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Chapter 5 Lecture - Part 3

Currents and forward biasing currents what drives

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Unformatted text preview: ng the concentration of excess electrons in this region. Currents and Forward Biasing Currents • What drives current flow (majority carriers) ? – Near the junction • Drift and diffusion, with drift dominating • Inhomogeneities in carrier distribution set up a diffusion Inhomogeneities component component – Far from the junction • Drift only Drift • Homogeneity of charge carriers in this region means no Homogeneity diffusion diffusion Currents and Forward Biasing Electron and hole components of current in a forward-biased p-n junction. In this example, we have a higher injected minority hole current on the n-side than electron current on the p side because we have a lower n doping than p doping. Charge Depletion and Reverse Biasing Charge • From the minority carrier perspective: Δpn ~ -pn which indicates that p(xn0) = 0 • Reverse biasing leads to a depletion of minority Reverse carriers at the boundaries carriers – Called minority carrier extraction • Physical interpretation – Minority carriers are initially swept across the junction – These carriers are not replaced by diffusion of carriers These to the junction boundaries to • The quasi-Fermi levels adjust to reflect depletion – The levels will fall outside the bands The Reverse Biased Junction (a) minority carrier distributions near the reverse-biased junction; (b) variation of the quasi-Fermi levels. The Quasi-Fermi Levels The • Recall that the Fermi level is a construct to Recall represent the distribution of charge carriers wrt the energy band structure in equilibrium • Quasi Fermi levels are the equivalent for steady Quasi state situations such as biasing state • The quasi-Fermi leve...
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