Geek help: Gate Electrode?

I'm trying to understand resonant tunneling transistors so I can make a question about them.

They apparently use a circuit element called a "gate electrode." The description talks about a voltage applied to this thing, and it has a + sign in the diagram. Is the voltage being applied ACROSS the electrode? The picture doesn't indicate two parts to the gate electrode, so I'm confused about this voltage which is supposedly across it.

Anyone able to enlighten me on this?