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ee3161 spring10 lecture note 6 - Pierret Ch 5(pp 195-234...

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U NIVERSITY OF M INNESOTA EE3161 Semiconductor Devices Sang-Hyun Oh Lecture Note 6 P-N Junction: Electrostatics 1 Overview Qualitative PN Junction Depletion approx. Poisson Equation Barrier Height Barrier E-field Biased PN Junction Diode n p Depletion region Pierret Ch 5 (pp. 195-234)
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U NIVERSITY OF M INNESOTA EE3161 Semiconductor Devices Sang-Hyun Oh PN Junction Devices 2 Blue GaN LED Avalanche Photodiode MOS transistor Bipolar Junction Transistor (BJT) Silicon solar cell
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U NIVERSITY OF M INNESOTA EE3161 Semiconductor Devices Sang-Hyun Oh Introduction: P-N Junction The vast majority of semiconductor devices are based on diodes: transistors, photodetectors, light-emitting diodes (LED) and lasers. The most important property of a pn-junction diode is its ability to rectify, i.e. to pass current strongly in one direction but not in the other. In its simplest form, the pn junction diode is made by joining two semiconducting materials, one p-doped and the other n-doped. 3
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U NIVERSITY OF M INNESOTA EE3161 Semiconductor Devices Sang-Hyun Oh Metallurgical Junction Typical doping profile of a pn junction is shown. The dividing line between the p and n regions, known as the metallurgical junction, occurs where N A =N D (or net doping = 0). 4
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U NIVERSITY OF M INNESOTA EE3161 Semiconductor Devices Sang-Hyun Oh PN junction: Equality of Fermi Level 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. 5 conduction band valence band Fermi level (electro-chemical potential) ? ?
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U NIVERSITY OF M INNESOTA EE3161 Semiconductor Devices Sang-Hyun Oh 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. 6
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U NIVERSITY OF M INNESOTA EE3161 Semiconductor Devices Sang-Hyun Oh Basic P-N Junction 7 Ionized impurities are fixed in space. Electrons and holes are mobile. Balance between drift and diffusion. J N = n nE ( x ) + qD N dn dx
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U NIVERSITY OF M INNESOTA EE3161 Semiconductor Devices Sang-Hyun Oh PN Junction as a capacitor Depletion regions are established, where oppositely signed space-charge regions give rise to a voltage. Charge neutrality: We expect that a (forward or reverse?) bias will increase stored charge.
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