Tim Wescott wrote:
. . .
The "saturation" in bipolar transistor terminology means "current
saturation", but it could just as well mean "carrier saturation". When
the transistor is saturated the base region is stuffed full of carriers
(holes, for an NPN transistor). It takes a while for those carriers to
go away, during which the transistor stays on. This is a very nonlinear
effect, and can be very slow. The old 74Sxx series logic put a
schottkey diode from collector to base on the transistors to keep them
out of saturation, and sped them up considerably.

The very slow saturation recovery you see in saturated switch
applications (unless using a gold-doped transistor) is largely absent in
typical RF power applications. The reason is the bipolar drive usually
employed -- there's typically a large amount of negative base current
available to suck the stored charge out of the base region in a hurry.

. . .

Roy Lewallen, W7EL