How a power transistor works
Solid-state microwave power

The traditions and characteristics of tubes and solid state have led to differences in vocabulary. For example, tubes have a bell-shaped curve P _out (P _in ): past a certain point, they are oversaturated and output power decreases as input power increases. Corresponding to a precise input power, the top of the curve gives the saturation output power P _sat . A transistor doesn't exhibit this bell-shaped response, and it's always possible to increase output power even very slightly by increasing input power. But, this is at the expense of gain, efficiency, linearity and even reliability. Since transistors exhibit little gain compression...