Hi.
Does such animal exist ? A Zener diode emitting light when the zener knee conducts ? Like a Zener and an LED in series in the same package

 

Not yet. Silicon junctions have been shown to emit light during avalanche breakdown, but not enough to be useful. Silicon emitters would have some useful applications in integrated circuits.

The phenomenon was observed before integrated circuits were invented.

 

all PN junctions emit light when forward biased. they may not be efficient and radiation may not be in the visible spectrum but they all do.

 

In a high voltage Zener during avalanche breakdown, carrier recombination can produce visible EM radiation. I would think the silicon structure requirements for visible photon emission and voltage regulation might not be exactly mutual.
https://www.intechopen.com/chapters/47465

Since electron-hole pairs are produced during avalanche breakdown, some radiative recombination occurs. Both the electrons and the holes can be heated by the electric field. Since it is attributed to the hot-carrier population under the condition of avalanche breakdown, the light emission can be used for high-speed light-emitting devices, high-speed light amplifiers, and the analytic investigation of hot carrier distribution

https://www.laserfocusworld.com/las...-optical-coupling-in-cmos-integrated-circuits

 

Does such animal exist ? A Zener diode emitting light when the zener knee conducts ? Like a Zener and an LED in series in the same package

Silicon junctions generally emit radiation in the near IR.
For visible light, you need higher band-gap semiconductor material like gallium, which are not generally compatible with silicon junction fabrication.

Why do you want a Zener to emit light?

 

if indication that zener is conducting is useful (clamping voltage) then it is not a problem to add indicator LED and perhaps adjust zener rating. but this is rather obscure and rare requirement so i do not see justification to produce single component that includes both of mentioned devices. one can easily make own assembly that does just that. same is done with switching regulators that are replacing LM7805. couple of SMD components on a tiny PCB and you are done.