Electronic – Can a BJT base-collector junction be safely used as a diode

bjtdiodesnpn

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.

Best Answer

(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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