Bipolar Junction Transistors (BJTs)

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Lecture NotesBipolar Junction Transistors (BJTs)Outline • BJT structure and I-V characteristics • Physical operation of power BJTs • Switching characteristics • Breakdown voltage • Second breakdown • On-state voltage • Safe operating areas

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Basic Geometry of Power BJTsFeatures to Note • Multiple narrow emitters - minimize emitter current crowding. • Multiple parallel base conductors - minimize parasitic resistance in series with the base.

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BJT Construction ParametersFeatures to Note • Wide base width - low (<10) beta. • Lightly doped collector drift region - large breakdown voltage.

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Darlington-connected BJTs• Composite device has respectable beta.

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Power BJT I-V CharacteristicFeatures to Note • 2nd breakdown - must be avoided. • Quasi-saturation - unique to power BJTs • BVCBO > BVCEO - extended blocking voltage range.

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BJT Internal Current Components• Ine and Ipe flow via diffusion. Inc and Ipc flow via drift. • Ine >> Ipe because of heavy emitter doping. • Ine ≈ Inc because Lnb = {Dnb tnb}1/2 << Wbase and collector area much larger than emitter area. • Ipc << other current components because very few holes in b-c space charge region.

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• IC ≈ Inc since Ipc very small : IB = - IC - IB = - Inc+ Ine + Ipe • IB/ IC = 1/b = (Ine - Inc)/Inc + Ipe/Inc • (Ine - Inc)/Inc represents fraction of electrons injected into base that recombine in the base. Minimize by having large values of tnb (for long diffusion lengths) and short base widths Wbase • Ipe proportional to pno = (ni)2/Nde ; Minimize via large Nde • Short base width conflicts with need for larger base width needed in HV BJTs to accomodate CB depletion region. • Long base lifetime conflicts with need for short lifetime for faster switching speeds • Trade-offs (compromises) in these factors limit betas in power BJTs to range of 5 to 20 Power BJT Current Gain b

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Beta versus Collect Current• Beta decrease at large collector current due to high level injection effects (conductivity modulation where dn = dp) in base. • When dn = dp, base current must increase faster than collector current to provide extra holes. This constitutes a reduction in beta. • High level injection conditions aided by emitter current crowding.

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Emitter Current Crowding• IB proportional to exp{qVBE/(kT)} • Later voltage drops make VBE larger at edge of emitters. • Base/emitter current and thus carrier densities larger at edge of emitters. So-called emitter current crowding. • This emitter current crowding leads to high level injection at relatively modest values of current. • Reduce effect of current crowding by breaking emitters into many narrow regions connected electrically in parallel.

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Quasi-saturation in Power BJTsActive region VBC < 0Quasi-saturation VBC > 0 but drift region not completely filled with excess carriers.Hard saturation VBC > 0 and drift region filled with excess carriers.Power BJT• Beta decreases in quasi-saturation because effective base width (virtual base) width has increased.

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Generic BJT Application - Clamped Inductive Load• Current source Io models an inductive load with an L/R time constant >> than switching period. • Positive base current turns BJT on (hard saturation). So-called forward bias operation. • Negative base current/base-emitter voltage turns BJT off. So-called reverse bais operation. • Free wheeling diode DF prevents large inductive overvoltage from developing across BJT collector-emitter terminals.

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Power BJT Turn-on Waveforms

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Excess Carrier Growth During BJT Turn-on• Growth of excess carrier distributions begins after td(on) when B-E junction becomes forward biased. • Entrance into quasi-saturation discernable from voltage or current waveform at start of time tvf2. • Collector current “tailing” due to reduced beta in quasi-saturation as BJT turns off. • Hard saturation entered as excess carrier distribution has swept across dirft region.

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Turn-off Waveforms with Controlled Base Current• Base current must make a controlled transition (controlled value of -diB/dt) from positive to negative values in order to minimize turn-off times and switching losses.

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Controlled Turn-off Excess Carrier Removal• ts = storage time = time required to remove excess charge Q3. • trv1 = time to remove charge Q2 holding transistor in quasi-saturation. • trv2 = time required for VCE to complete its growth to Vdc with BJT in active region. • tfi = time required to remove remaining stored charge Q1 in base and each edge of cut-off.

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Turn-off Waveforms with Uncontrolled Base Current• Excessive switching losses with collector current tailing.

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Uncontrolled Turn-off Excess Carrier Removal• Uncontrolled base current removes stored charge in base faster than in collector drift region. • Base-emitter junction reverse biased before collector-base junction. • Stored charge remaining in drift region now can be only removed by the negative base current rather than the much larger collector current which was flowing before the B-E junction was reverse biased. • Takes longer time to finish removal of drift region stored charge thus leading to collector current “tailing” and excessive switching losses.

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Darlington Switching Behavior• Turn-on waveforms for Darlington very similar to single BJT circuit. • Turn-on times somewhat shorter in Darlington circuit because of large base drive for main BJT. • Turn-off waveforms significantly different for Darlington. • Diode D1 essential for fast turn-off of Darlington. With it, QM would be isolated without any negative base current once QD was off. • Open base turn-off of a BJT relies on internal recombination to remove excess carriers and takes much longe than if carriers are removed by carrier sweepout via a large collector current.

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Darlington Turn-off Waveforms

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Power BJT Breakdown Voltage• Blocking voltage capability of BJT limited by breakdown of CB junction. • BVCBO = CB junction breakdown with emitter open. • BVCEO = CB junction breakdown with base open. • BVCEO = BVCBO/(b)1/n ; n = 4 for npn BJTs and n = 6 for PNP BJTs • BE junction forward biased even when base current = 0 by reverse current from CB junction. • Excess carriers injected into base from emitter and increase saturation current of CB junction. • Extra carriers at CB junction increase likelyhood of impact ionization at lower voltages , thus decreasing breakdown voltage. • Wide base width to lower beta and increase BVCEO . • Typical base widths in high voltage (1000V) BJTs = 5 to 10 and BVCEO = 0.5 BVCBO .

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Avoidance of Reach-thru• Large electric field of depletion region will accelerate electrons from emitter across base and into collector. Resulting large current flow will create excessive power dissipation. • Avoidance of reach-thru • Wide base width so depletion layer width less than base width at CB junction breakdown. • Heavier doping in base than in collector so that most of CB depletion layer is in drift region and not in the base.

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Second Breakdown• 2nd breakdown during BJT turn-off in step-down converter circuit.• Precipitious drop in C-E voltage and perhaps rise in collector current. • Simultaneous rise in highly localized regions of power dissipation and increases in temperature of same regions. 1. Direct observations via infrared cameras. 2. Evidence of crystalline cracking and even localized melting. • Permanent damage to BJT or even device failure if 2nd breakdown not terminated within a few µsec.

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2nd Breakdown and Current Density Nonuniformities• Minority carrier devices prone to thermal runaway. • Minority carrier density proportional to ni(T) which increases exponentially with temperature. • If constant voltage maintained across a minority carrier device, power dissipation causes increases in temp. which in turn increases current because of carrier increases and thus better conduction characteristic. • Increase in current at constant voltage increases power dissipation which further increases temperature. • Positive feedback situation and potentially unstable. If temp. continues to increase, situation termed thermal runaway.• Current densities nonuniformities in devices an accenuate problems. • Assume JA > JB and TA > TB • As time proceeds, differences in J and T between regions A and B become greater. • If temp. on region A gets large enough so that ni > majority carrier doping density, thermal runaway will occur and device will be in 2nd breakdown.

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Current Crowding Enhancement of 2nd Breakdown Susceptibility• Emitter current crowding during either turn-on or turn-off accenuates propensity of BJTs to 2nd breakdown. • Minimize by dividing emitter into many narrow areas connected electrically in parallel.

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Velocity Saturation and Second Breakdown• Moderate current in drift region - BJT active • Electric field E1 = Jc/(qµnNd) < Esat• Large current density in drift region - BJT active. • Jc > qµnNd Esat . Extra electrons needed to carry extra current. • Negative space density gives rise to nonuniform electric field. • Emax may exceed impact ionization threshold while total voltage < BVCEO.

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Contributions to BJT On-State Losses• Pon = IC VCE,sat • VCE,sat = VBE,sat - VBC,sat + Vd + IC(Rc + Re)• VBE,sat - VBC,sat typically 0.1-0.2 V at moderate values of collector current. • Rise in VBE,sat - VBC,sat at larger currents due to emitter current crowding and conductivity modulation in base.

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Last Updated: 8th March 2018

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