[citation needed] For example, the internal structure of an NPN bipolar transistor resembles two P-N junction diodes connected together by a common anode.
In normal operation the base-emitter junction does indeed form a diode, but in most cases it is undesirable for the base-collector junction to behave as a diode.
If a sufficient forward bias is placed on this junction it will form a parasitic diode structure, and current will flow from base to collector.
A common parasitic structure is that of a silicon controlled rectifier (SCR).
Once triggered, an SCR conducts for as long as there is a current, necessitating a complete power-down to reset the behavior of the device.