Analog Electronics
The current gain of a bipolar transistor drops at high frequencies because of

The early effect
Parasitic inductive elements
High current effects in the base
Transistor capacitances

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Analog Electronics
Pinch-off voltage in a JFET is

 the gate to source voltage that gives zero drain current.
 the drain voltage that gives zero drain current.
 the drain voltage that gives infinite drain current.
 the gate to source voltage that gives unity drain current.

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