Google and Wiki are surprisingly unhelpful.
In what context is it being used?
As a layer of . . . what? between two doped semiconnductor regions.
This is the area in which the actual “transistor effect” occurs. It’s where the holes and electrons are influenced by the electric field of the gate (in a FET), or by the base current in a bipolar transistor.
Makes sense. So, just so I’m clear – it is within one or both of the semiconductor layers, as opposed to being some third layer of additional different material?
As **beowulf **says, this is the layer under the gate oxide (or nitride) of a MOSFET. It’s lightly doped so as to be a poor conductor, unless the gate is charged up, in which case it becomes a good conductor. The source and drain are heavily doped and are good conductors.
In your basic FET, the current-carrying silicon is all one layer. Different regions are defined using photomasks, and then dopants are introduced. Here is a schematic diagram. For the simplest designs, there are only 3 layers - Silicon, oxide, and metal. Note that nothing is that simple these days.
The active layer is the layer that the charge carriers flow through from the source to the drain in a field effect transistor.
Got it?
Let’s assume that the charge carriers are electrons, since that’s what most people are used to. A voltage is applied between the source and the drain, but since they are semiconductors it may or may not be enough voltage to get the electrons to move through the active layer. The gate voltage will determine if electrons can pass through or not.
Now I do, and should have before.
It’s really the base-emitter *voltage *for a BJT. V[sub]BE[/sub] controls the width of the depletion region, and, well, you know the rest…