Depletion zone
In semiconductor physics, the Depletion Zone or Depletion layer is an insulating region within a conductive, doped semiconductor material where the charge carriers have been swept away. Whereas P and N-doped semiconductors are conductors, the Depletion Zone is an insulator. And since the removal of charge carriers exposes the opposite-charged dopant ions, a Depletion Zone contains a space charge and is the source of significant capacitor phenomena. The existence and shape of Depletion Zones is easily controlled by e-fields, i.e. by voltages applied to the electrodes contacting the semiconductor.
Depletion Zones figure largely in the explanation of the on/off switching of diodes, in the control of the Emitter junction barrier in bipolar junction transistors, in the control of the width/length of the conductive channel in field effect transistors, and in the control of the width of the dielectric layer in variable capacitance diodes, "varactors" or "tuning diodes".
A Depletion Zone is essentially a charged insulator of programmable shape; a nonconducting balloon which invisibly grows and shrinks within a solid block of silicon. Modern electronics is based on transistors, and transistor operation is based upon movable depletion zones.