EECS 105 Spring 2004 Lecture 22 Lecture 21 BJTs Bipolar Junction Transistors Prof J S Smith Department of EECS University of California Berkeley EECS 105 Spring 2004 Lecture 22 Prof J S Smith Context In 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 devices Department of EECS University of California Berkeley 1 EECS 105 Spring 2004 Lecture 22 Prof J S Smith Reading Today 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 Berkeley EECS 105 Spring 2004 Lecture 22 Prof J S Smith Lecture Outline z z z z BJT Physics 7 2 BJT Ebers Moll Equations 7 3 BJT Large Signal Models BJT Small Signal Models Next Circuits Department of EECS University of California Berkeley 2 EECS 105 Spring 2004 Lecture 22 Prof J S Smith Currents in the BJT z z A BJT is ordinarily designed so that the minority carrier injection into the base is far larger than the minority carrier injection into the emitter It is also ordinarily designed such that almost all the minority carriers injected into the base make it all the way across to the collector Department of EECS University of California Berkeley EECS 105 Spring 2004 Lecture 22 Prof J S Smith Current controlled z So the current is determined by the minority current across the emitter base junction IC I S e z qVBE kT But since the majority of the minority current goes right through the base to the collector IC I E z And so the amount of current that must be supplied by the base is small compared to the current controlled I C I B Department of EECS University of California Berkeley 3 EECS 105 Spring 2004 Lecture 22 Prof J S Smith BJT operating modes z Forward active z Saturation z Emitter Base forward biased Base Collector reverse biased Both junctions are forward biased Reverse active Emitter Base reverse biased Base Collector forward biased Transistor operation is poor in this direction because 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 Berkeley EECS 105 Spring 2004 Lecture 22 Prof J S Smith Collector Characteristics IB Saturation Region Low Output Resistance Breakdown Linear Increase Reverse Active poor Transistor Forward Active Region Very High Output Resistance Department of EECS University of California Berkeley 4 EECS 105 Spring 2004 Lecture 22 Prof J S Smith The origin of current gain in BJT s z z z The 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 collectorbase junction The 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 The current which does have to be supplied by the base contact comes from two main sources z Recombination in the base can often neglect in Silicon Injection of minority carriers into the emitter If we find the ratio of the current to the current that must be supplied by the base that will give us the current gain Department of EECS University of California Berkeley EECS 105 Spring 2004 Lecture 22 Prof J S Smith Diffusion Currents Minority holes in emitter they recombine at the contact Minority electrons in the base Base collector depletion extracts the minority carriers from the base The 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 Department of EECS University of California Berkeley 5 EECS 105 Spring 2004 Lecture 22 Prof J S Smith Diffusion Revisited z z z Why is minority current profile a linear function The diffusion current is proportional to the gradient diffusion constant Since current is constant gradient is constant Note that diffusion current density is controlled by width of region base width for BJT 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 Density at the contact is equal to the equilibrium value strong G R Wp z Decreasing width increases current Department of EECS University of California Berkeley EECS 105 Spring 2004 Lecture 22 Prof J S Smith BJT Currents Collector current is nearly identical to the magnitude of the emitter current define F 999 I C F I E Kirchhoff I E IC I B DC Current Gain I C F I E F I B I C IC Department of EECS F IB F IB 1 F F F 999 999 1 F 001 University of California Berkeley 6 EECS 105 Spring 2004 Lecture 22 Prof J S Smith Origin of F Base emitter junction some reverse injection of holes into the emitter base current isn t zero Some electrons lost due to recombination E B C F 100 Typical F 99 Department of EECS University of California Berkeley EECS 105 Spring 2004 Lecture 22 Prof J S Smith Collector Current Diffusion of electrons across base results in J diff n qDn n pB 0 qVkTBE qDn e dx WB dn p qDn n pB 0 AE IS WB IC I S e Department of EECS qVBE kT University of California Berkeley 7 EECS 105 Spring 2004 Lecture 22 Prof J S Smith Base Current In silicon recombination of carriers in the base can usually be neglected so the base current is mostly due to minority injection into the emitter Diffusion of holes across emitter results in J diff p dpnE qD p pnE 0 qVkTBE qD p 1 e dx WE qD p pnE 0 AE qVkTBE 1 IB e WE Department of EECS University of California Berkeley EECS 105 Spring 2004 Lecture 22 Prof J S Smith Current Gain qDn n pBo AE WB IC Dn n pB 0 WE F I B qD p pnEo AE D p pnE 0 WB WE Minimize base width ni2 N A B N D E n pB 0 2 ni N A B pnE 0 N D E Maximize doping in emitter Department of EECS University of California Berkeley 8 EECS 105 Spring 2004 Lecture 22 Prof J S Smith Simple NPN BJT model z A simple model for a NPN BJT C I B t B iB t VBE t IB E Real diode not an ideal diode C VCE VBE IE Department of EECS University of California Berkeley EECS 105 Spring 2004 Lecture 22 Prof J S Smith Ebers Moll Equations Exp 6 measure E M parameters Derivation Write emitter and collector currents in
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