EECS130 Integrated Circuit DevicesAnnouncementsPop QuizPunchthroughBase-Width Modulation by Collector VoltagePunchthroughAvalanche MultiplicationEmitter Bandgap NarrowingHeterojunction BJTsHeterojunction BTJsNarrow-Bandgap (SiGe) BaseEXAMPLE: Emitter Bandgap NarrowingEECS130 Integrated Circuit DevicesProfessor Ali Javey11/27/2007BJTs- Lecture 5Reading Assignment: Finish Chapter 11Announcements•HW10 is due Thursday, 11/29.Pop Quiz• Draw the minority and majority carrier concentration profiles for a n+pn BJT at equilibrium and in the saturation mode. Assume W<<LB . Clearly label the emitter, base, and collector regions. Label pE , nE , pB , nB , pC , nCPunchthroughPunch-through can be viewed as base width modulation carriedto the extreme, i.e., punch-through occurs when W → 0. ForC-B voltage beyond punch-through, the E-B barrier lowers and results in large increase in carrier injection from emitter to collector.Large increase in collector currents at high VCE0 occurs dueto two reasons:Punchthrough orAvalanche multiplication(Depletion region in collector is not shown)How can we reduce the base-width modulation effect?N+ PN emitter basecollector VCECEWB3WB2WB1 xn'} reduction of base width VCE1< VCE2<VCE3B VBEBase-Width Modulation by Collector VoltagePunchthroughAvalanche Multiplication• Avalanche may be important:1. If it occurs before punchthrough2. As an amplification mechanism in a phototransistor• Inject a photon into the CB depletion region to cause avalanche multiplication of it22iEiBBEnnNN∝βTo raise β, NE is typically very large, so22iiEnn >(called the heavy doping effect).kTEVCigeNNn/2−=Since heavy doping can reduce Eg , this effect is also known as band-gap narrowing. kTEiiEgEenn/22Δ=ΔEgE is negligible for NE < 1018 cm-3, is 50 meV at 1019cm-3, 95 meV at 1020cm-3, and 140 meV at 1021 cm-3.Emitter Bandgap NarrowingHeterojunction BJTsHeterojunction BTJsnpn heterojunction BJT with wide band gap emitter22iEiBBEnnNN∝β22iEiBBEnnNN∝βTo further elevate β , we can raise niB by using an epitaxial Si1-η Geη base.With η = 0.2, EgB is reduced by 0.1eV.Narrow-Bandgap (SiGe) BaseAssume DB = 3DE , LE = 3W , W<<LB , NB = 1018 cm-3, and niB2 = ni2. What is βfor (a) NE = 1019 cm-3, (b) NE = 1020 cm-3, and (c) NE = 1020 cm-3 and a SiGe base with ΔEgB = 60 meV ?(a) At NE = 1019 cm-3, ΔEgE ≈50 meV,(b) At NE = 1020cm-3, ΔEgE ≈95 meV(c) EXAMPLE: Emitter Bandgap
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