What is a depletion zone and why is it important?
When a PN junction is reverse biased, a region of high resistance (or a region depleted of carriers) is formed. See the figures below .The extent of this depletion is proportional to the reverse voltage applied. In figure A, this region, devoid of carriers, forms a junction capacitance and that capacitance is often a major limitation insofar as switching is concerned. On the other hand, it is the basis of the varactor diode; wherein the junction capacitance can be varied by varying the reverse voltage. Figure A In another example, a depiction of the junction FET is shown in figure B. Again, as the reverse gate junction voltage is increased, the depletion region (high resistance region) extends into the conducting n-channel eventually pinching off the N-channel current.
