Small-signal current gain ?o are both highly In practice, the DC current gain ?F and the To find the change in base current due to change The base terminal of a bipolar transistor Unlike MOSFET, there is a DC current into Differentiating and evaluating at Q (VBE, VCE ).The transconductance is analogous to diode.Saturation both diodes are forward-biases ? Terms of internal currents at two junctionsīuilding blocks diodes and I-controlled Iī-C junction is not forward-biased ? IR is very Holes into the emitter ? base current isnt zeroĭiffusion of electrons across base results inĭiffusion of holes across emitter results in (magnitude) of the emitter current defineīase-emitter junction some reverse injection of
Portion due to the minority carriers injectedĬollector current is nearly identical to the Higher, this current swamps out the current Minority carriers in base form a uniformĭiffusion current.Why? Base width much smaller than diffusion.Magic Most electrons cross the base junction.Holes since the doping of emitter is much higher Electrons emitted into base much more than.Base-emitter junction is forward biased andĬollector-base junction is reverse biased.Saturation Region (Low Output Resistance)įorward Active Region (High Output Resistance) Collector is controlled by base-emitter voltage.Collector current is control by base current.Multi-finger device also possible for reduced.Emitter area most important layout parameter.Base width determined by vertical distanceīetween emitter diffusion and base diffusion.n buried layout is a low resistance contact to.Vertical npn sandwich (pnp is usually a lateral.How does current flow? Base is very thin.NPN or PNP sandwich (Two back-to-back diodes).Time to cross junction (or minority carrier By charge neutrality, the source voltage must.Increasing forward bias increases minority charge.On average additional charge is stored in diode.Minority carriers injected into p and n regions.But another charge storage mechanism comes into.Grows and shrinks in response to the appliedįield. We have already seen that a reverse biased diodeĪcts like a capacitor since the depletion region.The I-V relation of a diode can be linearized.Title: Lecture 14: Bipolar Junction Transistors
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