lecture33 - Lecture 33 Wednesday April 27 EE 332...

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Lecture 33 Wednesday April 27 EE 332 Semiconductor Devices Problem Assignment Monday ! 9.19 and 9.23 Effects of Base Resistance Effects of Base Resistance Base current must flow through r’ b +r b to reach the active region. Design can virtually eliminate r’ b but r b (base spreading resistance or intrinsic base resistance) is always a problem. The forward bias will not be uniform across the base region and will be largest at the edges of the emitter (Effect called emitter crowding ). Geometries such as interdigitated stripes reduce these effects. However, base resistance, r b is always a problem because of the thinness and light doping of the base. Interdigitated Geometry The geometry minimizes emitter crowding. p +
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Modern BJT Process for double poly-silicon BJT. a) n + buried layer; b) n epitaxy followed by LOCOS (local oxidation of silicon) isolation; c) base/emitter window definition and p + poly- silicon is deposited with p diffused for base conduction; d) intrinsic base implant using self-aligned oxide sidewall spacers; e) self aligned formation of n + emitter and n + collector contacts. Base Narrowing W B is not independent of collector voltage. Early Effect - base narrowing. Early Voltage - Extrapolation of I C curves in common point. I-V curve slope increases as W b decreases. Schottky Diode Clamp Schottky diode put in parallel with collector- base junction so base storage charge is small. Schottky diode is a majority carrier device and has no storage charge. S.B.
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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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lecture33 - Lecture 33 Wednesday April 27 EE 332...

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