transistor leakage currents

[m7b52000]

I have been trying to understand transistor leakage current with a view to experimenting with leakage simulation with large resistors across B-C of Russian transistors with zero leakage. (Tonebender experimentation).  My problem is that there appear to be, theoretically, leakage currents possible in many places in a transistor.

So - which is the leakage that we are typically talking about with Ge transistors? B-E, B-C or C-E?

cheers,

Tony

[Rob Strand]

I posted this list the other day,
https://www.diystompboxes.com/smfforum/index.php?topic=120653.msg1222743#msg1222743

We are normally concerned the consequences of Icbo and that's the main one you want to emulate.

The leakage measurements jigs measure Iceo (much larger in value than Icbo).    The measurement jig measures Iceo = (hFE + 1) * Icbo.    This is largely a consequence of Icbo.   It's a special case when the base is open.

If you shorted the base to the emitter you would still see Icbo (technically Ices).   That should give you a hint that Iceo isn't the primary mechanism.

if you put a resistor across b and e, like in a circuit, the effect is largely modelly by Icbo.   The emitter current would be between Ices (~Icbo) and Iceo.  Some datasheets gave this as Icer, perhaps even giving a plot of Ie vs R.

If you dig around for info on leakage you will find the leakage depends on the collector voltage so the equivalent is neither a large valued resistor or a current.

Here's a pic,

[m7b52000]

Thanks, Rob. I'm still trying to picture in my mind what is going on. Is the leak B>C or C>B?

cheers,

Tony

[Rob Strand]

Thanks, Rob. I'm still trying to picture in my mind what is going on. Is the leak B>C or C>B?

It's like a current from C to B inside the transistor.    For example like a resistor or current source between B and C.
Think of it like an ideal transistor and a leakage current inside one transistor package.   The leakage current
feeds into the base but it's occurring  inside the transistor package.

When you don't have leakage you might see this type of circuit where RB provides the base current,


However when you have a lot of leakage you might see something like Q3.


In this case the leakage going in the base is stronger than what you need to bias the transistor.
Adding the BE resistor diverts some of the CB leakage current from going into the "ideal transistor's" base.
So that leakage current inside the transistor package now comes out of the base pin and down the resistor.