Tunnel diodes Zener breakdown
The typical IV curve of a tunnel diode shows that the breakdown voltage is positive:

That's from the RCA Tunnel Diode Manual that I linked to in my answer to one of your earlier questions about tunnel diodes.
The solid curve is from a normal diode.  A Zener diode breakdown would be similar.
The tunnel diode hits breakdown at a positive voltage - tunnel diodes conduct in reverse mode, always.