In the Microsemi 2N918 NPN RF transistor datasheet, the absolute maximum ratings are given as:
The fact that \$V_{CBO}\$ is so much greater than \$V_{CEO}\$ has me confused. Additionally, the datasheet specifies \$V_{CB}\$ at 25V and 30V for certain parameter test conditions later in the document.
My question is: for a BJT, what in the construction/manufacturing process makes \$V_{CBO} > V_{CEO}\$?
Best Answer
Just to make the terms explicit, \$V_{cbo}\$ is the maximum voltage that can be applied between the collector and base of a BJT while the emitter is left unconnected, and \$V_{ceo}\$ is the maximum voltage that can be applied between the collector and emitter of a BJT while the base is left unconnected. There is a huge difference between these situations.
With the \$V_{cbo}\$ case, some leakage flows between the collector and the base and this may cause the part to warm up a little. This is just a diode involved, though. So in this case, they just specify the voltage below which the part isn't damaged and from which the BJT can recover itself.
With the \$V_{ceo}\$ case, it's similar in a lot of ways. But now we aren't talking about a diode, but the entire transistor structure. So with the base open, any leakage current will also become an enabling base current, which of course is then amplified by the transistor's \$\beta\$. So that will lead to still more current, etc., heating it up still more.
That's the main reason I'm aware of why it's often true that \$V_{cbo} > V_{ceo}\$.