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Lecture 03-04 Solid-solid Junctions - PN Junction Reference. 1. R. Memming, Semiconductor Electrochemistry, Wiley-VCH, 2000 2. R. F. Pierret, Semiconductor Device Fundamentals, Addison Wesley, 1996 3. S. M. Sze, Physics of Semiconductor Device, Wiley, 2006 Todayβs Agenda β’ PN junction β’ Illuminated pn junction PN Junction Majority carriers: holes Minority carriers: electrons Majority carriers: electrons Minority carriers: holes When n-type and p-type semiconductors are brought together, electrons flow from n-type to p-type by diffusion holes flow from p-type to n-type by diffusion Then, what happens in a region where the carriers leave away? When n-type and p-type semiconductors are brought together, electrons flow from n-type to p-type by diffusion holes flow from p-type to n-type by diffusion But a region where electrons and holes diffuse away is left with a fixed ionized space charge with E-field. electrons flow from p-type to n-type by drift holes flow from n-type to p-type by drift In a pn junction diode at equilibrium, these diffusion and drift currents are opposite direction with the same magnitude. No Net Current Flow! and A Constant Fermi Energy Level Band bending charge density Charge neutrality NAxp = NDxn Depletion width xp+xn If you have charge distribution, there is a electric field given by Poisson Equation ππΈ π = (π β π + ππ·+ β ππ΄β ) ππ₯ π charge density Depletion width xp+xn πΈ π₯ =β π π ππ₯ π π΄ πΈ π₯ = π π ππ₯ π π· charge density E-field is highest at the junction πΈ π₯ =β π π ππ₯ π π΄ Depletion width xp+xn πΈ π₯ = π π ππ₯ π π· E-field is zero outside the depletion region If you know E-field distribution, potential distribution is given by E = -dV/dx (definition) charge density Depletion width xp+xn π π₯ =β πΈππ₯ Vbi : built-in potential Band Bending vs. Potential Distribution Higher hole energy Higher electron energy Higher hole energy Energy band is presented in a electron energy scale Energy band is simply the opposite of potential energy distribution in a semiconductor ππππ = πΈπ β πΈ1 β πΈ2 = ππ ππ΄ ππ· ln( 2 ) π ππ qVbi A system at equilibrium must have one Fermi energy level (or chemical potential)! Carrier Distribution at Equilibrium 1.P-N junction diode is integral for all electronic devices to operate, aggregating all forms of carrier transport, generation, and recombination. 2.Majority carriers can diffuse across the P-N junction depletion region, even though the electric field impedes their crossing. Minority carriers that reach the junction are swept across the depletion region due to drift. 3.At equilibrium, the net current (diffusion and drift current) is zero for both electrons and holes because the diffusion current is equal and opposite to the drift current for both carriers. Effect of External Bias Effect of External Bias Vbi Effect of External Bias Vbi Vbi - lVAl (+) for the p-type and (-) for the n-type Effect of External Bias Vbi Vbi - lVAl (+) for the p-type and (-) for the n-type Vbi + lVAl External bias simply changes the height of the energy barrier between pn junction (-) for the p-type and (+) for the n-type Vbi act as a energy barrier for electrons and holes to go to the other side External bias simply changes the height of the energy barrier Carrier Distribution in a Diode without Bias Carrier Distribution in a Diode with Forward Bias Minority carrier injection Carrier flow by diffusion A pn diode is a minority carrier device and the current flows by diffusion of minority carriers! Diode Equation πππ΄ πΌ = πΌ0 [exp β 1] ππ I0 : saturation current density Solar Cell I-V Characteristics Current from light absorption πππ΄ πΌ = πΌ0 exp β 1 β πΌπΏ ππ I-V curve of a solar cell is a superposition of a diode I-V curve and the photocurrent Next time Solid-solid junctions β’ Metal-Semiconductor junction
