...or more accurately avalanche breakdown.
Parts tested: 2N3904 from different makers, both in normal transistor
mode (E,B,C), and in inverted mode (C,B,E); the prefix "R_" is used to
indicate this inverted or reverse mode.
All parts were bought in March 2006 and the same type and maker may
differ greatly now.
1) Central Semiconductor CMPT3904
2) Central Semiconductor CMPT3904E
3) Fairchild KST3904MTF
4) Fairchild MMBT3904
5) Diodes Inc MMBT3904-7-F
6) ON Semiconductor MMBT3904LT1G

Standard / normal mode, 20V/div horizontal, 200uA/div vertical.
For the reverse / inverted mode, 2V/div horizontal, 200uA/div
vertical; the base offset current was adjusted for a minimum observed
current (lowest trace). 0.1mA/step on the base.
Note the decided negative resistance curvature on the "zero" base
current step and none seen elsewhere (at these currents; there was
enough self-heating in the normal mode for me to dare "pushing" it).