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lecture31

# lecture31 - Transistor Gain can be written as the product...

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Lecture 31 Wednesday April 20 EE 332 Semiconductor Devices Problem Assignment Monday Æ Problems 9.2 and find the depletion widths at collector and emitter junctions for the transistor in problem 9.5 Wednesday Æ 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 γα α = = = α can be written as the product of three gains γ = Injection Efficiency α 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 / 0 bias / 0 1 ) 0 ( B B =  → = normal Using the standard pn -junction equations ( ) 0 bias / 0 1 ) ( B B n e n x n kT V B B BC =  → = normal AB i N n n 2 0 = B In the case of nondegenerate doping

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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.
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lecture31 - Transistor Gain can be written as the product...

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