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Unformatted text preview: conductor Devices Sang-Hyun Oh 4 UNIVERSITY OF MINNESOTA PN junction: Equality of Fermi Level
conduction band Fermi level (electro-chemical potential) valence band ? ? Fermi level must be equal throughout a system in thermal and diffusive equilibrium. Otherwise particles would move so as to equalize the chemical potentials. Well away from the junction, the materials each should look just like they did before we joined them.
EE3161 Semiconductor Devices Sang-Hyun Oh 5 UNIVERSITY OF MINNESOTA Formation of the Depletion Region
Because of the imbalance of # of electrons between n and p regions, electrons tend to move (diffuse) from the n into the p material, leaving behind a net positive charge in the n material. In the p material, the electrons will recombine with holes, reducing # of holes near the edge. Net electric ﬁeld is generated, which stops the transfer process. EE3161 Semiconductor Devices Sang-Hyun Oh 6 UNIVERSITY OF MINNESOTA Basic P-N Junction Ionized impurities are ﬁxed in space. Electrons and holes are mobile. Balance between drift and diffusion.
EE3161 Semiconductor Devices Sang-Hyun Oh dn = qµn nE (x) + qDN dx
7 UNIVERSITY OF MINNESOTA PN Junction as a capacitor + V=Q/C V=Q/C -xp xn Q=NA xp = ND xn Depletion regions are established, where oppositely signed space-charge regions give rise to a voltage. Charge neutrality: Q=NA...
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- Spring '10