Why BJTs do not switch well at higher frequencies?

[nnayak82]

I was going through a book and it was mentioned BJTs are minority carrier devices and hence they do not switch well at higher frequencies [roughly above fT/10]

Can anyone explain it to me please? Does it have anything to do with mobility factor 'u' of carriers ?

 

[biff44]

BJTs store charge in the base-emitter junction, and to turn the device from on to off, you have to pull that charge out of the device first. The B-E region has some physical size, and you are limited to the voltage you can put on the base (typically zero volts, but certainly no more than -4 volts), so the electric field you can apply to sweep that charge out is limited, and the velocity of those electrons and holes is fixed. At some point in frequency, you simply can not inject charge into or out of the base fast enough to switch it anymore.