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Berkeley ELENG 130 - BJTs - Lecture 2

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EECS130 Integrated Circuit DevicesAnnouncementsSchematic representation of pnp and npn BJTsCircuit ApplicationsQualitative BJT OperationModes of OperationBJT Physical StructureQuestionBJT AmplificationSlide Number 10Performance ParametersQualitative Solution - DefinitionsEmitter Region FormulationBase Region FormulationCollector Region FormulationEECS130 Integrated Circuit DevicesProfessor Ali Javey11/13/2007BJTs- Lecture 2Reading Assignment: Chapters 10 and 11Announcements•HW9 is due Tuesday (Nov. 20) in class.•MT2 results•Class project is now assignedSchematic representation of pnp and npn BJTsEmitter is heavily doped compared to collector. So, emitterand collector are not interchangeable.The base width is small compared to the minority carrierdiffusion length. If the base is much larger, then this willbehave like back-to-back diodes.Circuit ApplicationsNotice IB vs. IC !!Qualitative BJT OperationModes of Operation• Cutoff: Base diode not turned on (VBE < 0.6V) so IB , IC = 0• Saturation: Base diode turned on, but IC limited by low VCE .• Active: IC controlled by IB , no effect of VCEBJT Physical StructureB EC p+p+ P base N collectorN+ subcollector P− substrateN+polySiN+DeeptrenchDeep trench ShallowtrenchP+polySiP+polySiQuestion• How can you increase IC /IB ?BJT AmplificationAs base width decreases, more currect is collected at collector, i.e., IB << ICBCII=βIncreasing βPerformance Parameters• Emitter Efficiency:– Decrease 5 relative to 2 to increase gain• Base Transport Factor:– Decrease 1 relative to 2 to increase gain• Common Base dc Current Gain:– (2) + (3) vs. (1+2) + (5)• Common Emitter dc Current Gain:– (2) + (3) vs. (4+5)EnEpEpIII+=γEpCpIIT=αTIIdcECγαα==dcdcBCIIdcααβ−==1Qualitative Solution - DefinitionsNE = NAEDE = DNτE = τnLE = LNnE0 = np0 = ni2/NENB = NDBDB = DPτB = τpLB = LPpB0 = pn0 = ni2/NBNC = NACDC = DNτC = τnLC = LNnC0 = np0 = ni2/NCEmitter Region Formulation• Diffusion equation:• Boundary ConditionsEEEndxndEDτΔΔ−=22"0)1()0"(0)"(/0−==Δ=∞→ΔkTqVEEEEBenxnxnBase Region Formulation• Diffusion equation:• Boundary ConditionsBBBpdxndBDτΔΔ−=220)1()()1()0(/0/0−=Δ−=ΔkTqVBBkTqVBBCBEBepWpeppCollector Region Formulation• Diffusion equation:• Boundary


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Berkeley ELENG 130 - BJTs - Lecture 2

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