Breaking down the BE junction of a 2N3904 with a series 1Meg resistor
from 12Vdc used to yield something like 1 Vpp of very flat noise over
the audio range. Cheap noise source for electronic games, sounds like
applause in the background. However, did NOT know the technique would
get something flat out to 1GHz! What transistor?
I used a 2n918 --this was over 15 years ago-- with 20kOhm to +18V
and the load impedance must be low: 50 Ohms, since we're doing RF.
Bare ENR is in the 30dB ballpark. For serious measurements it's good
practice to sacrifice a bit of noise in exchange for good reverse
matching by adding a 12 to 15dB attenuator.
The upper cutoff is set by the impedance at the emitter node,
basically the BE junction capacitance in parallel with the load
impedance. I never tried to find the impedance of a BE junction
in reverse breakdown. I should take a look, some day.
As I learned how to make lower noise amplifiers, I 'upgraded'
to quieter noise sources. These days, I measure noise by
switching back and forth between a termination resistor at
room temperature and another in LN2 at 77K. The bandwidth
over which those are flat should extend to many GHz. (Albeit
surely not as far as Boltzmann and Planck would say.)
Jeroen Belleman