energysurface

P-N Junction

Posted on: November 17, 2010

•A Method of increasing the number of mobile electrons and holes is to dope the material.
•Donor atoms contribute extra electrons that are less well bound than the usual valance electrons
–n-type, excess of negative carriers over positive
•Acceptor Atoms contribute too few electrons and so can trap electrons to contribute holes into the valance band.
–p-type, excess of positive carriers over negative.
•Electrons and holes move within the semiconductor in two ways
•Diffuse
–Driven by concentration gradients
•Drift
–Driven by potential difference.
Carriers move to reduce their electrical potential energy
•p-type and n-type material bought together
•On the n side we have an excess of electrons
•On the p side we have an excess of holes
•This creates a concentration gradient across the junction
•Electrons and holes diffuse across the junction
–Holes from p to n
–Electrons from n to p
•The leave behind layers of fixed charge on either side
•This sets up an electric field that resists further migration across the junction until  a dynamic equilibrium is reached
•The region around the junction is called the depletion region as it is now depleted of free carriers
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