A Working Transistor:
The On/Off States of a Transistor
From the Intel website

Transistors consist of three terminals; the source, the gate, and the drain.
In the n-type transistor, both the source and the drain are negatively-charged and sit on a positively-charged well of p-silicon.
When positive voltage is applied to the gate, electrons in the p-silicon are attracted to the area under the gate forming an electron channel between the source and the drain.
When positive voltage is applied to the drain, the electrons are pulled from the source to the drain. In this state the transistor is on.
If the voltage at the gate is removed, electrons aren't attracted to the area between the source and drain. The pathway is broken and the transistor is turned off.