Transistor Breakdown Voltages
The breakdown voltage ratings of a transistor are the maximum voltages that a transistor can handle for each of its 3 junctions. If voltages are fed to the transistor exceeding this rating, the transistor can be destroyed.
A datasheet for a transistor lists the breakdown voltage ratings for the emitter-base, collector-base, and collector-emitter junctions.
For example, a 2N3904 small signal transistor has the following breakdown voltage ratings:
The first 2 letters in the subscript indicate the two transistor terminals for which the voltage rating applies, and the third letter is in reference to the third unmentioned terminal which is left open.
The first voltage, VCBO, indicates the maximum allowable collector-to-base voltage with the emitter open. The second voltage, VCEO, is the maximum allowable collector-emitter voltage with the base open. The voltage rating, VEBO, is the maximum allowable emitter-base voltage with the collector open.
Exceeding any of these voltages can destroy the transistor.