Simulating transistor junction breakdown

[alec_t]

Does anyone know if LTSpice has a bipolar transistor model which handles reverse-voltage breakdown of the base-emitter junction?

 

[crutschow]

I believe the standard Spice transistor model does not support base-emitter reverse breakdown.

You can simulate it by placing a zener diode of the appropriate voltage back-to-back with a standard diode in parallel with the base-emitter junction. (The standard diode prevents forward conduction of the zener when the base-emitter junction is forward biased).

 

[alec_t]

I thought there was no standard support but was hoping someone knew of a freely downloadable model which did. Thanks for the tip, Carl. My transistor theory is rather rusty (and Google's not very helpful on this); do you happen to know if reverse breakdown of the base-emitter junction is accompanied by reverse collector current (which would also need to be modelled) ?

 

[moffy]

I once put an NPN BJT in a circuit inverted i.e. collector and emitter swapped. The base was floating and the C was connected to a resistor/capacitor in parrallel to gnd while the E was connected to +15v. The circuit oscillated as a sawtooth, at a few kHz. The E-B would break down injecting current into the B which would turn the inverted NPN on, and it would stay on until the current decayed to a certain level then the transistor would reset. I asked an old Prof why it didn't recover immediately when the voltage dropped below the E-B breakdown voltage and he pointed out that C-E breakdown voltage is current dependent. All those SOA curves for BJT's. So a B-E breakdown can have other effects.

 

[alec_t]

Ah, so reverse collector current does flow when the B-E junction breaks down. Thanks for the info, Moffy.

 

[moffy]

I'm not sure it would flow in all cases, as the biasing on my circuit was rather weird. It provided base current because the b-c junction was the new b-e junction, so the current was in the right direction. For a normal biasing arangement with say taking the base 7v below the emitter I'm not sure you'd get c-e current as the current flow is inverted to normal.