EE105 Fall 2005 Microelectronic Devices and Circuits Lecture 25 Bipolar Junction Transistor Announcements Homework 11 due after Thanksgiving No lab this week Lab 9 next week Reading Chapter 7 7 1 3 2 1 Midterm 2 Hi 50 Lo 12 Average 38 Median 39 5 3 Midterm 2 25 20 Midterm 1 15 Midterm 2 10 5 0 10 10 15 15 20 20 25 25 30 30 35 35 40 40 45 45 50 50 4 2 Lecture Material Last lecture Multistage amplifiers Cascode amplifier design example This lecture Finish cascode amplifier Bipolar junction transistor 5 Ideal BJT Structure IC Collector N Base P Emitter N IB VCE VBE IE Emitter P VEB IE VEC Base N IB Collector P IC NPN or PNP sandwich Two back to back diodes How does current flow Base is very thin A good BJT satisfies the following IC I E I C I B IC I S e qVBE kT 13 3 Actual BJT Cross Section Vertical npn sandwich pnp is usually a lateral structure n buried layout is a low resistance contact to collector Base width determined by vertical distance between emitter diffusion and base diffusion 14 BJT Layout Emitter area most important layout parameter 15 4 BJT Schematic Symbol IC I B IB IC I S e VCE VBE IE qVBE kT VC VB VE Collector current is control by base current linearly Collector current is an exponential function of the baseemitter voltage 16 BJT Collector Characteristic Ground emitter Fix VCE Drive base with fixed current IB Measure the collector current 17 5 Collector Characteristics IB Saturation Region Low Output Resistance Breakdown Linear Increase Reverse Active Bad Transistor Forward Active Region Very High Output Resistance 18 Base Emitter Voltage Control Saturation Region Low Output Resistance 0 3V Breakdown Exponential Increase Reverse Active Bad Transistor Forward Active Region High Output Resistance 19 6 Transistor Action recombination Collector n VCB 0 e Base p h h h VBE 0 e Emitter n Base emitter junction is forward biased and collector base junction is reverse biased Electrons emitted into base much more than holes since the doping of emitter is much higher Magic Most electrons cross the base junction and are swept into collector Why Base width much smaller than diffusion length Base collector junction pulls electrons into collector 20 Diffusion Currents Minority carriers in base form a uniform diffusion current Since emitter doping is higher this current swamps out the current portion due to the minority carriers injected from base 21 7 BJT Currents Collector current is nearly identical to the magnitude of the emitter current define F 999 I C F I E KCL I E IC I B DC Current Gain I C F I E F I B I C IC F IB F IB 1 F F F 999 999 1 F 001 22 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 Typical F 99 B C F 100 23 8 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 qVBE kT 24 Base Current 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 25 9 Current Gain qDn n pBo AE WB IC D n pB 0 WE n 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 26 Ebers Moll Equations Derivation Write emitter and collector currents in terms of internal currents at two junctions I E I ES eVBE Vth 1 R I CS eVBC Vth 1 I C F I ES eVBE Vth 1 I CS eVBC Vth 1 F I ES R I CS 27 10 Ebers Moll Equivalent Circuit Building blocks diodes and I controlled I sources 28 Forward Active Region B C junction is not forward biased IR is very small Typical Values VBE 0 7 VCE 0 2 29 11 Simplified Ebers Moll Forward Active Case IB B IC C IC F I B VBE 0 7 E Saturation both diodes are forward biases batteries IB B VBE 0 7 IC C VCE 0 1 E 30 12
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