Re: A diode?
Except that diode noise may not be random!
Zener/Avalanche diodes can develop a negative resistance characteristic. When used in a circuit to produce random noise, they can develop into a Relaxation Oscillator. Such a relaxation oscillator produces a very predictable waveform, which is not at all random. That's not good.
Take a look at Figure 5 on page 19 of On Semi's "Zener Theory and Design Considerations" handbook:
Notice those little zigzags in the expanded portion of the I-V curve, and then realize that those represent regions of negative resistance. They explain this as an artifact of "Microplasma discharge theory". If the load line happens to go through one of those points, the result will be a relaxation oscillator.
Another discussion I've found by some researchers indicates that approximately 75 percent, or more, of off-the-shelf Zener/Avalanche diodes could be turned into relaxation oscillators with the right load/voltage being applied!
Now, for the really disgusting part. Even if the original design doesn't oscillate, the devices can be subject to a parametric shift, perhaps caused by the formation of Frenkel Pair Defects, or perhaps caused by trapped charge in the surface passivation layer, which will shift the characteristics of the device slightly. Thus, a design which doesn't oscillate initially very well may drop into oscillation after, oh say 5000 hours of use. So says the Voice of Experience!