Gregory's Server
As an EE I would guess that at 6V on a forward biased PN junction the power dissipation would result in the silicon de crystalizing (aka melting). In our solid state circuits class we had to compute why 20V was the breakdown voltage for MOSFETs and it was basically because the electron mobility overcomes the gate at that point and conducts current (resulting in the gate being shorted from then on).
@ChuckMcManis reverse biased PN junction, in this case. what's odd is that discrete transistors are nearly always limited to 6V, but transistors on a chip have VBE ratings that are all over the place and often much higher (30V, etc)