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11/9/20101LECTURE 4: PN JUNTION DIODESEECS 170A11/9/20101Preparation for Devices•After understanding the quasi Fermi levels and non‐equilibrium condition wemay proceed to simple semiconductor devices.•Before we move on let’s remember the basic postulates of thermal equilibrium•There are two types of charged carriers in a semiconductor, electrons and holes,and current is generated ONLY by diffusion or drift of these carriers11/9/20102JJJqDnqEn•Semiconductor reaches equilibrium when diffusion, drift, generation andrecombination balances each other and hence the probability of finding electronat a given energy level is always constant at any point along the crystal. Hence0FFFdEdEdEdxdydz||||nn diffn dripp diffp driftptnnpfJJJqDpqpE Preparation for Devices (2)•Semiconductor is under non‐equilibrium condition if there is an externalperturbation in the form of generation, diffusion, drift and recombination. Theseperturbations add excess carriers that varies in space and in time as defined bycontinuity equations.11/9/20103Diffusion by carrier gradientDrift due to external fieldRecombinationCarrier generation22PpLdpddpDdxdxpGdtpDrift due to external fieldDiffusion 22pnNdnDdxdndxdndtCarrier generationby carrier gradienRecombinationnLtGnP‐N Junctions: Basic postulates•So far we were dealing with bulk semiconductors with fixed type (p type or n‐type)•Now we would like to understand what will happen if we have two differentsemiconductors are brought together11/9/20104P‐TYPEN‐TYPEXECEVEiEFECEVEiEF
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