# Lecture4 - 2 Afterunderstanding thequasiFermi condition we...

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11/9/2010 1 LECTURE 4: PN JUNTION DIODES EECS 170A 11/9/2010 1 Preparation for Devices After understanding the quasi Fermi levels and non equilibrium condition we may 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 carriers 11/9/2010 2 J J J qD n q E n  Semiconductor reaches equilibrium when diffusion, drift, generation and recombination balances each other and hence the probability of finding electron at a given energy level is always constant at any point along the crystal. Hence 0 F F F dE dE dE dx dy dz | | | | n n diff n dri p p diff p drift p t n n p f J J J qD p q pE    Preparation for Devices (2) Semiconductor is under non equilibrium condition if there is an external perturbation in the form of generation, diffusion, drift and recombination. These perturbations add excess carriers that varies in space and in time as defined by continuity equations. 11/9/2010 3 Diffusion by carrier gradient Drift due to external field Recombination Carrier generation 2 2 P p L d p d d p D dx d x p G dt p   Drift due to external field Diffusion 2 2 p n N d n D dx d n dx d n dt  Carrier generation by carrier gradien Recombination n L t G n  P 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 different semiconductors are brought together 11/9/2010 4 P TYPE N TYPE X E C E V E i E F E C E V E i E F

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