lecture31 - Lecture 31 Wednesday April 20 EE 332...

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Unformatted text preview: Lecture 31 Wednesday April 20 EE 332 Semiconductor Devices Problem Assignment Monday Problems 9.2 and find the depletion widths at collector and emitter junctions for the transistor in problem 9.5 Wednesday Problem 9.5 and 9.12 Transistor Gain M I I I I I I I I T nC C nE nC E nE E C = = = can be written as the product of three gains = Injection Efficiency T = Transport Efficiency M = Collection Efficiency nE pE E nE I I I I / 1 1 + = = nE rec nE nC I I I I = = 1 nC pC nC C I I I I / 1 + = = BJT Model Assumptions 1. Base width, W B , which is measured between edges of depletion region, is assumed small compared to the minority diffusion length in the base, L nB . 2. Length of emitter is assumed small compared to the minority diffusion length in emitter, L pE . 3. Concentration of injected electrons into the base is assumbed small compared to the majority concentration. This is called the low- injection condition. Carriers Injected Across pn-Junctions ( ) kT V kT V B BE BE e n e n n / bias / 1 ) ( B B = = normal Using the standard pn-junction equations ( ) bias / 1 ) ( B B n e n x n kT V B B BC = = normal AB i N n n 2 = B In the case of nondegenerate doping Collector Efficiency 1 1 + = = nC pC nC C I I I I M I pC is the reverse bias current due to holes ( npn ) being injected into the base, and the holes resulting from electron hole pair generation. As we found with diodes, both of these currents are normally negligibly small. Injection Efficiency nE pE...
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This note was uploaded on 06/30/2010 for the course EECE 332 taught by Professor Constable during the Spring '08 term at Binghamton University.

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lecture31 - Lecture 31 Wednesday April 20 EE 332...

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