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Lets build a rectangular potential barrier to scatter electrons. We know that that
electrons traveling in the conduction band are approximately free particles with an
effective mass. What if we insert a small region of a different semiconductor, B, into an
otherwise homogenous semiconductor A. In Figure 4, A is Gallium Arsenide and B is
Aluminum Arsenide. Note that B has a larger band gap than A. So an electron travelling
in A that impinges on B will see an approximately square potential barrier since
the electron will have to jump up into B's conduction band to continue flowing. This
type of device geometry is called a heterojunction. Now that we can construct
potential barriers we are ready to discuss resonance.
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