Hi neilhao, can you share something more about the design? I couldn't find it in your web page.
By looking at the small photo on the top right of your attachment, it seems much like the double-zener design appearing in IEEE Microwave and Wireless Components Letters, vol. 28, no. 4, April 2018 "A broadband Microwave Noise Generator Using Zener Diodes and a New Technique for Generating White Noise", Arslan and Yıldırım. Is it inspired by that?
Personally, I have tried zeners as noise generators, but I have been a bit disappointed by the low-pass character. Sure it depends on diode selection: small voltage diodes are based on zener effect and are not particularly noisy (low ENR), while above 6 V avalanche effect prevails and noise increases; however, avalanche tends to happen as small bursts of electrons followed by a relaxation time, which is an intrinsically low-pass phenomenon. Besides, all kinds of zener diodes tend to have large junctions in order to be able to dissipate power, so capacitance is not small, again an high frequency limitation. Best diodes are probably in the 6-12 V zone, minimal power. At least, this is my personal experience, but I'd like to hear other people thoughts.