs max) is 1A. \$\endgroup\$
    – Spehro Pefhany
    Mar 25, 2019 at 17:45
  • \$\begingroup\$ Yes, I forgot that not all transistors will have Ic_max = Ib_max. \$\endgroup\$
    – G36
    Mar 25, 2019 at 17:57
  • \$\begingroup\$ I'm afraid I don't understand your answer (3); Is it guaranteed that Vcbo > Vces? (I'm honestly not even sure I understand the Vces rating...) \$\endgroup\$
    – marcelm
    Mar 25, 2019 at 19:25
  • \$\begingroup\$ Vcb"O" is a collector base voltage with the emitter "O"pen. And Vce"S" is a collector-emitter voltage with a base terminal "S"horted to the emitter. \$\endgroup\$
    – G36
    Mar 26, 2019 at 14:44
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    \$\begingroup\$ @G36 Recheck part 5 of your accepted answer .The current comes out of the base ?! .The limit would be the Base current rating which is often not stated and when it is it is a fraction of Icm \$\endgroup\$
    – Autistic
    Aug 5, 2020 at 21:18

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