1Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 22Lecture 21: BJTs(Bipolar Junction Transistors)Prof J. S. SmithDepartment of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 22 Prof. J. S. SmithContextIn Friday’s lecture, we discussed BJTs(Bipolar Junction Transistors)Today we will find large signal models for the bipolar junction transistor, and start exploring how to use transistors to make amplifiers and other analog devices2Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 22 Prof. J. S. SmithReadingToday’s lecture will finish chapter 7, Bipolar Junction Transistors (BJT’s)Then, we will start looking at amplifiers, chapter 8 in the text.Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 22 Prof. J. S. SmithLecture OutlinezBJT Physics (7.2)zBJT Ebers-Moll Equations (7.3)zBJT Large-Signal ModelszBJT Small-Signal ModelsNext: Circuits3Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 22 Prof. J. S. SmithCurrents in the BJTzA BJT is ordinarily designed so that the minority carrier injection into the base is far larger than the minority carrier injection into the emitter.zIt is also ordinarily designed such that almost all the minority carriers injected into the base make it all the way across to the collectorDepartment of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 22 Prof. J. S. SmithCurrent controlledzSo the current is determined by the minority current across the emitter-base junctionzBut since the majority of the minority current goes right through the base to the collector:zAnd so the amount of current that must be supplied by the base is small compared to the current controlled:CEII≈−CBII>>BEqVkTCSIIe≈4Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 22 Prof. J. S. SmithBJT operating modeszForward active–Emitter-Base forward biased–Base-Collector reverse biasedzSaturation–Both junctions are forward biasedzReverse active–Emitter-Base reverse biased–Base-Collector forward biased–Transistor operation is poor in this direction, becauseβ is low: lighter doping of the layer designed to be the collector means that there is a lot of minority carrier injection out of the Base.Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 22 Prof. J. S. SmithCollector Characteristics (IB)Forward ActiveRegion (Very High Output Resistance)Saturation Region (Low Output Resistance)Reverse Active(poor Transistor)BreakdownLinear Increase5Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 22 Prof. J. S. SmithThe origin of current gain in BJT’szThe majority of the minority carriers injected from the emitter go across the base to the collector and are swept out by the electric field in the depletion region of the collector-base junction.zThe base contact doesn’t have to supply that current to maintain the voltage of the base—the voltage which is causing the current in the first place.zThe current which does have to be supplied by the base contact comes from two main sources:–Recombination in the base (can often neglect in Silicon)–Injection of minority carriers into the emitterzIf we find the ratio of the current to the current that must be supplied by the base, that will give us the current gain β.Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 22 Prof. J. S. SmithDiffusion CurrentsThe minority carriers injected into the base have a concentration gradient, and thus a current. Since emitter doping is higher, this current is much larger than the current due to the minority carriers injected from the base to the emitter. This is the source of BJT current gain.Base-collectordepletion extractsthe minority carriers from thebaseMinority holesin emitter-theyrecombine at the contactMinority electronsin the base6Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 22 Prof. J. S. SmithDiffusion RevisitedzWhy is minority current profile a linear function?zThe diffusion current is proportional to the gradient×diffusion constant. Since current is constant→gradient is constantzNote that diffusion current density is controlled by width of region (base width for BJT):zDecreasing width increases current!Density here fixed by potential (injection of carriers)It is proportional to the number of majority carriers on The other side of the barrier, and is exponential with the(lowered) barrier height.WpDensity at the contact is equal to the equilibrium value (strong G/R)Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 22 Prof. J. S. SmithBJT CurrentsCFEIIα=−Collector current is nearly identical to the (magnitude)of the emitter current … defineKirchhoff: ECBIII−=+DC Current Gain:()CFEFBCIIIIαα=−= +1FCBFBFIIIαβα==−.999Fα=.9999991.001FFFαβα===−7Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 22 Prof. J. S. SmithOrigin of αFBase-emitter junction: some reverse injection of holes into the emitter Æ base current isn’t zero E BCTypical:Some electrons lost due to recombination.99Fα≈100Fβ≈Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 22 Prof. J. S. SmithCollector CurrentDiffusion of electrons across base results in0BEqVpnpBdiffkTnnBdn qD nJqD edx W⎛⎞==⎜⎟⎝⎠BEqVkTCSIIe=0npB ESBqD n AIW⎛⎞=⎜⎟⎝⎠8Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 22 Prof. J. S. SmithBase CurrentIn silicon, recombination of carriers in the basecan usually be neglected, so the base current is mostly due to minority injection into the emitter.Diffusion of holes across emitter results in01BEqVpnEdiffnEkTppEqD pdpJqD edx W⎛⎞⎛⎞=− = −⎜⎟⎜⎟⎝⎠⎝⎠01BEqVpnE EkTBEqD p AIeW⎛⎞⎛⎞=−⎜⎟⎜⎟⎝⎠⎝⎠Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 22 Prof. J. S. SmithCurrent Gain00npBoEpBBCnEFpnEoEBpnEBEqD n AnWIDWqD p AIDpWWβ⎛⎞⎜⎟⎛⎞⎛⎞⎛⎞⎝⎠== =⎜⎟⎜⎟⎜⎟⎜⎟⎛⎞⎝⎠⎝⎠⎝⎠⎜⎟⎝⎠20,,20,,ipBAB DEinE A BDEnnNNnpNN⎛⎞==⎜⎟⎝⎠Minimize base widthMaximize doping in emitter9Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 22 Prof. J. S.
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